Macroscopic Editor, Progression, and Principles

Deus · March 28, 2025, 7:42pm

CONSTRAINTS

As I discussed before, I really do think having constraints which the player must cleverly deal with fundamental challenges is going to result in the greatest amount of engagement, excitement, and diversity, with clear lessons learned from KSP. And I do think that managing these constraints should be among the largest components of editor gameplay.

Again though, it is important to have these constraints be interrelated as much as possible, and have them be approachable and universal so as to not throw 50 different factors at the player to consider at a moment’s notice.

Relating these general constraints to secondary stats can help us understand how they can manifest in gameplay, and will better inform us on how to approach providing tools in the editor.

Biodynamics

Revolves around the movement and mobility of a creature. I think this should be one of the more “active” and engaging constraints a player faces, encouraging a bunch of tweaking and playtesting. I think such a system will ultimately need to represent these aspects:

Center of Mass

If possible, assessing where along a creature’s length their center of mass would result in really interesting emergent dynamism with the editor.

Marine animals with a center of mass closer to their front (sharks, crocodiles, rays):

Have stronger limbs in the front, optimizing striking and lunging capabilities.
Find it easier to swim downwards, being generally better optimized to diving.
Spend less energy swimming since they are better able to “cruise”, reducing movement costs.
Are generally less nimble and agile.
Have more prominent fins in the front to help balance movement.
Are better able to stay attached to the ocean floor.

Marine animals with a center of mass closer to their rear (tuna, sailfish, flying fish):

Have stronger limbs in the rear, optimizing movement and explosiveness in movement.
Find it easier to swim upwards, being generally more buoyant in propulsion.
Spend more energy stabilizing orientation, thus increasing movement costs.
Are generally more nimble and agile.
Have more prominent fins in the rear to help stabilize.
Are better able to breach the water surface, escaping predators.

If we’re looking for a mechanic which makes limb location influential and dynamic, this is a good one. Along with influencing stats, including a measure of center of mass can result in incredible divergent gameplay in the editor, influencing choices made and the positioning of limbs and providing greater depth.

Note that having center of mass be relevant vertically as well will be important for land, but is generally less important for marine animals. Discerning where the center of mass is along the x-axis of an animal is good for the early macroscopic stage.

“Streamline” Measure

Having some sort of measure of being “streamline” would be an important thing to include. It is something most marine animals should generally prefer, but is traded for in certain abilities, such as armor or certain limbs. So players should balance being streamlined between potential improvements in other avenues, such as combat and maneuverability (burrowing, ability to reach land, etc.).

In general, being more streamline reduces the impact of currents, and increases top movement speed. Being less streamline means currents can grab you more, and decreases how fast you can go.

Otherwise/concurrently, the more traditional stats derived from biodynamics include:

Speed: The top speed you can reach.
Agility: Ability to turn, as well as acceleration and deceleration.
Buoyancy: How easy it is to swim upwards or float as opposed to sinking or diving. Influential in benthic v. pelagic lifestyle choices. Neutral buoyancy would probably be desired by most swimmers. Note that we should generally represent buoyancy through movement costs associated with going up or down as opposed to actually making the player sink or float upwards unless buoyancy measure is extremely towards a certain direction - it could be annoying for players to constantly press space or control.
Movement Costs: In general, more explosive movement styles require a lot more energy. Your body plan will also influence movement plan efficiency, with smart placement of limbs in response to constraints placed on your body plan going a good way in making movement more or less efficient.

Certain adaptations, such as swim bladders, can generally reduce the impact of constraints such as center of mass if a player wishes to avoid them. They won’t be impacted by the negatives, but they also won’t benefit from the positives.

Mass

Mass is probably one of the more intuitive constraints a player will face. In general, as a player grows bigger, their mass will increase.

Influences how strongly center of mass affects animal; less massive animals won’t have to worry about their center of mass as much.
Influences total health, with larger animals being better able to take damage.
In general, reduces agility and slightly increases movement speed, up to a certain point. Reduces general buoyancy, requiring more adaptations to remain neutral.
In general, increases the force production of an animal, increasing damage. Note that smaller muscles are more efficient at creating strength however, so there is a tradeoff; beyond a certain size, strength is lost.

Mass also has a strong influence on your metabolic rates. Larger animals generally need more food in terms of sheer quantity, but have reduced metabolic rates, meaning the same amount of food lasts a bit longer. However, I believe metabolism should be more specifically discussed before discussing this aspect of things.

Surface Area to Volume Ratio

Surface area is multi-faceted, affecting metabolism, respiration, and movement. It will be more important for soft-bodied animals and for certain types of skins/thickness, influencing evolutionary strategy. Movement effects will be a bit more universal. Note that for every bullet, the opposite statement is true (if higher surface area to volume boosts x, then lower surface area to volume reduces x)

Higher surface area to volume increases buoyancy.
Higher surface area to volume reduces health.
Higher surface area to volume increases the impact of currents, as there is more of a “sail” to catch.
Higher surface area to volume decreases speed and increases agility due to drag.
Higher surface area to volume decreases tolerance to cold, but increases tolerance to heat.

Questionable

Square-Cube Law: The square cube law is an important constraint to represent that we surely should put some thought into. I put it here though because it is a multi-faceted feature which influences other constraints in dynamic ways, influencing metabolism, movement, growth, and more. As such, a lot of thought would need to be put into explaining this feature.

Ultimately, I think creating really well-made mechanics centered around these constraints would be really worthwhile and would go a long way in making the Macroscopic Stage really fun. Hopefully it’s manageable for auto-evo as well.

Deus · March 31, 2025, 6:51pm

Here is an example of how basing our editor around solving problems posed by constraints can lead to some really engaging editor gameplay, with a reduced need of just adding an endless number of tools for the player to interact with.

Center of Mass

Relevant information from prior post if refreshment is needed:

Marine animals with a center of mass closer to their front (sharks, crocodiles, rays):

Have stronger limbs in the front, optimizing striking and lunging capabilities.

Find it easier to swim downwards, being generally better optimized to diving.

Spend less energy swimming since they are better able to “cruise”, reducing movement costs.

Are generally less nimble and agile.

Have more prominent fins in the front to help balance movement.

Are better able to stay attached to the ocean floor.

Marine animals with a center of mass closer to their rear (tuna, sailfish, flying fish):

Have stronger limbs in the rear, optimizing movement and explosiveness in movement.

Find it easier to swim upwards, being generally more buoyant in propulsion.

Spend more energy stabilizing orientation, thus increasing movement costs.

Are generally more nimble and agile.

Have more prominent fins in the rear to help stabilize.

Are better able to breach the water surface, escaping predators.

Center of “gravity” is used more often in biomechanic circles. I’m choosing to name this factor as center of mass in order to more clearly associate it with mass itself. The center of mass in an organism is really important in determining biomechanics and gait, as well as the arrangement of a body plan.

Tools Derived from Center of Mass

Mobility Appendages Proximity to Center of Mass

In marine animals, fins closer to the center of mass often help with agility and thrust at the cost of some stability, while fins farther from the center of mass often help with stability at the cost of some leverage.

For now, I’m going to say that stability can be a stand in for a measure of ATP costs associated with movement. Learning from this, we can say that:

The closer to the center of gravity an appendage is, the more it benefits mobility at the cost of greater ATP cost. The farther it is, the more energy efficient the limb is at the cost of some mobility.

Note that the effects of a limb are additive as long as it doesn’t negatively impact your streamline measure too much. When I say “more efficient at the cost of some speed,” I don’t mean the speed of your organism goes down, I mean the limb’s speed bonus isn’t as fast as it could be.

Below are examples of fish with the same center of gravity, pretty far behind their heads. The top fish really emphasizes stability at the cost of maximum speed, while the bottom fish really emphasizes speed at the cost of some stability. As a result, the first fish is generally better suited for distance and efficient cruising, while the second fish is generally better suited for quick movement in bursts as opposed to efficiency.

Note that in actuality, stability does have an important impact on speed. So perhaps another interpretation of stability could be as a sort of coefficient to speed as well as energy, but that could be complicated to manage.

Mass-Bearing Features & Appendages

“Heavier” parts of the organism will shift the animal’s center of gravity more or less dorsally or posteriorly. An example of an aquatic organism with a high-mass section of their body is Dunkleosteus, which notably has a heavily-armored skull, shifting the center of mass forwards:

This suits its ecological role very well; an animal that was likely built for inflicting a ton of damage on armored organisms, having a forward-set center of mass really helps it to hit like a freight train. Note the strong pectoral fins, providing control to the center of gravity, and the prominent, backwards shifted tail and dorsal fin, providing greater stability.

Varying Torso Size

Animals that have segments of their body enlarged tend to have their center of gravity shift that way. In Thrive, making a segment of your torso wider can shift the center of mass in a responsive way.

Varying Torso Length

Lengthening the torso, creating a sort of “tail”, is a common strategy utilized by many animals to shift their center of gravity backwards. Representing this mechanic in Thrive can implicitly lead to similar phenomena occurring organically in Thrive.

Here, from representing a dynamic constraint, we give ourselves a framework to understanding what kind of tools could be useful in an engaging editor, and can set up these tools to result in gameplay experiences which can seriously reflect a lot of the pressures real life organisms face in a very interesting way.

I can easily imagine such a system resulting in fun personal goals for the player to achieve: build the fastest animal by shifting the center of gravity backwards, shift the center of gravity as much as you can forward and try to build an animal able to succeed with that sort of pressure, etc. It also leads to many interesting “stories” behind the body plan of an animal. Why does this clade have very prominent fins in the back? Because its ancestor was really front-heavy, or it wanted to maximize speed.

There is only so much of “finetune this mouth/leg/eye/hand" etc. we can provide to the player without it being micro-managey to the player, or without us turning development of the macroscopic stage into “how many additional things can we fit here”. Having these significant and clear constraints can naturally result in the sort of dynamism players face, without throwing their attention to 5 different things on their creature that they must adjust sliders on or shape differently in an editor trip.

I am starting to think that this is an ideal way of creating an engaging editor, and think that nailing down the macroscopic factors, parameters, mechanics, etc. dealt with in the editor will make conceptualizing it a lot easier. There are some things we need to be wary of, however:

There should be very little actual “restriction” placed on the player surrounding these mechanics; that is, instances where the editor flat out says “you can’t do that, and we won’t let you leave the editor until you fix it” should be as absent as possible surrounding these constraints. Even if there are only three or four of these larger constraints, situations where the player has an organism with multiple issues can emerge, and this can easily lead to confusion and frustration.
Information and UI elements surrounding these systems should be clear as to the statistical impact of your choices. Bonuses should be highlighted and minuses should be pointed out, and you should be able to see what your surface area, mass, where your center of gravity is, etc.
There should be ways for the player to alleviate the pressure of some of these constraints if they wish, so as to create some sort of accessibility. This can be done in a way that doesn’t harm the depth of Thrive, as long as the bonuses of these constraints are also nullified to the same extent the negatives are. It can also be done in ways that are scientifically accurate: animals with less mass don’t have to worry about their center of gravity as much, and the presence of a swim bladder in some fish helps to alleviate the buoyancy effects of their center of gravity.
It should be clear to the player when a certain constraint is more important to their organism. Animals with a certain type of skin will need to look at their surface area a bit more, while animals with another type of skin will be less affected. Smaller animals won’t have to worry about their center of gravity as much, but larger ones should. This will also reflect progression: early, primitive animals often had to focus a lot more on surface area, as an example, then different strategies related to integuments reduced this pressure.
It’s better to have three well-designed, intuitive, and engaging constraints which are interrelated than it is to have a hundred constraints, addressing the tiniest of details.

Deus · April 9, 2025, 2:09am

The Marriage of “Appendages”, “Limbs”, and “Extremities”

Here is a take on how much of the functionality revolving around limbs and appendages could work.

Appendages vs. Limbs

An “appendage” is an extension of an organism’s body, while a limb is a type of appendage which often denotes mobility or weight-bearing functionality; often, but not necessarily always, with some sort of skeleton.
Appendages and limbs are treated with the same sculpting system, with more functionality attached to limbs, and more diversity attached to appendages. Players can designate appendages into numerous tools, while limbs are among the most dynamic and important tools an appendage can be, requiring detailed thought. Appendages also offer detailed customization of a creation.
Designating an appendage as a “limb” will require the player designating that appendage as bearing a joint. Once that limb is designated, a joint will be placed at the connection point between the torso and the appendage, where movement functionality will be conferred.

Joints - Specifying What a Limb Is

In Thrive’s editor, joints will designate a “mobility” function. Designating an appendage as a “limb” will require the player designating that appendage as bearing a joint. Once that limb is designated, a joint will be placed at the connection point between the torso and the appendage, where movement functionality will be conferred.
A joint in the editor doesn’t necessitate the presence of an actual, skeletal joint, bony or not. Soft-bodied limbs, such as those of velvet worms and Hallucigenia, would have a joint in the Thrive editor. Skeletal joints have their own bonuses applied to them, and have more diverse function options.
In benthic animals and terrestrial animals, jointed limbs without an extremity attached to them are assumed to work like ciliated, footed, or sticky pads, flatly pressed against the ground and creating crawling movement similar to snails, starfish, and slugs. In pelagic, free-swimming animals, jointed limbs without an extremity attached to them are assumed to work like fin folds (soft-bodied/endoskeleton) on primitive fish or lobbed flaps (exoskeleton-bearing organisms) like Anomalocaris, undulating or flapping rhythmically.
After a certain level of progression, players will be able to designate the placement of additional joints on a limb. The proportion of each segment in a limb is influential to the function of an appendage, depending on the extremity.

Extremities - Attaching Function to an Appendage

An “extremity” is the absolute end of an appendage. This includes graspers, feet, fins, feathers, stingers, external gills, webbing, and more.
Placing an extremity on a limb will transform the appendage, applying different rules, behaviors, and abilities on the appendage. They will be applied on the ends of an appendage, and will alter the appearance of underlying segments of the appendage. Different extremities will emphasize different stats; for example, surface-area becomes very important for appendages with an extremity focused on flight, mass becomes very important for appendages with a weapon on the end, etc.
Certain extremities, like feet and graspers, can only be placed on jointed appendages (limbs). Other types of extremities will be locked behind certain extremities; for example, wings.
Extremities will by default be rather vague, but can be customized extensively through manipulating both the root appendage and characteristics in the extremity itself. Variations in extremities, as well as the different form of extremities, will generally alter the importance placed on certain characteristics of an appendage, requiring different handling of mass, surface area, streamline, and more.

An Emphasis of Constraints: Why Appendages, Joints, and Extremities?

Right now, I am thinking that an extremity denotes the function of an appendage, and the appendage’s form influences the extremity’s stats and effectiveness. I am approaching it this way to tie it back into the idea of dynamic constraints, where certain constraint factors of the appendage are emphasized or de-emphasized depending on the extremity.

I am doing this for these reasons:

Engaging Editor Gameplay - If done well, I think doing it this way can result in a really deep gameplay experience. Instead of flatly attaching a different type of limb or part which has fixed stats, the player will have to deliberately alter their morphology to maximize function.
Anatomy - Certain structures require different alterations of the underlying limb or appendage to become more effective. By manipulating the stats of a limb via constraints, we can reflect these anatomical considerations in a way that is approachable to the player. And by having this classifying system of extremities denoting function, differentiation of anatomy is much easier to understand.
Reflection of Evolutionary History - This could be a nice way to incorporate evolutionary story-telling, and reflect certain principles of evolution. For example, by making high surface area limbs beneficial for swimming, auto-evo naturally forces various marine animals to evolve convergent fin structures. I hope it can also lead to some cool behavior from auto-evo: if surface-area is beneficial for flight, then perhaps auto-evo will be more likely to transform a high surface area limb into a wing, therefore emphasizing the transformation of underlying structures into different functions.

In summary: I think thinking of extremities, appendages, and limbs this way will allow for some really cool emergent gameplay. By measuring the parameters of the appendage and attaching stats to these parameters - greater mass increases damage and is beneficial for x, greater surface area increases suitability for flight and is beneficial for y, etc. - we can get a really cool system which reflects evolutionary design and history. It also offers a great compromise between functionality - making sure the player is able to designate exactly what they want of something, and making sure us developers can rationally approach certain features - and customization - allowing players to apply cool visuals and shape the features of their organism.

I can go into more detail as to how constraints and more exactly interacts with appendages. But for now, I want to focus more on how that we can even go about portraying certain features before we go over how to attach gameplay function.

Additional Notes

Here are some additional concepts branching from this joint, appendage, and limb conceptualization.

Tails

Tails are posterior segments of an organism which often are adapted to assist an organism’s movement, occasionally being used as weapons.
Tails are designated by the placement of a joint on an organism’s torso. If a joint is placed on a torso, then every part of the torso behind that joint is considered a tail. Players cannot place additional limbs on segments of the torso designated as a tail. They can place additional appendages.
In water, longer tails will provide greater agility, while shorter tails will provide greater speed and explosiveness. On land, tails will have much less of a direct effect on mobility, but will have a stronger effect on the center of gravity of an organism.
Note that “length” here refers to the portion of the torso which is tail or not tail, not necessarily the length of the extremity on the end of the tail. A long-tailed Thrivian organism would be an “eel” or rattail fish, which uses the majority of its body as a tail while swimming. A big-eye thresher shark would be considered to have a shorter tail, similar to other sharks, as the majority of its tail’s length is on the extremity rather than the actual, musculated tail.
Attaching different extremities on a tail can alter the function of the tail. For example, placing a fin on a tail indicates its use in active propulsion in marine environments, while attaching a stinger or weapon informs a function as an offensive or defensive tool, such as in scorpions, stinging insects, and stegosaurs/ankylosaurs. Certain extremity attachments can also attach items to the sides of the tail in total, allowing things like ribboned fins.

General Progression Ideas

Appendages will first appear in the macroscopic stage as a soft-bodied extension of the body which primarily affect attributes like surface area, volume, and movement. Players will be able to broadly manipulate the shape of the appendages and attach some different functions, such as stinging extensions seen in jellyfish/other marine animals, very basic limbs/fins, and antennae/sensory organs. However, placing multiple joints on a limb, which is needed for more advanced functionality, will be locked behind a skeletal structure.
Evolving a skeletal structure will unlock more advanced extremities and limb options, as well as generally enhance the abilities of a limb. Before the evolution of a skeletal structure, tails will be really important for pelagic movement. Tails will remain powerful tools for creating propulsion, but there will also be greater options for the player to utilize.

SAMPLE CATALOGUE OF EXTREMITIES

Feet & Graspers

Feet are structures used by benthic marine animals to stay rooted to the ocean floor, and by terrestrial animals for general mobility. Graspers are extremities which allow the ability to grasp, enabling grappling with other animals and interaction with the environment. Graspers generally evolve from feet.

These parts indicate the use of an appendage as a limb, and may only be placed on jointed appendages.

Weaponry

Certain extremities enable offensive, or defensive, capabilities, such as stringers and spikes. Attaching a spike to the end of an appendage transforms that entire appendage into a large spike or horn, similar to those seen on trilobites, certain reptiles, and mammals. Attaching a stinger on an appendage enables a more active, offensive capability, allowing animals to strike at range with their appendage, similar to scorpions, spiders, and wasps. Greater mass in these parts can strongly influence stats.

Sensory

Certain extremities transform an appendage into a sensory organ. For example, attaching sensilla to an appendage transforms that appendage into an antenna, similar to those seen on arthropods, enabling greater detection of a certain input. Electroreceptors, whiskers, and other tools can also provide detection abilities. Surface area can have a strong effect on sensory organs.

Mobility

Certain extremities transform a limb into a fin, wing, or digging structure. These emphasize certain attributes, such as streamline measure, surface area, and more.

Questions and Considerations

Should a tail be dealt with like a limb, or just as an extension of the torso? I assume a tail being an appendage can make it really easy to represent multiple things, such as fins on a tail, stingers, capabilities, and more. However, it can also lead to confusion on the player’s end if not explained well, or might be an unneeded complication of things.
When imagining a sort of “editor catalogue” this approach with appendages and extremities, though dynamic, does result in a model which might make including other categories of structures awkward if they don’t fit well with appendage sculpting. For example, eyes are an important sensory organ which frequently aren’t attached to the evolution of an appendage; where does that part fit in with this? There are other parts - ears and noses jump out immediately to me, but I’m sure we’d find other such structures as the stage progresses - which would probably need to be treated differently. This isn’t necessarily something that is a huge or unavoidable flaw, but it does mean we need to be thoughtful about what goes into the catalogue of tools in Thrive so that we don’t just end up with a hodgepodge section of the editor.
We need to be mindful of how interrelated many extremities often are, and the fact that certain structures evolve from other structures. For example, wings come from more traditional limb structures, feet from fins, graspers from feet, etc. etc. A lot of thought would need to be put up in portraying this progression and

Applications

Attached below are some mediocre concepts illustrating some points I tried to make above.

A pretty traditional fish body plan. The lighter green indicates an appendage; here, there are two unmotile appendages assisting with the movement of an organism in a dorsal and anal fin, and another front appendage with dark green circles in them. The darker green circles represent the placement of a jointed limb segment, which are the fins on which movement animations will be applied, and where stronger movement boosts will be applied. The dark red indicates the part of the body identified as a tail, and the dark red circle indicates a placed joint, indicating the start of a tail. The orange (or light red?) indicates an extremity, which is providing information on the function of a limb; indicating a dorsal tail fin, used for movement, and a pectoral fin, also used for movement.

An organism with a body plan like an Anomalocaris, with limbs represented as simple jointed pelagic limbs with no extremity attached to its end. Since the limbs have no appendages, they will undulate, similar to the animal I was trying to replicate. It also has a shorter tail.

An organism with a body plan like very primitive chordates, having a long, undulating fin attached to its very long tail. This illustrates the benefit of handling the tail link up with extremities; it allows the player to customize their organism in a way that would be difficult with traditional appendages and limbs.

An organism with a body plan similar to a scorpion. The light red indicates an extremity focused on stinging, thereby transforming the tail into an offensive weapon. Illustrates the diversity of features which can be applied to a tail.

An organism with a body plan similar to a trilobite. The light red indicates the placement of a spike extremity on the end of an appendage, therefore transforming the entire underlying appendage into a spike structure. Illustrates the customization options available should we diversify the function of extremities to be beyond just hands and feet, and the benefit of unifying appendages and limbs.

Introducing: a weird crab thing with legs. Note the orange extremities offering differentiation of appendage functions; all are limbs, but the front bottom limb operates as a claw/grasper, the bottom three posterior limbs operate as legs, and the top limb operates as a wing, transforming the appearance of the appendage into a wing. Note that for the appendage operating as a wing, surface area is maximized, whereas for other limbs, surface area isn’t as important. Meanwhile, we can infer that the front claw probably isn’t used very offensively, as it has little mass. Illustrates how we can attach constraints to this feature to increase play diversity.

A jellyfish-like organism. It has five appendages, four serving as offensive, stinging appendages, and the larger middle one serving to increase surface area to assist with a metabolic process. Note that the stinging appendages are designated as offensive by the placement of stinging extremities, therefore making constraints behave differently for those appendages as opposed to the more traditional, surface-area boosting appendage. Illustrates differentiation of appendage functions.

Deus · April 12, 2025, 9:21pm

Mouths, Mandibles, and Teeth

Mouths and jaws are something I imagine to be more modularized/parametric-part based than appendages just going off the fact that the majority of mandibles and mouths are slight alterations to existing structures.

There are clear evolutionary chapters on our planet’s history regarding the use and diversity of mouths. They reveal a story on how the majority of animals got their food. I’ll go over these briefly to provide context.

STATS AND ABILITIES

These are the major abilities and stats which mouths and modifications to mouths will generally revolve around. Note that the general constraints are also at play here, but I will be focusing on the actual, mouth and mandible-specific tools.

Abilities

These are the major abilities attached to the jaw of an animal.

Eat - Ability to engulf items. Available to all mouths; prior to the evolution of a mouth, eating works through contact of plankton clouds and substrate with skin.
Suction - Ability to absorb material and small enough prey. Available early on to some animals without and with jaws, though effectiveness dissipates at larger masses.
Bite - General chomp ability, inflicting damage on other animals. Available to mouths with some sort of jaw.
Grasp - The ability to grab items and, if discrepancy in mass is big enough, prey. Requires a jaw.

Attributes

These are the major stats influenced by mandibles. For more on ingestion rates, look at this post: Metabolic Rate System - #5 by Deus

Ingestion Rate - Essentially what component of whatever you eat is accessible or digestible by your organism. Eating plants with carnivore teeth results in only 40% of the nutrition, eating armored animals with teeth not able to break the armor results in only 20% of the nutrition,eating shrubbery with teeth designed to graze grass results in 50%, etc.
Raw Damage - How much damage your jaw structure is able to apply. Shorter, wider jaws generally are able to create more baseline raw damage.
“Effectiveness Coefficients” - Essentially, stats which changes damage applied to an organism depending on your jaw. Wider jaws are more effective against tougher material, while narrower jaws are more effective against flesh.
Range - The range at which effects can be applied. Longer snouts provide greater range.
Grasp-Related Factors - How well a mandible is at grabbing items. Generally revolves around how long you can hold an item, and the maximum size of item you can hold. Duration of grasp is more relevant to prey items, while maximum size is influential for both carnivores and animals which can interact with their environment. Narrower jaws are generally better for grasp.

MOUTH CATALOGUE

These are the general major types of “mouths” available to players.

Mouthless - Absorption of material occurs through the skin, meaning other stats, like surface area and appendages, are more important.

Jawless

Simple jawless mouth which basically enables engulfment, with no bite, no suction, and very little range.

Suction - Boost to suction, though reduces size of engulfable matter.
Toothed - Believe it or not, teeth-like structures likely evolved before mandibles, with conodonts especially displaying these capabilities. Allows for a very limited bite function.

Jawed

In general, jaws can be modified in a parametrized way like this:

Length - Longer jaws decrease bite damage, but increase range. Shorter jaws increase bite damage, but decrease range.
Width - Wider jaws increase max damage and impact on tougher skin as opposed to narrower jaws. Narrower jaws increase grasp strength and impact on less tough skin as opposed to wider jaws.

Arthropod Mandibles - Accessible relatively early on, the mandibles of arthropods will be extremely customizable, with the potential for more extensive abilities attached to them. More modification and specialization options available to arthropods in comparison to animals with an endoskeleton. Arthropod mandible raw damage doesn’t scale up with mass as strongly as high mass endoskeletal animals.

Proboscis - A more advanced mouthpiece available later on in the Aware Stage. Takes away bite ability, but opens up interactions with unique flora, as well as parasitism. Allows access to ubiquitous food sources, such as productive plant species and parasitism, but with practically no capability for predation.

Endoskeletal Mandibles - Generally allows the highest amount of stats when specialized, though more elaborate and expensive to develop/modify than arthropod mandibles, and generally less adaptable.

Teleost Jaw - Extendable jaw, like in the majority of extent fish today. In general, strong boost to suction, range, and grasp ability underwater in exchange for raw damage. Increased streamline measure. Incredibly useful for highly-adapted marine animals who pursue nimble prey, less efficient for larger animals which hunt larger prey or terrestrial animals, where suction is completely nullified.
Beak - Less raw damage and no access to teeth, but strong boosts to ingestion and efficiency rates, and much less impact on mass and center of gravity. Incredibly efficient for animals that are highly specialized around a food source and for animals which prioritize mobility.

Chitinous Beak - Available to soft-bodied organisms. High damage with some suction ability, and limited grasping. Much less diversifiable than either of the other two mandible options; provides decent stats, but the stats, with perhaps a tiny bit of modification, are largely what you’re gonna get.

TEETH

Teeth will be available to animals with an endoskeleton that don’t have beaks, and will allow for further diversification or amplification of the stats of the mandible. Adaptations to chitinous beaks and arthropod mandibles can reflect the function of teeth - for example, serrations in beetle mandibles - but are alterations to the underlying mandible structure itself. True teeth allow for an additional layer of customization to a mandible beyond its shape, allowing for more diverse function.

Once players unlock teeth, a modification option opens up for the player’s jaw. Players will be able to specify the shape of teeth, as well as their distribution in the jaw. Teeth placed near the front of the mouth are more influential on stats related to interaction with the world and other animals (grasping, damage, effectiveness coefficients, etc.) while teeth placed in the back of the mouth are more influential on ingestion rate. The shape of teeth will alter the stats discussed below in different ways - conical teeth increasing grasping, thicker teeth being more effective against armored animals, shorter teeth being more efficient for plant matter, etc.

Teeth augment and manipulate these stats:

Ingestion Rate - Influential in the diets you can eat, with flatter teeth better at grinding plant matter, and sharper teeth better at tearing meat.
Raw Damage - Effective teeth can damage prey items well.
Grasp-Related Factors - Certain teeth arrangements enhance grasping ability, such as conical teeth.
Effectiveness Coefficient - Certain teeth are better at damaging certain mediums; for example, wider teeth being better at damaging armor, and skinnier teeth being better at cutting through flesh.

When a tooth type is specified, players will be able to place and drag that tooth onto the mandible. Players will have sliders that allow control on the location and frequency of teeth. In the below concept, players can increase the distribution of teeth by pulling the arrows on the end of the lines, and can move the location of the teeth by dragging the circle in the middle of the range:

Players will be able to create additional teeth types once a certain level of progression is unlocked. Similar to the multicellular editor, creating another tooth type starts as a duplication of an existing tooth. Players will then drag that tooth onto the jaw, which will replace existing underlying teeth, or populate un-toothed areas:

Additional Notes on Teeth

Increasing the size of a tooth type visually means less of that tooth in a given region of the jaw, and decreasing the size visually means the opposite.
An unpopulated space on your mandible can mean forgoing some stat bonuses, but can also increase the max size of whatever teeth you have in your jaw, which can also enhance stats. That opens up space for animals like Dunkleostus, who have prominent, giant bony teeth in the front of their jaw, but no such teeth in the back.
Certain teeth can be modified to attach certain benefits; for example, being able to envenomate prey, or the ability to chop through wood. These will be more important in later portions of the stage.

Differentiated vs. Regrowable

Differentiated teeth are a unique characteristic of synapsids (proto-mammals and mammals), who have sections of their mouth uniquely catered towards different functions. Though some non-synapsid animals have differentiation in dental structure - fangs in certain fish and reptiles - the degree of differentiation is much less, and there is much less sectional specialization.

Differentiated teeth obviously allow for a lot more diversity in mandibles, but also come with a cost; in general, the more your teeth are differentiated, the more significant losing a tooth is. Animals with less differentiated teeth, like sharks, are often able to regrow lost teeth rapidly, while animals with more differentiated teeth have limited ability to replace teeth. For animals like sabretooth cats, losing one of their fangs was essentially a life-long disability.

It’s difficult to think of a way to proxy this in Thrive. An immediate idea popping into my head is that trying to bite material that your organism doesn’t have a good ingestion rates or coefficient factors for can debuff your stats derived from teeth temporarily, and this debuff can last longer the more different your teeth are. That way, it is implied that less differentiated teeth are repaired quickly, while more differentiated teeth don’t recover as fast. This concept sounds fine on paper, but it involves another mechanic, and it might be bothersome or too much detail for the type of detail we’re focused on.

Another idea I have is that the more differentiated your teeth are, the more severe the disparity in ingestion rates and effectiveness coefficients are. So for animals with undifferentiated teeth, you might be able to get atleast some nutritional value from items that fall under your digestive system’s diet even if you don’t have well adapted teeth for that purpose, and can more generally inflict damage on more types of armor and flesh than animals with specialized teeth. On the other hand, for animals with differentiated teeth: if your teeth aren’t suited for it, then you get basically nothing from it. This fits in well with the stats I brought up, but it might have a weird effect on balancing; with this system, would animals which first evolve teeth be the most versatile, and animals which have more evolved teeth be less versatile?

So that’s one area I’m not super certain on. I am more inclined to second idea just because it fits neatly, but I think this is something we’ll have more answers for as we get a clearer understanding of the macroscopic stage.

Concluding Thoughts

I think the above concepts are pretty straight-forward and concise. Jaws, teeth, and all that sort of stuff related to the mouth are one of the more obvious “form = function” trends we see in nature, and there’s a good amount of comparative anatomy research surrounding jaws.

The more confusing part with jaws revolves around progression. As I mentioned, adaptations in jaws are the result of adaptations even more incremental in nature than adaptations to appendages and limbs. With appendages, there’s a lot more of a “place and tweak” application there. By that I mean in arthropods, limbs are pretty frequently added and removed; and even in vertebrates, who do regularly stick with the two or four limb body plan, segments on the limb are added and removed somewhat frequently. So providing the player with more significant tools related to limbs is pretty intuitive.

With mandibles, the mandible is much less frequently messed around with; once a mouth is placed, alterations are frequently very slight. So the tools we provide to the player will also likely have to be slight as well, and progression through different “variants” of jaw - for example, going from a mouth opening to a jaw, then from a jaw to a beak - is pretty binding. We’ve seen the progression and reversion of a limb into a foot and back into a fin much, much more frequently than we’ve seen the progression and reversion of a mouth from a mandibulated, advanced jaw to a simple opening, or a beak back into a normal, toothed, jaw, etc.

Writing this out and thinking about this more, I realize that this isn’t really something that requires super-specific handling, or hard locks, or anything like that. If we set it up so that jaws are really beneficial, and make it so that jaws are pretty influential, players and auto-evo will naturally treat them as significant pieces of anatomy and reside in small tweaks.

I think I’m just trying to say that, even though we are arguing for a more part-driven, parametrized system, we cannot think of it as just placing and dropping another part to increase a jaw like in Spore. Variations in appendages, jaws, and limbs have to revolve around altering existing structures in a way that unlocks a different variation. For example, instead of going into a “Variant” menu like we have with organelles right now, turning your beak into a jaw will have to involve the player losing teeth, then turning the segment of jaw without teeth into a keratinized bony structure, with different bonuses applied.

MirrorMonkey2 · April 15, 2025, 8:05pm

These two posts are really great!

Deus · May 10, 2025, 7:37am

Integumentary System Customization

Here is the next component of an editor concept, this time focused on integumentary customization. Note that I do not discuss “brushing” or “patterning” tools here, and instead focus on a general concept/system allowing us to represent complex skin types and navigating through some of the more difficult conceptual areas to cover. Customization tools such as brushing and color customization can be discussed once we understand where and how to best apply them I think.

Discussing integumentary customization gives us base concepts for almost every part of the external editor when combined with the past few posts, so it’s an important topic to give specific focus to.

Attributes and Constraints

Here are the primary stats which are heavily influenced by the integumentary system, as well as how the system can connect to conceptualized constraints.

Attributes Involving Skin

Protection - How much your skin provides defense from attacks and harmful elements. Basically, an armor effect.
Gas Exchange - How much your skin augments respiration.
Dehydration Rate - How quickly your body loses moisture through skin, limiting terrestrial capability.
Mobility Stats - Boosts to movement stats such as buoyancy, agility, etc., more important in water, some marginal effects on land.
Environmental Tolerances - Especially for land animals, integumentary systems have a strong impact on their tolerance to climate.

Constraints

Mass - Certain skin types and customization can boost mass. For example, thicker skin, dense fur, and more defensive scales make animals bulkier.
Surface Area - Certain skin types are much more influenced by surface area. Surface area is particularly important for animals which rely on gas exchange via skin, such as amphibians, and for animals without keratin, impacting dehydration and gas exchange. It is also influential on tolerances.
Streamline Measure - Certain adaptations can increase or decrease how streamline an animal is. Teleost fish typically have integumentary adaptations to improve maneuverability in water, while bony fish trade some streamlining for better defense.

Complications

This concept was kind of complicated for me to think through, so I wanted to first bring up some of the issues I was ruminating over for a while so that my thought process might make sense in this post before getting to implementation details.

Connections to Skeletal Structure: The complicating factor is the fact that a chitinous integumentary structure leads to the evolution of an exoskeleton, but other integumentary structures do not necessitate the evolution of an endoskeleton. This has odd implications on the progression of the game, as it’s confusing to lock a skeletal structure entirely behind an integumentary style.
Transitions: There is also the question of transitioning through different integumentary bases. Variations to underlying structures exist, but so too do big jumps between different skin types. We’ve seen reptiles and mammals lose fur and feet to adapt to marine environments, but we haven’t seen reptiles and mammals lose the keratin which makes them waterproof.

Evolutionary “Checkpoints”

There appear to be certain integumentary adaptations which are much more binding and impactful.

Keratin - Generally indicates adaptation for land. Significantly reduces gas exchange in skin, but significantly reduces the rate at which you lose thirst, allowing much more terrestrial lifestyles. Allows for the evolution of reptilian scales, fur, glandular skin adapted for land, and feathers.
Chitin - Generally indicates the evolution of an exoskeleton, allowing arthropod skeletal structures and advantages and disadvantages brought by that.

Note that keratin and chitin are not exclusive to the described groups. For example, keratin is found in fish teeth and parts of their scales, as well as in the nails of amphibians; collagen, important for non-exoskeletal organisms, is present in arthropods, important for their muscular tissues and ligaments; and chitin is found in many animals who don’t necessarily have an exoskeleton, such as velvet worms.

Implementation

Skin Types

“Skin Types” are the broadest categorization of your integumentary system, typically referring to their characteristic structure. For example, one skin type is “Dermal Scales”, referring to the scales of fish, while another is “Keratinized Scales” referring to the scales of reptiles.
Each different skin type gives a fundamentally different baseline impact on your stats, and emphasizes/de-emphasizes certain constraints. For example, skin type A gives you more baseline mobility than skin type B, and skin type A is more sensitive to surface area.
Your choice of skin type defines what customization options are available to you.

Attributes

Attributes are special designators of the general morphology of your skin type, providing unique adaptations besides those designated by your skin type. They allow us a convenient way to represent integumentary adaptations.
You can assign an “Attributes” to your skin type, augmenting that skin and providing bonus stats. These abilities can make your skin look different, and provide bonuses depending on your body plan and constraints. Only one attribute can be assigned to your skin type (?).
For example, if you have Gelatinous skin, an attribute available to you is “Mesoglea”. The Mesoglea attribute grants you buoyancy depending on your surface area, allowing you an easier time to float at such an early part of the Macroscopic Stage, but generally reduces your streamline measure. It makes your creature look more bloated when equipped, reflecting jellyfish and comb jellies.
Attributes have customization options attached to them. For example, you can customize how much Mesoglea is in your Integumentary System if it is equipped.
Your access to certain attributes depends on your skin type. For example, animals who don’t have gelatinous skin cannot equip mesoglea. Unlock conditions can also be applied to attributes if desired. For example, being hunted three times with dermal scales unlocks armored scales, reflecting the evolution of armored fish.
Attributes define the general characteristic of your skin type rather than unique adaptations. For example, a fish with a single armor plate on its skull doesn’t have the “Armored” attribute - the “Armored” attribute, once equipped, indicates that your skin as a whole is hardened, and is an additional layer of customization besides placing individual armor plates. This lets player apply stats, adaptations, and appearances to their organism which having to spam a specific customization, part, or tool on their entire organism, which would be annoying, a hassle, and not realistic to how such integumentary structures evolved.

Attributes allow us to represent pretty complicated structures and adaptations in a relatively gamified and simple way. Selection should be concise, mindful of what we choose to represent as a brush/part and what we choose to represent as an attribute.

In my head, an Attribute notifies the system of the general characteristic of your animal’s integumentary adaptations, while individual sculpting/parts are the more “unique” and individualized sections of your animal which provide an individual stat bonus. For example in the “Armored” attribute, the attribute essentially denotes the skin as being characteristically hardened and attaches a cool texture, and the player has an option to place a more intricate plate decoration which more significantly confers protection on specific sections of your body plan and gives more influential boosts. But it’s something to be mindful of for every attribute, and particular attributes I threw out there above are more vulnerable to that question: the “Spined”, “Quilled” and “Ornate” attributes, among others, are particularly sticking out in my head.

Catalogue

Here is a catalogue of skin types and attributes. As mentioned above, skin types refer to the general nature of the integumentary system, whereas attributes are customization options for the skin type which confer unique stats. Bulleted items are attributes, whereas bolded items are skin types.

Gelatinous

Our stand in for skin types before collagen and chitin. Encompasses diploblastic and simple triploblastic animals. Strong sensitivity to surface area, though may be amplified. Most simple skin type, only available to animals without a skeleton.

A potential goal to aim for with gelatinous is having the player automatically unlock/start with an attribute that reflects their membrane choice in the Microbe/Multicellular stage. This can do a good job of connecting the two stages.

Mesoglea - Gelatinous substance in skin which helps with floating. Surface area dramatically enhances buoyancy, reduces streamline measures.
Filter-Feeding - Surface area enhances uptake and ingestion rate of materials. Much more sensitive to surface area impacts.
Chitin Capsules - Bonus to defense; automatically equipped if chitin was present in microscopic stages. Reduced sensitivity to surface area, and increased mass.

Dermal

The default skin type for animals which evolve an endoskeleton, a stand in for fish integument. General boost to streamline measure and weak boost to protection compared to gelatinous skin. Without an equipped attribute, dermal skin looks smooth, such as that of a lamprey and ancestral fish. Dermal skin is the most well-suited for marine environments, having the strongest baseline boost to the streamline measure. It is also pretty widely customizable, with its attributes able to apply strong bonuses.

Armored - Skin becomes generally plated and hardened. Provides significant protection, but hampers streamline measure. Increases mass.
Placoid - Provides decent protection, less than armored, but with minimal impact on mass and a boost to streamline measure.
Leptoid - Minimal protection, but great boost to streamline measure. Reduces mass.
Spined - Similar to a pufferfish, your integumentary surface is covered in spikes which can be retracted. Damage upon contact when enabled, but considerable hit to streamline measure.
Mucuseous - Being attacked generally reduces the mobility of the attacker, similar to hagfish. Streamline and defensive measures are impacted.

Mucuseous Glands

Proximating the skin of amphibians, provides boost to respiration based on surface area. Regardless of respiratory organs, respiration rate is boosted on both land and water, allowing even animals with gills some capability to breath on land, and allowing animals with lungs some capability to breath in water. Dehydration rate compared to fish scales is reduced, but still very sensitive, meaning terrestrial life is limited by access to water. Impact of surface area greatly increased.

Poisonous - Skin damages other animals if attacked. Reduced surface area impacts. Reduced surface area impacts, both good and bad.
Keratinization - Dehydration rate reduced, allowing longer habitation on land. Reduced surface area impacts, both good and bad. Enables unlocking of scales.
Armored - Hardened skin, allowing general boost to protection. Reduced surface area impacts, both good and bad. Effect not as strong as protection from amniote or fish scales.

Fur

Proximating the skin of mammals, fur provides a boosted tolerance to cooler environments, and generally minimizes the impact of surface area on the organism. Despite boost to cold tolerance, tolerance to high temperatures, though impacted, isn’t shifted significantly due to the ability to dynamically shed. Due to its unique tolerance to colder temperatures and ability to tolerate warm environments, fur is generally the most widely-adaptable skin in Thrive.

Water-Tight - Proximate for aquatic mammals who have not lost fur. Streamline measure improves.
Keratinized - Proximate for skin such as that of armadillos and pangolins. Boost to protection, mass increases.
Quilled - The fur of the mammal is generally hardened, damaging other animals upon being attacked. Increased mass, and increased surface area impacts.
Wooly - Fur is highly adapted to cold environments, strongly increasing tolerance but decreasing tolerance to the heat. Mass is significantly boosted.

Keratinous Scales

Approximating the skin of reptiles, keratinous scales provide a sizable boost to protection, and a boosted tolerance to warm environments depending on surface area at the expense of a sensitivity to cold environments. Dehydration rate is minimized. Combination of dehydration rate reduction and tolerance to heat makes this skin well-suited for arid and warm conditions.

Water-Tight - Proximate for reptiles who are aquatic. Streamline measure improves.
Osteoderms - Skin is generally covered with thickened scales, strongly enhancing protection but increasing mass.
Pycnofibers - Skin is generally covered with primitive, feather-like fuzz. Increased tolerance to cold, and increased streamlining for flight, though reduction in protection and increased sensitivity to surface area. Enables unlocking of feathers.
Glandular - Scales are minimized, replaced with smooth skin and the presence of skin glands, similar to that of synapsids. General boost to tolerance to colder environments and mobility and reduction of surface area impacts, though noticeable loss to protection. Enables unlocking of fur.

Feathers

Proximating the skin of furs, feathers provide a great boost to mobility and reduce at the cost of protection and some sensitivity to surface area effects. Strong boost to streamline measure on land, boosting advanced mobility options such as flight and gliding.

Water-Tight - Proximate for aquatic birds. Streamline measure improves considerably, but reduced bonuses to mobility.
Ornate - Feathers are iridescent and flamboyant. Reduced bonuses to mobility, but general discount for MP due to enhanced sexual pressures.

Chitin

Proximating the skin of arthropods, chitin provides great boosts to protection with a noted, but minimal impact to mobility and mass. Animals with chitin appear to be encased in hard armor, akin to arthropods in real-life, and have appendages which look more geometric.

Having chitin as an integumentary structure denotes your animal as having an “exoskeleton”, which applies unique appearances and stats to your organism’s appendages and torso, organs, mandibles, and unique discounting in the editor. In general, it is much cheaper for players with an exoskeleton to place down and modify appendages. Surface area impacts are reduced, and size-related costs are boosted, capping size a bit more, though can be mitigated through advanced respiratory structures.

Chitin also has an additional customization slider with integument thickness - rigidity - which provides a split between mobility and protection. More rigid exoskeletons appear less segmented, while more fluid exoskeletons appear more segmented, such as the skin of a trilobite or centipede. Despite having less “options” than collagen-based skin types, chitin by itself is much more customizable than any of the other skin types.

A more advanced discussion of exoskeletons, as well as their unique behavior in the editor, will be held when discussing skeletal structures.

Water Dynamic - Similar to the skin of early sea scorpions, lobsters, and trilobites, segments are well adapted for water, smoothly extending. Increases streamline measure dramatically, and visually represents grooved plated segments, similar to Anomalocaris and other aquatic arthropods.
Armor-Plated - Similar to the skin of scorpions and trilobites, exoskeleton is incredibly adapted towards withstanding damage. Very strong impact on mass, and reduced streamline measure.
Lightened - Exoskeleton is made of lightweight material, increasing streamline measure and reducing mass, but decreasing protection. Surface area becomes a bit more impactful.
Calcium-Enriched - Skeleton embedded with calcium carbonate, strongly boosting protection with little impact on streamline measure. Strong impact on mass.

Additional Potential Skin Types and Attributes

The only thing I can think of currently is something to proximate the skin type of mollusks, such as snails and octopi. I can see it being linked to the unlocking of a “hydrostatic skeleton”, but that is something that should be conceptualized when we have a better understanding of progression in the Macroscopic stage.

In general though, I think the attribute system is really beneficial because it lets us add different unique adaptations with a relatively simplified tweaking of stats, constraints, and in some cases, abilities. There are additional attributes I can think of on the spot now which would be cool to represent - the skin of chameleons and other animals which can change skin color for example, or the skin of hagfish - but those depend on our priorities at the time of implementation. In general, I wanted to illustrate the premises of the concept with the suggested attributes and skin types; it isn’t meant to be a conclusive or comprehensive list.

Solving the “Exoskeleton” Problem

As I mentioned above, one problem we face with the integumentary system is related to chitin - choosing chitin as your primary skin results in a fundamental change to your skeletal structure, giving you an exoskeleton. That complicates things, as it’s an awkward way to deal with an important piece of progression in Thrive. So, how do we deal with this?

At first, we don’t really give much information about integumentary structures to the player. We start them off by assuming that their skin is gelatinous, akin to a very basal macroscopic animal.
The first determiner of your integumentary evolution isn’t presented as related to the skin itself, but instead a choice about the skeletal structure. That way, the more apparent binding nature of your skeleton also intuitively leads to more bound integumentary evolution. Once you choose a skeleton structure, your skin changes to a basal chitin or dermal covering depending on your choice.
This fits in nicely with theory anyways; chitin obviously corresponds with an exoskeleton, but collagen and fish scales - which started the evolutionary path of keratinized and collagen-based skin, such as dermal scales, feathers, fur, and reptilian scales - are heavily tied to mineralized tissues as well.

That takes care of the most major “binding” between integuments, if you wish to revert to the other big integumentary type, you have to undo all skeletal structures associated with your skeleton. Once you build onto your skeletal structure, it becomes harder to switch to the opposite skin type.

We can apply similar principles to more “minor” binding questions - for example, reverting from fur to scales, scales/fur to amphibian skin, etc. Certain structures can be placed only with certain integumentary structures, and must be replaced before reverting. But that is a more delicate matter which I think would best be discussed later, and can be dealt with in other ways.

Questions, Concerns, and Concluding Thoughts

I think this is a solid way to deal with pretty complicated morphological structures, setting up a system that can be added upon pretty easily compared to creating very specific, handled sculpting mechanics, or requiring the placement of many, many smaller parts. It does bring up some questions regarding the philosophy of our editor however; particularly, in the “sculpting v. parameterized” area.

The ultimate question: what is dealt with via the more intricate sculpting tools we offer the player, and what is dealt with via the “Skin Attribute” system? For example, the “Armored” attribute for fish scales: how do we allow detailed sculpting of prominent dermal plating, such as is seen in many basal fish species?

All in all, this system is meant to make it easy to represent certain things which might be difficult to represent traditionally. This could make certain things, like a very dynamic brush system where you can apply a texture on your organism, less influential. I think both can be implemented - attributes informing the system of the general characteristics of your skin, and brush providing bonus stats and customization options - but such a parametrized system does ultimately reduce some customization options.

There are also lingering questions on how to limit progression and switching between the “collagen-based” skin types. It’s pretty hard to de-evolve fur and feathers and revert to being completely scaled, it’s pretty hard to revert to amphibious skin if you have keratin, it’s pretty hard to revert to fish scales if you have amphibious skin, etc. I do have some ideas there, but I want to make sure the basic premises of this idea don’t have any glaring issues first.

This can be a starting point for discussing a complex topic, and can be clarified further as we understand the goals and expectations for our integumentary system. Nailing this down alongside the prior posts gets us further along understanding having an initial master concept for how our editor will deal with the “External” parts of an organism - the internal editor, which focuses on organs, is another conversation which should be discussed once we understand what the sculpting tools will look like.

Deus · June 26, 2025, 6:01am

EXTREMITIES IN DEPTH

Stats for “Jointed” Extremities

Extremities will be influential in stats, and will be one of the most important ways to gain relevant abilities in the macroscopic stages. Placing a certain type of extremity offers certain abilities to the player, and these abilities have their own baseline stats which are able to be customized. Keep in mind that the shape of the appendage the extremity is attached to will have an extreme effect on stats as well, but this will be discussed more later this post.

I am going to primarily discuss jointed extremities here, because I am more certain about the application of this “appendage-extremity” dynamic to them than the more diverse anatomical structures.

These are the primary stats and abilities which jointed extremities primarily deal with.

Speed - How fast your organism can move.
Agility - How good at changing directions and turning your organism is.
Grasp - How capable of grasping prey or items your organism is.
Damage - How much damage your organism can apply to another.

Note that stats can be more detailed than this: agility can involve acceleration which is another stat in itself, speed can be altered by the medium through which you are moving, damage can be modified by the defenses of the organism you are trying to attack, etc. But you get the basic idea here.

Extremity/Appendage Interaction with Constraints

Extremities change the nature of the appendage itself, transforming its visual appearance and function. As such, different extremities will make certain appendage shapes more important. We can express this through our constraint system.

Mass - Appendages with more mass in the distal (farther) regions are typically optimized for striking, while appendages with more mass in the proximal (closer) regions are typically more optimized for movement. Appendages with motile extremities will generally be more massive proximally, while appendages with more offensive extremities will be more massive distally.
Center of Mass - The appendage itself won’t have a center of mass, but your organism’s overall center of mass will influence the effectiveness of your limbs. I discussed this in an earlier post in this thread more in detail (Macroscopic Editor, Progression, and Principles - #41 by Deus), but organisms with a center of mass closer to their front are generally better at inflicting damage, while organisms with a center of mass closer to their back are generally more motile. So the closer your center of mass is to a given limb, the better its stats will generally be, at the potential cost of stats in another limb.
Streamline - Certain appendages/extremities will have greater baseline stats, but will also interact very sensitively to streamline measures. This is much more apparent in water or in the air, but on land, this also has repercussions; swift animals tend to have skinny distal regions of their limbs to avoid getting caught on something on the ground. On the other end, organisms which reinforce appendages with weaponry tend to give up some streamline measure.
I will note that a stat like surface area to volume can be very important to limbs, but I want to hold off on this one a bit until we have a better understanding of other types of extremities. As it relates to motility, appendages with a greater surface area to volume in the water tend to help with floating planktonally while reducing streamline, while more voluminous appendages tend to help with more active propulsion.

Extremity Customization Tools

Appendage Alteration

Remembering that an extremity being placed on a limb transforms that entire limb, one way of altering extremities is by altering the underlying appendage. Appendages will have multiple degrees of customization, including…

Flattening or fattening for surface-area/volume and motility. Flatter appendages generally maximize surface-area and can increase buoyancy, though being too flat means a reduced streamline measure due to increased drag. Fattening appendages maximized volume and can increase the amount of force put into a limb, potentially assisting propulsion and damage while hampering streamline. Streamline is generally optimized somewhere in the middle, depending on the needs of your organism.
Adding/reducing metaballs for ideal segmentation. Certain types of extremities function better with a certain number of metaballs. Fins are generally optimized with one or two appendage segments, one for maximal speed and two for additional agility; weight-bearing limbs or weaponry often benefit from atleast two or three segments for additional leverage.
Increasing/decreasing the size of a segment to alter mass and streamline. Discussed above more in depth (distal v. proximal distribution), but altering the size of a segment alters the distribution of mass in your appendage, as well as how streamline your limb is.

As I mentioned, different customization options for an appendage will alter the stats of the appendage differently; certain extremities could be optimized better by being flatter, while others might be optimized better with more mass on the distal segment of the appendage.

Extremity Manipulation

Extremity customization will differ depending on the extremity itself, and across different skeletal types. In general, customization at first will be limited to appendage customization, but will eventually provide greater layers of detail offered to the extremities themselves.

Because extremities vary so much, I can’t provide an in-depth estimation of every single possibility right now; we can think of extremity customization as a macroscopic proxy of the “Modify” tool in the Microscopic Stages, so things vary. There are some things I do want to bring up, however, particularly related to the most relevant extremities.

Digits!

Representing fin spicules, fingers, toes, paws, and more, digits will be the primary way for endoskeletal organisms to customize their extremity itself. The first extremities available to the player won’t have digits, but when a dermal endoskeleton evolves, digits can be utilized.

Digits will alter stats and function of extremities. They will allow the player control over shape, which in turn strongly alters the importance of certain constraints. They can also combine multiple abilities into one appendage (lob fins that can bear weight and swim, hands that can strike and grab, etc.). Digits behave differently across different extremity types, so alteration of digit structure will be common; longer digits are useful for graspers, while more spread out digits are useful for ray fins.

Digits will remain as you switch across different types of extremities. If you switch out your lobbed fins with three digits for a ray fin, those three digits become three fin rays in the ray fin; if you switch that lobbed fin for a foot, those three digits become three toes; if you switch that foot into a wing, those digits become the finger guides for your wing. So on and so on.

If it is possible, the actual mechanism through which digits are added can be one of the more free-form segments of sculpting in the editor. Players can add a digit, then drag, tweak, and angle that digit as they please. If that proves to be immensely challenging, then we can parametrize as needed; adding a given number of digits via a more simple button, and allowing variations akin to modifications.

A bootleg concept of how digits can transfer between different types of extremities: from left to right, ray fins, lobed fins, and feet. Green indicates the metaball end of the appendage, while red indicates the digits themselves. Note that the underlying shape remains the same across the different extremity types. The only difference is the black outline, indicating a change of form.

Tail Customization

I think the important thing when it comes to tail customization is most relevant for the aquatic portion of the Macroscopic Stage. Digit customization for the tail can essentially be how you shape your tail fin. This provides a pretty intuitive way to represent different tail fin structures, and can represent more nuanced adaptations, like rotating digits resulting in a paddle that is oriented to flap up-and-down (like in cetaceans).

The concept above shows a pretty basic fin shape. The crimson red outline indicates the form of the extremity, while the lighter red/orange lines inside the tail fin indicates two digits angled in a way to give that fin shape. This gives a pretty easy method of customization. There’s a lot of function underlying tail fin shapes, but that’s beyond the point for right now; we can get into that once we have some common understanding of the underlying mechanics.

What gives me some pause for thought is how to deal with the more paddle-like fins and fin folds seen in primitive fish, such as the ones below:

For the bottom one, with the more defined paddle fin, that could be dealt with digit manipulation if customization works smoothly:

Top body plan indicates an animal with a fin extremity on their end without digits, and the bottom one with digits. Crimson and orange color scheme is the same as the prior concept, but this time, the green line indicates where the tail joint starts.

So with those basal fin folds - applying that specific extremity to your tail applies that fin to the entire length of your tail without digits, and digits can further customize the shape of your fin. Some general “sharpness” mechanic can also provide a general definition on your tail shape, whether or not it forms an explicitly fin or if it’s more of a fold. This can be a nice progression and introduction to more advanced digit manipulation of tails that comes with more advanced morphological structures.

Beyond that, I think we should note that the tail will require a bit more specific handling than other extremities, just for the fact that you can’t apply certain things on a tail (an “ear” extremity, for example, doesn’t make sense on a tail). The weaponry and tools we offer to the tail appendage generally shouldn’t be the same menu offered to other, non-torso appendage.

I will link this page for future reference: Fish locomotion - Wikipedia

Auto-Evo Consideration

I think an important thing to ask is how auto-evo will, one, understand what it is doing, and two, create organisms that don’t look like blobs. I think of this more specifically when it comes to the manipulation of extremities, such as digits, tails, etc.

I think presenting our edits via shape guides from a prior concept, where players are able to tweak their extremities via accessible sliders, are a good thing to build our extremities around. Players can drag sliders and such in a way that easily adjusts the shape, which in theory should make such manipulation a bit easier for auto-evo to tweak; auto-evo places those shapes, and tweaks them in certain ways to maximize certain stats.

Of course, I’d appreciate any feedback on what exactly auto-evo can and can’t “think through”. I think we can get away with some broader customization tools for torsos, appendages, and mouths while still representing an extreme amount of customization and depth - its extremities where more in-depth tweaking is seen, and where that level of customization would probably be requested by the playerbase.

Endoskeletal v. Exoskeletal Customization

There will be differences in how customization works between organisms with an exoskeleton and organisms with an endoskeleton which proximate the differences in body plan evolution between the two.

Endoskeletal organisms will generally have more fine-tuned customization over their limb-related parts - for example, digits. Arthropods won’t have a customization tool such as digits, but will generally have much greater ease in customizing the appendage itself, and will generally have more diverse extremity types they can place on their appendage.

I can see customization options for, say, pinching claws that can mirror digits in vertebrates, but that involves less the dynamic addition of parts and more the tweaking of the underlying shape of the two ends of the claw.

I want to handle arthropod customization with its own post, so I will share more detail on that later. The mechanics should generally be the same once everything is established, just with some smaller tweaks and different visuals.

Catalogue

Fins

Players will first be able to manipulate basic stats about their organism, such as streamline measure, mass, and center of gravity via extremity-less appendages to adjust their fluid dynamics. Controls such as flattening or fattening an appendage, increasing/decreasing its size, and manipulating length will adjust these constraints, loosely influencing movement.

Once their skeleton becomes proficiently advanced, players will be able to place fins on their appendages. Fins will strongly boost player movement based on constraints - for example, a streamlined appendage will provide much more of a movement bonus once that appendage becomes a fin.

Certain fins respond better to certain constraints - for example, ray fins respond extremely sensitively to streamline measures and surface area, but suffer more of a penalty from being on massive appendages, while lobbed fins behave the opposite way. As such, different fins will require different limb shapes to perform optimally.

Cartilagenous Fin - The first available fin for vertebrates, cartilaginous fins are generally well-rounded, responding in a balanced measure towards most of the constraints. Cartilagenous fins provide solid versatility, but due to their limited flexion and control compared to other fins, provide less extreme stat bonuses.
Lobe Fins - Needed for transitioning to land, lobe fins evolve from cartilaginous fins. Lobe finned organisms suffer much less of a performance reduction from mass, making it an ideal choice for larger primitive fish who don’t yet have an optimized body plan and armored fish. Lobe fins allow some basic movement on land and perform well in shallow and benthic lifestyles, making them the ideal choice for animals living in shallower waters who seek to transition to land. They have less response to being streamlined, meaning organisms with lobbed fins, unless very well-built, will be less nimble than ray finned or faster cartilaginous organisms.
Ray Fins - Evolving from lobed fins, ray fins are generally the best optimized fins for organisms which choose to stay in the oceans. Except at extreme sizes or in massive organisms, ray-finned marine life will enjoy peak movement in the water. Ray fins are extremely optimized towards being streamline, and also respond very well to surface area, making them well-suited for organisms that enjoy speed or drifting.

Weapons

Players will be able to place certain defensive or offensive capabilities on their appendages, turning that entire appendage into a weapon. Many of these structures will respond extremely well to mass, but the dimensions of the appendage can also be influential and can benefit customization.

A majority of players will likely utilize their mouth as their primary weapons, but can diversify their options via appendages.

Spike - Indicating the placement of a stiff spike, this turns the appendage into an extremity which inflicts damage upon contact. Benefits heavily from mass, but has a notable effect on center of gravity and streamline measures. The speed at which contact is made scales damage, as well as the dimensions of the spike; longer spikes are able to inflict damage at a wider area, but apply less damage. Are generally more defensive than stingers, indicating a constant potential for damage, but can be leveraged offensively with speed or creative body plans. This requires a skeleton to place down.
Cnidoblast - Indicating the placement of cnidoblasts on the surface of an appendage, this extremity can inflict an extreme amount of agents upon contact. Available very early on in the Macroscopic Stage, the effect scales with surface area. Cnidoblasts can be placed on appendages with joints, but unlike stingers, they do not apply base damage and only apply the effect. Alterations to this extremity can enable a temporary grappling of any organisms which come in contact with the appendage.
Stingers - Indicating the placement of barbs, this turns the appendage into a weapon able to jab at other organisms, like in stingrays, scorpions, and other arthropods. As opposed to cnidoblasts and spikes, allows an offensive “strike” ability which applies base damage, enabling an intentional leverage of weaponry. Can be altered to include agent damage. More mass indicates more baseline damage, though it also slows down the speed at which the strike happens. Stingers require a jointed appendage to place down. Longer appendages generally attack more slowly, but have a wider reach and can enable more mass to be put into the appendage.

Utility

Other appendages can just generally influence constraints, add different uses and abilities, or provide alternative movement methods. This is sort of a catch all for now, but with further conceptualization, more clear patterns can emerge.

Sensory Antennae - Proximating a whisker or antenna, placing this extremity on an appendage turns it into a wispy and skinny rod focused primarily on sense. Exactly how senses will work is a topic for another day, but I think something we can guess is that the antennae are extremely responsive to surface area, improving sensory capabilities with increased surface area but also strongly impacting health and tolerances.
Ears - Available after a certain point, a certain time of extremity allows the attachment of ears to an appendage, transforming that appendage into an ear similar to those found on mammals. Before then, serve as simple parts, able to be placed on an organism freely. Heavily respondent to surface area.
Filter-Feeding Surface - Proximating a very think area of the body, applying this extremity will transform the entire appendage into one capable of gaseous exchange and absorption of nutrition, such as in jellyfish and certain ecdysozoans. Heavily influenced by surface area, this extremity is extremely useful if the player has an integument that inherently has a high responsiveness to surface area, but is less useful for organisms with thicker/less-sensitive skins.

I think for now, that’s all I’ll list. The list of extremities can obviously go on forever, and I do think we should have a good amount more. But my point is that even with these few options, we can create really interesting gameplay if we tie them into constraints. And also just illustrating how the concept can work with different types of extremities.

What’s Next

Looking through existing posts, I think we are reaching a very solid point for a concept on the macroscopic external editor. I’m not saying this is an absolute commitment right now, but I do think it is the most robust and comprehensive take on a potential Macroscopic Editor, with decent mechanics to encourage robust gameplay and creativity.

I think my next step will be a concept focused specifically on how editing an organism with an exoskeleton will differ. Again, as I mentioned above, it shouldn’t be that different from the editor as a whole, I think I just need to flesh out the catalogue and do some more research to ensure that what has currently been written is consistent with arthropod evolution.

Then, I’m going to focus a bit on the internal, organ system editor. The thing with that is that the internal editor is heavily correspondent with progression in the stage, so I will need to flesh out expected experiences for macroscopic gameplay as a whole and general progression mechanics.

The Skeletal System I think is the most easy organ system to think of through since it is so relevant to the External Editor (integument, appendages, and jaws), but Sensory and Respiratory Systems are also important since it will clarify a bit more about the type of extremities we offer (things like ears, gills, antennae, etc. being individual parts, or extremities).

Beyond that, I appreciate all feedback on these concepts. I am obviously looking for input from programming and the graphics team on how feasible aspects of this concept are, but would also appreciate input from those working on Auto-Evo as to whether or not certain aspects of these concepts might be too much for the algorithm to handle.

Regardless, the Macroscopic Editor is nowhere close to being “solved”, but I think this is a solid spot to be in. I think some baseline ideas, like the constraints and torso editing, can be prototyped at this point.

Deus · June 30, 2025, 6:51am

Refining Digit Customization

Upon feedback and reflection, the current digit customization concept is very dependent on sculpting. Though this is favorable for customization, it makes the system very difficult for auto-evo to deal with. So I went back to the drawing board.

For convenience and future reference, I will be copy and pasting @GameDungeon’s guidelines on how auto-evo thinks, which will be useful for concepts related to the editor:

The Rules of Auto-Evo

Auto-Evo is random. It is a controlled randomness, but it can and will generate anything. The system needs to be able to handle any arbitrary set of parameters that Auto-Evo can generate.
Auto-Evo is an optimizer, but it only works if and only if it has a heuristic to optimize against. Auto-Evo can handle any situation where a sliders value has a direct effect on the overall fitness score.
As a corollary to rule 2, Auto-Evo cannot handle any situation that does not directly affect fitness. These things will either be random, per rule 1, or programmatic though some other system. This includes anything that does not have a gameplay effect, eg cosmetics. Sculpting without a gameplay effect is cosmetic.
Even in cases where sculpting and the like has a measurable gameplay effect, they will be done either randomly or programmatically, without artistic talent, in blind pursuit of the heuristic. Unless the fitness score for the sculpting somehow encodes aesthetics, it will be random and optimized to said score.

Going off that, I will set out a new framework for customizing extremities that I think will be much more manageable for auto-evo.

Rework

Somethings Stay the Same

First, I will note that I think my concepts from the prior posts on the differences between ray finned, lobe finned, cartilaginous fins is still valid. I will get into that more in my next post, which focuses more on traditional limb extremities like fins and feet/hands as opposed to the tail. But in general, your skeletal structure type in your limbs - cartilaginous, lobed, or ray-finned - will alter how constraints interact with your limbs, so that dynamic can still result in interesting divergent gameplay, and can provide great control over progression.

Variants and Modifications

Kind of similar to how the integumentary system can be dealt with, I think we can offer variations which can further be manipulated. There are very clear evolutionary patterns and structures across numerous of the more important extremities, especially with wings, graspers, feet, fins, tail fins, etc.

So we can apply those general structures, and give them different functionality and interaction with the constraints we set up. They will have to be broken down more in depth across the various types of extremities, so this will serve more like a catalogue.

I will also start by going into more depth about tails, as I am now more certain that they should be dealt with like appendages/limbs.

Tails & Caudal Fins

Players will be able to determine how much of their body operates as a tail by placing and moving a joint alongside their torso. The closer to their head their tail joint is, the more of their body will undulate; the closer to the rear of their body their tail joint is, the stiffer their body will appear in motion. This will implicitly represent their locomotion strategy, and will affect the underlying animation as they move. Any part of the body which is a tail can have additional appendages on it, but cannot have additional limbs.

Here is a basic description of the various types of fish locomotion. I will sort them by how close to the head the tail joint would be in a Thrivian organism…

Anguilliform - Similar to eels, certain catfish/lungfish species, and others, almost the entire body undulates in a slithery motion. This is energy efficient, provides solid mobility, and makes more advanced movement, such as digging or sheltering, easier. However, because the body more loose, it is hard to generate enough force to swim quickly.
Subcarangiform - The body as a whole is relatively flexible, but undulation increases more to the end of the body, with the majority of the work and undulation is done by the rear end of the organism. Compared to anguilliform organisms, it enables more speed at the cost of some maneuverability. This is seen in trout, and might be more frequent in fish which inhabit freshwater or brackish environments.
Carangiform - More of the body is stiff, enabling a blend between maneuverability and speed. Many Carangidae, fast-moving fish in the open ocean, utilize this locomotion style.
Thunniform - Seen in some of the fastest marine organisms, such as in tuna and certain species of sharks, thunniform fish have a very stiff body, relegating movement to the tail segment only. These organisms are very quick, but often require a lot of energy to sustain such an active lifestyle.
Ostraciiform - The body does essentially no undulation, with only the caudal fin itself moving, such as in pufferfish and sunfish. This allows advanced movement strategies, like hovering and proficient swimming backwards, but limits speed. In Thrive, this can be implemented in organisms where the joint is at the absolute last segment of the torso, enabling a different mode of swimming. It can allow much more equal movement speed backwards and forwards and enhanced rotation, but much less speed.

So in general for aquatic locomotion - the closer to the head the tail joint is, the more agile the organism is, the more energy efficient, and the more effective at certain advanced locomotion styles, like digging and burrowing, they are. Closer to the rear, and organisms are less nimble, energy-efficient, and effective in advanced movement styles. However, except for ostraciiform movement - where the joint is at the most posterior segment, enabling a different movement style - organisms are by default faster. We don’t need to explicitly have these be labelled in the editor; they just guide how we apply stats for the position of the tail joint.

This can be made so by having the streamline constraint be more impactful on certain movement types. For example, the streamline constraint can be extremely responsive to speed for organisms with a tail joint close to their rear, but less responsive for maneuverability. The other way around, the streamline constraint is more responsive to maneuverability and in measurement for things like burrowing and other advanced movements we implement, but less so for raw speed. That makes streamline an extremely definitive stat for the player to be aware of, while also enabling it to be dynamic.

Also, undulation is much more noticeable with a joint near the front of the body, and much less noticeable with a joint near the back of a body. That should hopefully make procedural animation much more manageable.

I will note that the effects of the different locomotion styles isn’t universal; tuna, which move thunniformally, are very maneuverable at high speeds. It just gives the base movement, which will be able to be manipulated via additional limbs or modifications to the caudal fin itself. Organisms with a tail joint closer to their rear will generally have greater customization due to the fact that they have more room to place limbs on their organism.

Caudal Fins

Onto caudal (tail) fins themselves. First, if a player has a tail without an extremity attached to it in a marine environment, the fin will be assumed to work like that of an eel or primitive vertebrate, with the entirety of the tail being used as a propellant rather than a fin.

Appendage customization will already be a tool that the player can utilize, primarily…

Flattening/Fattening - Similar to basic appendage customization tools, players will be able to flatten or fatten the tail segment to affect constraints. Flattening the segment increases streamline and surface area to volume ratio, but decreases mass. Streamline of course is a very important stat, but if the player flattens the appendage too much, they risk their center of mass being pushed away from their tail fin, thus counteracting the benefit of streamline. This visual effect will apply to the entire tail segment.
Shape Manipulation - Players will be able to lengthen and widen their segments as needed. Stretching the appendage increases surface area and affects streamline, but also affects mass.

They will also be able to alter the orientation of the appendage to make the tail undulate up-down or side-to-side.

The Caudal Fin Variants include…

Protocercal - Symmetrical tails with unexpanded surface fin area, as is seen in some of the earliest fish, lancelets, and other more basal swimmers; mass of the tail itself is used for propulsion, meaning this type of tail is likely optimized by having a longer tail. Simple and efficient, but not well optimized for thrust and acceleration. This would be equivalent to a tail without an extremity attached to it for marine organisms in Thrive.
Diphycercal - Seen in many bony and Palaeozoic fish, lungfish, and coelocanths, vertebrate is extended to the tip of the tail and is expanded, creating the appearance of a very extensive protocercal tail. Diphycercal are able to be manipulated to create various lengths and expansion. They also seem to provide greater agility and manueverability, enabling some backwards movement and navigation around tight spaces. However, they aren’t as streamline as terminal fins, and aren’t able to reach as high of a speed.
Terminal - The more “traditional” tail fin, terminal fins appear as an extension to the end of the tail fin. They tend to be the most optimized for movement in open areas, allowing a combination of solid maneuverability and speed. They also are generally pretty customizable, with very dynamic uses. Overall, probably the most optimized tail fins for open water swimming.

All fin types can be manipulated via sliders in these ways…

Surface Area - This increases or decreases how “fanned out” the fin appears to be, altering the surface area of the part. This impacts how much drag a fin has. More surface area on a fin appears to reduce maneuverability at high speeds, but increase it at lower speeds. Less surface area appears to increase maneuverability at high speeds, but decrease it at lower speeds. A dynamic movement system is obviously desired with in depth nuance to movement at various speeds. But if that’s too difficult, I think representing this by having different maneuverability for sprinting and base movement can be ideal, with high surface area improving base maneuverability at the cost of sprint speed and maneuverability, and low surface area improving sprint speed and sprint maneuverability at the cost of base movement maneuverability.
Vertical Slider - This determines whether or not the fin is longer on the top or the bottom. Adaptations in this manner generally seem to help with generating lift or maintaining neutral buoyancy. We probably have to simplify it in a way that isn’t 100% realistic in Thrive since there isn’t a clear “top fin results in this unilaterally, bottom fin results in this unilaterally” dynamic. But in general, we can make a longer top fin make it easier for the player to swim upwards, and a longer bottom fin making it easier for the player to swim downwards. It can be flipped if that seems to look wrong.

Terminal Fin Types

Rounded - Optimized well for stamina, though not very effective for speed.
Truncate - Well optimized for acceleration and maneuverability, but not for distance and efficiency.
Emarginate - Well suited for maneuverability, but not as optimized for speed.
Forked - Among the most common caudal tails, balancing maneuverability, speed, and efficiency.
Lunate - Typically seen in the fastest fish, well optimized for speed over distance.

We can probably make these a slider with some customization options as opposed to a pure toggle or switch via these means…

Horizontal Slider - How indented a fin is. Dragged maximally inwards results in a lunate fin, while pointed outwards results in an extended fin shape. More indented fins are better optimized for speed, while more extended fins are better optimized for agility.
Fin Shape - Pointed or rounded. Rounded fins are better for stamina, while pointed fins are better for maneuverability.

Conclusion/Reflection

This should probably be a better system for auto-evo to think through, with more clear parametrization and control. If these methods are more valid, then I will apply this new principle to fins.

I will briefly note here to clarify: I think the nature and use of “Streamline” as a constraint is beginning to become more clear and nuanced the more I add onto this concept.

I think ultimately, different types of movement will respond to streamline in different ways. By that, I mean: how streamline an organism is can be very important for numerous movement patterns, such as flying, digging, swimming, and to a lesser extent, walking/running. But obviously, different extremities have different uses: the most streamline fin in the world would be pathetic on land compared to the most ill-suited foot.

So, streamline can be used as a measure of how effective the movement tool is at achieving its purpose. If a limb is used for digging, then the streamline measurement dramatically affects effectiveness at digging, but gives much less of a boost to swimming speed. If a limb is used for flying, then the streamline measure dramatically affects mobility in the air, but not for running. This makes the streamline measure a very effective stat, but gives it nuance.

It could also potentially lead to cool cases of tools being transitioned between various anatomical structures in a more logical and easy way, similar to what @HyperbolicHadron mentioned. If a fin is streamlined and has proper qualities, perhaps it can transition to the fin of a flying fish more easily. Perhaps a wing can be altered in a way to give a boost to swimming as well, as seen in certain seabirds, making the transition to a fin more easy, a la penguins. I don’t want to throw that out there without thinking about it more, but I do wonder about the implications of it all.

Attaching images at the bottom to help better visualize types of fish tails:

Protocercal

Diphycercal