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.