Athyrmatheria

 

Athyrmatherians ("toy beasts"), are one of the dominant clades of large-bodied fauna that inhabit Athyrmagaia. From the surface, these organisms may resemble vertebrates in appearance. They have skin, liquid lens eyes, and even bones. In truth however, their biology is far stranger, unlike anything seen on Earth. An individual Athyrmatherian is not one, but multiple specialized animals ("zooids") that make up the body segments of an aptly named "composite animal". Unlike earthly modular animals such as siphonophores, which hatch from a single egg and grow specialized polyps, the body segments of an Athyrmatherian develop from separate embryos, starting out as identical larvae that either immediately or eventually form semi-permanent chains with their siblings. As the larvae grow, they quickly develop into specialized body segments via an extreme expression of polyphenism. Even more strangely, however, rather than fusing on a cellular level, these zooids passively (albeit very securely) fasten together via biological locking mechanisms derived from the mouthparts, which allows the zooids to separate if need be. Due to this, the modular bauplans of species can be highly variable, and have allowed for a number of species to develop some truly strange and unorthodox behaviors unlike anything seen on Earth.

There are two main "classes" of Athyrmatherians that call the two largest continents Borea and Comedia their home; the ectothermic reptile-grade Tagmatasaurians ("arranged lizards"), and the endothermic, physically active Euathyrmatherians ("prospering toy beasts"). The Euathyrmatherians are the most dominant of the two classes, filling a wide array of niches similar to those of warm-blooded mammals from Earth. The Tagmatasaurians, or "tinkertiles", in the meantime fill roles similar to those of reptiles and occupy terrestrial niches that are left unoccupied by their more energetically costly brethren.

Like tetrapods, Athyrmatherians have hydroxyapatite bones composed of calcium phosphate and collagen. The internal structure of these bones is also surprisingly convergent with the condition seen in earth vertebrates, with the long bones of the limbs containing a honeycomb-like matrix of cancellous bone and myeloid tissue (bone marrow). Despite this, the Athyrmatherians are not proper "vertebrates." They are actually members of the phylum Stegospondyla, which are Athyrmagaia's analogs to arthropods. However, they are as far removed from their distant insect-like relatives as we humans are from our closest invertebrate relatives, the lancelets.

The Athyrmatherian equivalent of a spinal column is formed from a series of internalized, ossified tergites, with the individual zooids latching together through a passive yet sturdy coupling joint system derived from the jaws and tail bones to form a unified "composite skeletal system." In addition to the "spine," each zooid has a cephalon and a telson (tail), which vary drastically in structure depending on the zooid. For the cranial zooid, the cephalon retains its function as a skull and almost resembles a bird's head. But for the rest of the zooids, the cranial bone is reduced to a vestigial, arched structure. Similarly, the telson only retains its function as a tail in the lower abdominal (aka pelvic) zooid, with the telsons of the other zooids being used as a part of the coupling joint system that connects the zooids. While the vertebral tergites fulfill a function similar to that of vertebra, the sternites function like either a sternum in the thoracic and upper abdominal tagmata or a pubis in the lower abdominal tagmata. Connecting the tergites and sternites together are the tagmal ribs, which evolved from muscles that lined a long-lost hemocoel. In addition to tagmal ribs, many Athyrmatherians also possess additional rib-like structures called lateralia, bony or cartilaginous rod-like structures on the sides formed from muscles in the body wall. These bones function like our own ribs by protecting the internal organs. In the cranial zooid, the vertebral tagmata are numerous and highly articulated, working similarly to the cervical "neck" vertebra.

Each zooid that comprises the body of an Athyrmatherian has two pairs of limbs: a front pair of "dewlegs" that is smaller and permanently raised off the ground, and a rear pair of larger "locomotor" legs that are typically used for walking. Both limb pairs bear a bone structure convergently similar to tetrapods, though they bear some fundamental developmental and morphological differences. Rather than developing from the fin bones of ancient fish, the long bones of these limbs evolved from ossified muscles and tendons that once anchored to the insides of exoskeletal segments. The presence of multiple digits on each foot, as opposed to the singular tarsus of their more insect-like cousins, suggests either the fusion of ancient limbs or a deviation in gene expression that was more beneficial than detrimental. Also, as opposed to ancestrally bearing joints that bend in opposite directions, both the dewlegs and locomotor legs take an arrangement that closest resembles that of a tetrapod forelimb, though the naming conventions of some of the forelimb and hindlimb bones remain the same as that of our own tetrapods for clarity. All four limbs have an upper bone called a coxa, which usually functions like a scapula (aka scapulate) and forms a shoulder. The upper long bone (humerus/femur) and two lower long bones (ulna + radius/fibula + tibia) bend backward to form an elbow, with the lower limb bones terminating into a wrist or ankle formed from four carpal/tarsal-like bones (carpomeres/tarsomeres). Attaching to the wrist/ankle is an additional set of long bones (metacarpals/metatarsals) that form a hand or foot and terminate into three digits that typically bear claws (one on the third digit, two on the first and second digit). Like the rest of the composite organism, homologous limb structures are modified for different purposes per zooid. For example, the dewlegs of the thoracic, abdominal and pelvic zooids are usually either vestigial or used for mating. However, those of the cranial zooid are hypertrophied and used as a pair of independently movable lower jaws called "gnathopods." The pelvic locomotor legs, in contrast to the forelimb-like structure of the thoracic and abdominal pairs, are modified to emulate a structure similar to that of a digitigrade animal from Earth; the coxa is femuroid (femur-shaped) rather than scapulate, and anchors directly into a ball-and-socket joint in the posterior pelvic tagma to function as a thigh rather than a shoulder. As a consequence, the actual femur of the pelvic leg is used as a shin, and the elbow is used as an ankle. On the cranial zooid, the locomotor legs are entirely absent, with the individual bones of these limbs being shrunken and internalized within the animal's neck to act as the organism's inner ear. Some clades even retain the phalanges of this otherwise internalized pair as structural support for external, vaguely mammal-like "ears" called auripods.

As opposed to earth colonial organisms such as siphonophores or coral, there is absolutely no tissue fusion involved in the attachment of Athyrmatherian zooids. Instead, they use tight, secure coupling joints derived from the mandibles and telson (tail bone). The specializations are so extreme that the mandibles of the zooids (with the sole exception of those possessed by the cranial zooid) have entirely lost their function as mouthparts, causing the pharynx (oral opening) to migrate just below them. Aside from the actual coupling joints themselves, there are also a number of other adaptations that ensure each zooid of an Athyrmatherian's body is securely locked together. The front and back connection surfaces are mildly tessellated, with the front being convex and the back being concave to ensure that both surfaces maintain direct physical contact at all times. To strengthen the connection even further and protect the exposed soft tissues from drying out or getting infected, the lip and outer rim of each connection surface is lined with rows of "adhesive pores", which secrete a fibrous and extremely sticky mucus that rapidly dries when exposed to oxygen to act as an extremely strong airtight seal. When the segments need to detach, an additional set of "loosening pores" secretes an enzyme that quickly breaks down the mucus. Both of the connective surfaces have a set of vascularized and permeable tissues called "distribution membranes", which allow the zooids to directly transfer nutrients, oxygen, hormones and enzymes to one another via osmosis. This is especially importants in regard to the consumption of food, as it allows for the nutrients that are drawn by one segment to be evenly shared with all the others. When the segments are detached, these membranes also double as sensory organs, with specialized cells around their edges allowing them to sense heat.

Perhaps the most impressive of these adaptations would be the so-called "zip-lock" skeletal muscles. By themselves, these muscles truncate into flattened mats of collagen fiber that are visible on the connection surfaces. When the zooids attach, however, the collagenous flat ends of these muscles are able to "zip" together, forming hybridized skeletal muscles that connect between each zooid. The connections that form when these muscles zip is so strong that no differences between them and regular muscle have been found in terms of tensile strength. Even more surprisingly, these muscles can just as easily "unzip" when the zooids detach, which suggests that the zipping mechanism is "toggled" by neurological impulses. How such a thing evolved in the first place is uncertain, though it has been found that these muscle groups are exclusive to the composite stage.

Athyrmatherians have a composite nervous system formed from the individual zooid's nervous systems, linked together by a series of neural conduits in the upper mandibles and telsons that transfer impulses and information between each segment. Though each segment has its own rudimentary "brain", that of the cranial zooid in particular is quite complex. This brain complexity is not necessarily a sign of intelligence, but a unique adaptation that allows it to act as the primary nerve center of the composite organism. As long as the cranial zooid is attached, it has full control of the motor functions of the body segment zooids. If the head zooid is lost and is unable to be recovered, the body zooids will be unable to eat and operate properly, and will eventually die of either starvation or predation.

For cranial zooids in particular, what would normally be a receiver conduit is often modified into a series of electroreceptors that line the edges of the labia (upper mandible). Other species, particularly carnivorous Tagmatasaurians, have modified these conduits into potent, electricity producing organs, with mineralized jaws made of iron to allow for a paralyzing electric bite. In a few highly specialized lineages, the conduit has evolved into an antenna-like organ used to send and receive radio signals, allowing for a unique form of long-distance communication.

Rather than having identical duplicates of a full digestive system in each zooid, the individual digestive tracts of an Athyrmatherian's segments form the specialized regions of a continuous system that runs through the entire length of the composite organism. Food is taken in by a mouth and esophagus in the cranial zooid, which is swallowed whole and passed into a gizzard located in the thoracic zooid, where it is further broken down by powerful muscles. The stomach, which is located in the upper abdominal zooid, breaks it down even further with gastric acids, and then it is passed into the small intestine of the lower abdominal zooid, where the nutrients are absorbed into the bloodstream. The leftover waste product is then passed through the large intestine. Athyrmatherians have two small livers in their upper abdominal segment (just behind the stomach), which serve a similar purpose to the singular large liver of earth vertebrates, helping remove toxins from the food consumed by the organism. A symmetrical pair of gallbladders help produce bile to aid in digestion.

Liquid waste is handled differently by the two main clades of Athyrmatherians. Cold-blooded Tagmatasaurians expel liquid and solid waste at the same time, in a manner similar to reptiles. Euathyrmatherians however, such as the one pictured here, expel solid and liquid waste separately, the latter in the form of a discolored, malodorous fluid (urine). Like the small and large intestine, the kidneys and bladder are located in the lower abdominal zooid. Despite the separation of liquid and solid waste, both are still excreted from the same orifice, the cloaca.

Athyrmatherian zooids each have a full respiratory and circulatory system, and the action of inhalation and exhalation is often synchronized. The lungs are paired, each lung consisting of three linear chambers containing bronchi, and air enters and exits the lungs via a pair of slit-like spiracles on the sides of each zooid. Ring-like muscles that form the outer lining of the lungs contract to exhale, while another set of muscles, which anchor from the lungs to the inner surfaces of the ribs, pull on the lungs to inhale. While the lungs of the body zooids are fully developed, those of the cranial zooid are comparatively small, mostly serving to allow the creature to smell by passing scent particles through a set of olfactory turbinates.

Though their earliest ancestors possessed open circulatory systems, Athyrmatherians have closed circulatory systems where blood is actively pumped through veins and arteries. Per zooid, there are two hearts (one per lung), each with two chambers, albeit in a rather different arrangement than what is seen in vertebrates. Much like us however, Athyrmatherian blood is red due to its use of hemoglobin as an oxygen carrier. Whatever oxygen is inhaled by the lungs is distributed to the other tissues by the blood, and the remaining carbon dioxide will be brought back to the lungs to be exhaled. Their blood also distributes nutrients, enzymes and other vital fluids to various organ systems, and with the aid of the distribution membranes, which have densely woven capillaries that are close to the surface, the zooids are able to exchange these substances with one another via osmosis.

Athyrmatherians have fairly standard reproductive systems with male and female sex organs. Most of the endothermic fauna are gonochoric (distinctly male or female), with all of the zooids within a single body being genetically the same sex. More basal ectothermic clades, however, tend to be hermaphroditic, bearing both male and female organs and reproducing via cross fertilization. Due to their modularity and high degree of polymorphism, the placement and even number of reproductive systems can vary drastically in both sexual systems. Many species only have reproductive organs in their lower abdominal segment and copulate in a much more conventional manner. Others may have reproductive organs all body segments except for the head, and reproduce by disassembling and exchanging zooids with one another. Most Athyrmatherians are oviparous, and lay leathery eggs that hatch into (initially) non-modular larvae. Males (as pictured here) have retractable penises, which evert from the cloaca via hydrostatic action.