Heteropoda

 

Heteropods ("different feet") are a terrestrial superclass of Enantignathan ("opposite jaws") Radiochordates, and one of the major clades of fauna native to Athyrmagaia. On Austrus, a small. isolated continent in the southern hemisphere, the Heteropods are the dominant clade of terrestrial fauna, and are represented by everything from meek, tiny "lizards", to hulking, megafaunal herbivores and carnivores. On Borea and Comedia, however, Heteropods are not nearly as prevalent; a mysterious mass extinction event had wiped out a majority of the Borea-Comedian Heteropod clades roughly 230 mya, with their vacant niches eventually being filled by the invertebrate Athyrmatherians. In the present, what remains of Heteropod diversity on Borea and Comedia are small, reptile-grade Saurophysans and flying, bird-like Pachyalatans, which descended from the minute number of species that managed to survive the mass death. Though they constantly face competition from Athyrmatherian copycats such as the Tagmatasaurians and Polypterans, these modern Borean/Comedian Heteropods still manage to thrive on this strange yet familiar world. In Austrus, the basal amphibious Uligoderms and the warm-blooded Therorhinans managed to avoid complete extinction, with the latter becoming the dominant land animals of the isolated landmass.

The instance shown here is a Saurophysan, more specifically a large herbivorous litopternadontid from the Orenstein Archipelago. On a surface level, Heteropods bear many remarkable similarities to tetrapods from Earth, and despite a much spottier fossil record it can be confidently assumed that they evolved due to very similar circumstances. Indeed, unlike the Athyrmatherians, they can more or less be considered "proper" vertebrates. They have four limbs supported by a hydroxyapatite skeleton, a closed circulatory system with hemoglobin blood, and an efficient respiratory system that actively pumps air. Of course, like all Enantignathans Heteropods possess an inverted jaw structure in which the position of the mandible and maxilla are opposite to the condition seen in Earth's gnathostomes. In effect, all Heteropods possess four nostrils on what would be considered their "chin", with one pair being used for breathing, and a specialized front pair called a "nasorium" (in honor of the literature of the late Gerolf Steiner), which can either be a specialized pair of inlets used to drink water, or fused into a second mouth of sorts. They also inherit the tetrameral spinal columns and four "tail lobes" of their Pisciphysan ancestors, though with heavy modifications to improve the viability of a terrestrial existence. The dorsal tail lobe functions much like a tetrapod tail, while the ventral tail lobe, or "genital tail", houses the animal's outer reproductive tract, functioning as either a penis or an ovipositor depending on the sex of the animal. The lateral tail lobes have evolved into the animal's hind limbs, and strongly resemble tetrapod hind limbs in form and function. Integumentary structures strongly vary among Heteropods depending on metabolism. Saurophysans, which are ectothermic, possess tough keratinous scales, whereas endotherms such as Pachyalatans and Therorhinans have a furry coat of insulating fibers.

 

Just like the fish-like Pisciphysans, Heteropods are members of the phylum Radiochordata, and are distinguished by the possession of an axial "basket skeleton" formed from tetrameral (four-fold) spinal columns. This condition is a result of the phylum's apparent ties to marine Spondylobrachians, which exhibit radial symmetry much like echinoderms. Due to this, they are also sometimes called " radial vertebrates" or "tetrameric vertebrates". Out of their four spines however, only the dorsal spinal column actually functions like a vertebrate spine, and the lateral and ventral spinal columns are heavily modified into new structures to allow for improved skeletal flexibility. In the neck region, the lateral and ventral vertebrae are nothing more than chain-like cables of loose bones loosely connected by ligaments and nerve bundles. In the thoracic region, however, the lateral vertebrae form elongated, protective ribs that fit into sockets in the dorsal vertebrae, and a sternum formed from fused ventral vertebrae serves as additional support for the pectoral girdle. A row of thin, rod-like "gastral vertebrae" lines the abdomen and belly, protecting the organs in a manner much like the gastralia of crocodiles and dinosaurs. Behind what appears to be the "pelvis", two separate tails derived from the dorsal and ventral columns fulfill different functions. The dorsal tail is used to maintain balance and stability, whereas the ventral tail serves as an outlet for the reproductive organs.

 The jaw structure of Heteropods can be crudely likened to an inverted version of that seen in our own jawed vertebrates. The mandible, which serves as the organism's upper jaw, opens upward like a hinge-jointed lid, and rather than using hydrostatic muscles like Picsciphysans, the action is accomplished using fibrous muscles that anchor from the back of the mandible to temporal fenestra under the eye sockets, as well as shallow depressions at the front of the cranium. The maxilla, which also contains the animal's nasal passages, serves as a both a stationary lower jaw and an attachment point for the muscles that close the mandible. Due to its reversed orientation, the maxilla is affixed to the cranium via two parallel struts of solid bone, making room for the buccal cavity and associated organs that would otherwise be accommodated by a lower mandible. Heteropods have two bones in their throat called hemihyoids, which are derived from the bones of lower gill opercula and act as muscle attachment points for a twin pair of tongues.

Though they have evolved a four-limbed condition comparable to that of Tetrapoda from Earth, the way Heteropods achieved this is different in one fundamental aspect: The two hind limbs are derived not from the pelvic fins, but from the lateral tail lobes and associated fin rays. It's very likely that the actual pelvic fins were long lost even before their ancestors started to breathe air, which would have forced stem-heteropods to rely on their lateral tail lobes in order to crawl on land. What appears to be the femur on each leg is actually an elongated lateral vertebra called a caudifemur, and the "pelvis" is a strong, sturdy girdle of fused lateral and ventral vertebrae called the caudipelvis.

 

The nervous system of a Heteropod is very unique in that it possesses characteristics of both a centralized and decentralized nervous system. It is a radial, basket-like structure comprised of four parallel cords of nerve tissue that extend from a semi-ganglionic brain in the cranium, with a hybridized peripheral nervous system that connects all four nerve cords together in a net-like mesh. The dorsal nervous system functions analogously to a vertebrate's central nervous system, and is comprised of the organism's spinal cord as well as the largest portion of the creature's brain. The lateral and ventral nervous systems, which are comprised of especially elastic bundles of nerve tissue, work in tandem with the dorsal nervous system in the gathering and transferral of information to and from the brain.

 This hybridized system has a number of unique benefits. If the dorsal spinal cord is severed or damaged to the point that its function is inhibited, the lateral nerve cords (if intact) can act as alternate pathways to allow information to still be exchanged between the damaged sections, negating the effects of body paralysis. Additionally, with the exception of the dorsal brain region, nearly all of their nerve tissues are able to heal from injury, with the approximate duration of tissue regeneration being dependent on the area. As a result, Heteropods are noticeably more durable than Earth tetrapods of similar dimensions.

Despite descending from radially symmetrical ancestors, the overall format of a Heteropod's digestive system is surprisingly similar to that seen in bilateral organisms such as vertebrates. They possess a mouth and esophagus, a stomach and gallbladder for breaking down food, and an upper and lower intestine for absorbing the nutrients. They also possess a liver for eliminating mild toxins from the food they eat. As one might expect, however, their alien evolutionary background has led to very peculiar deviations from what would be considered "normal" for animals from Earth. They have two tongues capable of independent movement, each with their own hyoid bone. In addition to a mouth, Heteropods also possess a unique organ called a nasorium, a modified pair of anterior nostrils at the tip of their lower jaw that leads directly into the buccal cavity rather than the larynx. The nasorium functions as a "second mouth" used specifically for drinking water, and allows Heteropods to hydrate themselves without having to risk flooding their airways. In species that almost exclusively feed on liquid foods (ex: nectarivores, hematophages), the nasorium is even more specialized, replacing the mouth as the primary feeding orifice. There are even species that use the nasorium to deliver venom. In the case of the Therorhinans in particular, the nasorium has replaced the true mouth in function, becoming a wholly new organ called a nasognath.

Like most obligately terrestrial organisms, Heteropods respirate aerobically, taking oxygen from Athyrmagaia's atmosphere and exhaling carbon dioxide. To accomplish this, they have developed a similar means of active gas exchange as air-breathing vertebrates; They possess four lungs, which they had evolved from the gas-filled buoyancy bladders of stem-Heteropod Pisciphysans. Each lung contains a bronchial tree with tubular branches that terminate in clusters of alveoli, which are used to diffuse gases in and out of the bloodstream. While four lungs may seem excessive for an animal that breathes an earth-like atmosphere, the combined oxygen intake of all four lungs per breath is roughly the same as that of a two-lunged vertebrate of equivalent size. The nasal passages and scent receptors of Heteropods are located in the lower jaw (maxilla), and air is primarily inhaled and exhaled through the posterior nostrils. Unlike Athyrmatherians, and a LOT like Earth vertebrates, the respiratory and digestive systems of Heteropods converge in the mouth and throat, with a valve-like epiglottis serving to prevent food from entering the airways as the organism swallows.

Heteropods possess a closed circulatory system where hemoglobin-rich blood is actively pumped through veins and arteries across the body. Their blood is circulated by a singular heart, though as a result of the organism having four lungs, the heart houses a total of eight chambers rather than the usual four seen in reptiles, mammals and birds.

 

Heteropods are gonochoric, which means they have two biological sexes that can be defined as "male" and "female". Like all gonochoric life forms, male and female Heteropods have reproductive organs that, while homologous in structure, serve very different functions. Males have testicles for producing sperm-like gametes while females have ovaries that produce oocytes (egg cells), and a zygote is created when the latter is fertilized by the former.

Unlike virtually all other terrestrial animals on Athyrmagaia, which possess a cloaca where both waste and gametes are secreted, obligately terrestrial Heteropod clades have a fully separated anus and gonopore. In both male and female Heteropods, the outer reproductive tract and associated gonopore are housed within a specialized ventral tail lobe called a "genital tail". The genital tail is a very unique and highly versatile structure, and its use varies depending on the sex of the animal. Males use their genital tail as a penis for internal fertilization, and the shape and look can vary depending on the sex positions of a given species. Females, on the other hand, use it as an ovipositor for laying their eggs. In some species of Pachyalatans, the female's genital tail bears a keratinized tip for boring nests into the hard, plastic-like bark of Olekirkophytes.