Cancer is one of the most feared diseases in the natural world, and animals from both Earth and Athyrmagaia are capable of falling victim to it. In short, cancer is an abnormal group of rogue body cells that start to grow and divide at an uncontrollable rate, to the point that it may potentially invade vital organs and kill the organism. Whether it be viral agents, radiation, or even just poor lifestyle choices, cancer can be caused by an innumerable amount of factors. Most cancers by themselves are non-contageous, but unfortunately this isn't always the case. Sometimes, albeit very rarely, a cancer can come to life, gaining the ability to spread between hosts without the aid of a virus. These are known as "clonally transmissible cancers", and they are essentially single-celled parasitic forms of the species they originated from. On Earth, they are known to plague a number of animals, such as dogs, bivalves, and Tasmanian devils. Some can even spread between two different species. In even rarer, more extreme cases, however, a transmissible tumor has the potential to evolve into its own dedicated animal clade. This is speculated to be the case for the myxosporeans, a clade of parasitic cnidarians that show evidence of having evolved from a variety of cnidarian transmissible tumor.
On Athyrmagaia, the most extreme case of this theoretical phenomenon has happened, and to a degree so unprecedented that it seems to defy the very laws of phylogenetics. About 230 MYA, a mass extinction occurred among the Heteropods of the Borea-Comedian landmasses, though the circumstances were unknown for quite a while since there were no indications of any sort of climatic shift that may have caused it. More complete remains from these strata, however, show signs of severe deformity, with various holes, bumps, and growths on the bones that seem to be directly responsible for the animal's deaths. Nearly all of the Heteropod remains from this rock layer, in all growth stages, share these deformities, and the growths themselves resemble a form of bone cancer. It stands to reason that the mass extinction that killed these animals was not caused by a climatic cataclysm (not directly, anyway), but by an extremely aberrant species of single-celled parasitic fauna. This parasite is believed to have originated from a type of Heteropod transmissible cancer that gained the ability to spread between different Heteropod species, speciating into an entirely new clade of aggressive, genetically chimeric parasites by quite literally stealing the homeobox genes and mitochondrial DNA of its hosts through horizontal gene transfer. This extinct family of transmissible Heteropod parasites is now known as "Catastrophically Mutagenic Heteropod Transmissible Tumor" (CMHTT), and while it is still unknown what caused such a malignant tumor to spread to such a catastrophic level, these parasites proved to be so devastating that virtually every single Heteropod in the northern and western hemisphere was wiped out. The sudden loss of megafaunal herbivores and predators resulted in the overgrowth of Olekirkophte flora, pumping the atmosphere full of oxygen to create a high-oxygen environment similar to Earth's Carboniferous Period. This era was called the Neocarbonic, and it lasted for well over 50 million years. These very conditions were what allowed for megacolonial stegospondyls to evolve into the first Athyrmatherians, which were quick to adjust to the lowering oxygen levels as they developed a more complex respiratory system. They, however, were not the only ones who benefited from such conditions. Though the original wave of CMHTT went extinct along with the original megafaunal Heteropods, it quite evidently managed to leave behind a perplexing legacy that would shape nearly all ecosystems to come.
Tumeofauna are an enigmatic, almost anomalous grouping of animals that are found in nearly all habitable regions of Athyrmagaia. As the systematic name implies, Tumeofauna are an offshoot of the same lineage of CMHTT that caused the Borea-Comedian mass extinction event, but they are so extremely derived from their forebears that they are considered a whole new form of animal life of their own. These organisms are very far removed from their carcinogenic ancestors, evolving from mere rogue body cells into multicellular, often colonial heterotrophs and mixotrophs, capable of reproducing independently of a host. Using homeobox genes stolen by their primordial ancestors, Tumeofauna have secondarily re-evolved the ability to form specialized tissues in an organized, non-abnormal manner, and possess a distinguishable body plan and archaic organ systems. Being descended from an HTC lineage, Tumeofauna are technically an aberrant offshoot of Heteropod, though their DNA is extremely chimeric as result of over millions of years of horizontal gene transfer from hosts, with genetic matches to innumerable Heteropod clades both extant and extinct, as well as various phyla of invertebrates, algae and even flora. Some species, most often parasites, even have Athyrmatherian DNA in their genome. Due to this, pinpointing their precise ancestry to any specific Heteropod order, family, or genus is practically impossible. Additionally, modern Tumeofauna also engage in horizontal gene transfer at a regular basis, sometimes with animals and plants, and most often with microbes and other species of Tumeofauna. As a result, their genome is so heavily mixed that it's difficult to determine if Tumeofauna are truly a unified clade or a paraphyletic group of organisms that evolved their current conditions independently from one another.
Typically, a single colony of Tumeofauna is comprised of multiple interconnected, radially symmetrical "nodules", each technically being its own individual animal. At the center of the mass is the central nodule, which branches outward into smaller "child" nodules which are linked together by either a carpet of tissue or a network of "internodular ligaments" where the circulatory and nervous systems of each nodule intertwine. The outside of each node is covered in an epidermis, which is perforated with spiracles that connect to a subdermal network of trachea used to passively respirate atmospheric oxygen. The integument of the epidermis often varies, ranging from bare, moist skin to scale-like plates made of chitin. Similar to fungi, some Tumeofauna protect themselves using toxins produced from glands in the epidermis. Their bodies are lined with layers and bands of of muscle, and most species can be observed visibly convulsing and contracting. Some larger species possess a solid, internal test made of hydroxyapatite, which serves as structural support. A rudimentary, nodular nerve net is scattered across the colony's biomass, which allows it to react and respond to external stimuli, and at the core of the mass is a blood-filled, heart-like hemocoel that branches out into blood vessels and smaller hemocoels across the body tissues. Surrounding the central "blood chamber" are specialized tissues called "factory tissues" that produce stem cells, which are released into the blood stream and distributed across the body as the organism grows. Tumeofauna often lack a proper digestive system, so they instead gain sustenance by absorbing nutrients through permeable, root-like tendrils on the underside. Most Tumeofauna propagate through a very novel form of asexual reproduction. Like a mold, it grows and spreads continuously through cell division, creating new child nodules. In most species, the child nodules are not clones, and and are genetically distinct from the central nodules thanks to a form of cellular automixis, allowing the organism to reproduce asexually while still maintaining genetic diversity. Some parasitic species, however, lack the ability to perform automixis, and instead reproduce "sexually" by stealing mitochondrial DNA from the cells of their animal hosts, using an invasive form of meiosis to self-replicate. In addition to this, Tumeofauna can propagate by forming fruiting bodies called "cysts" on various parts of their physical mass, using a similar process of automixis or meiosis to produce fertile, spore-like oocytes within the cysts ready for dispersal. Many species simply expel these gametes into the air like fungal spores, though others may rely on plants or animals to either passively or actively disperse them. Regardless of the method of dispersal, as soon as the oocyte finds a suitable place to live, it will begin to germinate and grow into an all-new colony of Tumeofauna.
Though the original CMHTT pandemic was almost certainly responsible for the 230 MYA mass extinction event, the Tumeofauna that descended from this infection are now some of the most ecologically important organisms on the planet, sharing many niches with the fungus-like Sphondamycetes. Tumeofauna are a very broad and variable bunch, with a vast diversity of species found on both the land and even the sea. Ecologically speaking, they can best be described as "semi-sentient, throbbing mushrooms". Being heterotrophs, they can grow essentially anywhere where a sufficient amount of sustenance is available, but they are especially abundant in moist environments. Tumeofauna, along with an unrelated phylum of fungi-convergent life forms, are often keystone species that play a major role in the decomposition of organic matter. There is an especially high abundance of species that are involved in the decomposition of carrion, growing on the outside and inside of animal cadavers while eating away at necrotic flesh with the assistance of bacteria and digestive enzymes. Tumeofauna also serve as a food source for a wide variety of animals. Some active predators opportunistically eat non-toxic Tumeofauna as an easily accessible source of meat, and there are even species of carnivorous "grazers" that feed on the fruiting bodies of Tumeofauna like a cow eats grass. Tumeofauna also frequently engage in symbiosis with other organisms. Multiple topical and substropical species have a mutualistic relationship with photosynthetic microbes that live in the upper layers of their skin, allowing them to become mixotrophs that gain energy from both the sun and organic substances. Similarly, some tumeofauna inhabit the gut microbiomes of certain herbivores, protecting their hosts from intestinal parasites. On the flip side, however, there are also a multitude of Tumeofauna that have secondarily regressed back into parasites. Many of these parasites are relatively non-lethal, simply feeding on either the blood and skin of an animal host or eating away at the cuticles and sap of Olekirkophytes. The deadliest parasitic Tumeofauna, however, have secondarily re-evolved a transmissible cancer-like lifestyle resembling a toned-down version of that exhibited by the extinct CMHTT. These parasites grow and spread in two ways; Some species use a symbiotic oncovirus to inject their DNA into surrounding cells, with the resulting growth of infected cancer cells making assimilation and clonal/non-clonal self-replication easier for the parasite. Other species, as mentioned in the paragraph above, do the inverse and use horizontal gene transfer to steal mitochondrial DNA from their host for cellular meiosis, growing budded "hybrid" offspring that are able to spread more freely across their host's tissues. Unlike their ancestors, these modern parasites are capable of infesting a wider variety of animals, including Athyrmatherians. Thankfully, for modern animal life on Athyrmagaia, this modern generation of carcinogenic parasites has not yet evolved to the same catastrophic degree as the CMHTT strain they descend from. Not yet, at least.
