The Osteopoda (lit: "bone legs") are the clade from which terrestrial sarcopods descend.
Osteopoda first emerged around 4190ma, evolving from sarcopods inhabiting coastal swamps. From there, the Osteopods ousted the Diplostomes for the role of terrestrial megafauna, causing the extinction of many Diplostome species in the process. As of yet, the Osteopods are the most diverse and thoroughly explored lineage of animals on TIRA 292b: Osteopods are present in every terrestrial biome, and later lineages have come to dominate pelagic environments as well.
The body of Osteopods consists of a head-like cephalothorax and a long abdomen. They have eight legs for locomotion and two pedipalps, the latter of which are lined with setae as a hearing mechanism. They possess four mandibles for masticating food.
Osteopods are the first clade of animals with a skeletal system, which evolved from toughened muscle fibers and are comprised of a bone-like polymer. This skeleton includes a skull plate, tri-jointed legs, and a hip-like limb girdle. Osteopods also possess a closed circulatory system with a single heart, as well as multiple kidney-like organs within their abdomen for blood filtration. Their digestive system includes a gizzard in their cephalothorax and a larger stomach in their abdomen. Their respiratory system has internalized, with two spiracles containing diaphragm-like muscle and olfactory nerves.
Osteopods possess selective hermaphroditism, allowing them to change sex in response to environmental or biological pressures. During gestation, fertilized eggs are contained within an ootheca developed from the mother's oviduct until they are ready to hatch: this ootheca will serve as their first meal upon hatching. Most osteopods are boneless at birth, and it is only once they grow to a large enough size that their skeletons ossify.
Like lophostomes, osteopods complete the majority of their development before they hatch.
There are two main branches of Osteopods, each of which differ primarily in skeletal structure.
- Polyschia: Possess a multiple flexible limb girdles. The most mobile terrestrial animals.
- Synischia: Possess a single fused limb girdle. Take up the role of megafauna.
- Many fans have compared the Osteopods (especially earlier clades) to spiders. In the 50K Subscriber Q&A, Biblaridion admitted that he did not realize the similarities until it was pointed out to him.