Osteoastera
Osteoastera is a clade of Astergnathiformes which includes all descendants of the Arsnoot, regardless of evolutionary grade or ranked taxon. They are joined by a variety of shared characteristics not found in their ancestors, such as relatively high intelligence, a bony endoskeleton, and a distinctive 6-boned leg structure. Aquatic species typically have horizontal tail fins, much like a dolphin.
Within Osteoastera, the terrestrial clade Tripoda (commonly known as "stinzers") also exists. This clade sports the 6-bone structure in three of their limbs instead of just one, but otherwise retain most of the same basic anatomical features.
Anatomy
Skeleton
Osteoasterans have bony, vertebrate-like skeletons. All species have a skull, a dorsal vertebral column, and ribs. However, the simple comparison to Terran vertebrates ends there.
In what could almost be called a parody of the concept of vertebrates, every single bone in the Osteoasteran body is derived from vertebrae, either primarily such as the bones of the skull or secondarily such as the scapular ribs, with the exception of the pelvic ring, jaws, teeth, and scleral ring. All of the limbs and each of their digits are, in essence, modified backbones. This bizarre anatomical quirk is the result of the lineage's ancestry and development: Their ancestors had tetraradial symmetry but secondarily developed bilateral symmetry in a very similar manner to the sea pigs of Earth, and thus each limb directly corresponds to an ancestral radial cut. This is reflected in Osteoasteran larvae and fetuses, which typically have external radial symmetry and rope-like "backbone limbs".
Jaws and Teeth
Osteoasterans have 4 jaws. The "normal" upper and lower jaws are comparable to the jaws of Terran vertebrates, though they can only move up and down. The muscles for these run under the eye, which has no bone socket protecting it, and as a result biting down hard puts significant pressure on the eyeball. As a result, species which need a strong bite to feed depend on the second pair of jaws--the lateral mandibles.
Each lateral mandible consists of two bones and an enlarged tooth. The two bones develop from the lateral jaw bone of radial larvae, splitting in two and bending inside-out during development. The upper bone is attached to the skull and can rotate, while the lower bone can slide. The enlarged tooth, sometimes called a tusk or a lateral mandible spike, is set on the sliding lower bone. In Stinzers, the sliding joint is taken full advantage of, allowing them to move the mobile tooth almost to the front of the snout as needed.
The teeth both inside the jaws and on the lateral mandibles are made of calcified keratin, with those of Stinzers also having a bone core. The teeth cover the tongue and the palate; mastication is typically accomplished by grinding these together.
Internal Organs
In most Osteoasterans outside of descendants of Rivet, most internal organs are placed in the anal arm or set within the pelvic ring. The pelvic ring has 4 lungs set inside of it, resting between each limb socket. On the inner side of the pelvic ring, near the lateral limbs, there is a pair of hearts which serves to pump blood throughout the body.
Behind the pelvic ring, or in front of it in descendants of Rivet, is mostly digestive organs, including a stomach and intestine. The stomach has an odd lumpy shape, with lobes that stretch towards the lateral and raptorial limbs, as a result of it originally being derived from the radial waste funnel found in more primitive members of Asterzalia. Near the end of the intestine is a gamete-producing and baby-incubating reproductive section, which is ancestrally bag-like but not fully disconnected from the intestine; Stinzers have altered this into a 3-lobed system corresponding to the branching of their anal fingers, where males have 3 internal testes and females have 2 ovaries and a womb. The cloaca is located at the end of the anal arm in the "palm" of the hand, except in Sagmalix which moved its cloaca to its pelvic region.
Behavior
Intelligence
Osteoasteran intelligence ancestrally includes memory and object permanence. They are capable of memorizing their surroundings, and Stinzers in particular have decent homing ability. Their ancestral intelligence, even in fish-grade members, is comparable to terran amphibians.
Parental Care
Osteoasterans are distinguished by their advanced parental care. While most other vertebrate asterzoans depend on their parents to some extent, the Osteoasterans instinctively keep track of their offspring and protect them from danger, including threatening or antagonizing potential predators.