| Cosesaurus Temporal range: Middle Triassic |
|
|---|---|
| Type specimen, with adhering jellyfish | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Prolacertiformes |
| Genus: | †Cosesaurus Ellenberger & de Villalta 1974 |
| Species: | †C. aviceps |
| Binomial name | |
| Cosesaurus aviceps Ellenberger & de Villalta 1974 |
|
Cosesaurus is a genus of prolacertiform archosauromorph reptile. It is known from a single, hand-sized fossil from the middle Triassic period of Spain. The fossil is a perfect impression of a largely articulated and complete specimen preserving soft parts, including an adhering jellyfish, but no bones remain in these impressions.
In 1977, Ellenberger proposed that Cosesaurus was an ancestor of modern birds.[1] That publication followed the description of the bird-like theropod, Deinonychus, but it appeared long before the theropod ancestry of birds had been widely accepted. In his large and highly detailed treatise, Ellenberger interpreted the following traits in the fossil: a strap-like scapula, a furcula (wishbone), a keeled sternum, beak-like jaws, a retroverted pubis and tail feathers. Some of these interpretations have not been supported by subsequent research. Padian and Chiappe (1998) regarded Cosesaurus as a member of the archosauromorph clade Prolacertiformes.[2]
Peters (2000) found that Cosesaurus nested with Sharovipteryx, Longisquama and pterosaurs in a clade, the Fenestrasauria,[3] though at least his inclusion of pterosaurs in this group has not been supported by any other researchers, who criticized his methods as flawed.[4][5] Like Ellenberger (1977), Peters found an antorbital fenestra without a fossa was a shared trait of these taxa, convergent with the antorbital fenestra with fossa found in archosaurs and archosauriformes. Later, Peters (2009) described a small pteroid in Cosesaurus.[6] This was reduced from the fenestrated-type of coracoid found in most lizards and the prehistoric taxon Huehuecuetzpalli, by simply increasing the size of those fenestrations. While a disc-like coracoid is suitable quadrupedal locomotion (all lizards have such a coracoid) stem-like coracoids are better for flapping, as in birds and pterosaurs. This would require at least occasional bipedal locomotion, as in certain lizards. Supporting that hypothesis, the feet of Cosesaurus have been matched to the sometimes bipedal and narrow gauge Triassic Rotodactylus tracks, which are distinguished by the impression of digit 5 far behind the digitigrade impressions of the other toes.[7] Cosesaurus also had four sacral vertebrae, two more than in any lizard. These may have helped support a bipedal configuration by adding strength at the fulcrum located at the hip joint.[3]