Coelurosauravus jaekeli (Piveteau 1926) Late Permian ~250 mya, ~40 cm in length, had elongated dermal ossicles supporting a wing membrane for gliding. Note the ribs. This is a primitive taxon that retained ribs that still looked like ribs, not like elongated transverse processes as in sister taxa like Mecistotrachelos (below), Icarosaurus and Kuehneosaurus. Coelurosauravus was derived from a sister to Palaegama and Saurosternon and phylogenetically preceded Icarosaurus and Kuehneosaurus.
Distinct from Palaegama, the skull of Coelurosauravus had further posteriorly displaced nares. The squamosal was enlarged and baroque with several ventral projections, all decorative. The supratemporal continued this crest-like structure in like fashion. The parietals extended further posteriorly maintaining their supratemporal connection. This made the once oval upper temporal fenestra into a slit in dorsal view, deeper in lateral view. The mandible had a more robust coronoid process.
The cervicals of Coelurosauravus were longer and more robust. More cervicals were present. The dorsal series was likewise longer. The dorsal ribs were shorter. The presacral number was increaed from 20 to 26. The caudals were more slender and more are preserved.
The scapula was relatively small and virtually identical to that in Owenetta (not well known in Saurosternon and Palaegama). The forelimbs were more gracile. The metacarpals decreased laterally after mc 2, the opposite of most lepidsosauriforms. Even so, digit 4 remained the longest.
The pelvis was smaller and more gracile. The hind limbs were relatively smaller. Like the metacarpals, the metatarsals decreased lateral to mt2.
Elongated dermal extensions framing a glding membrane had their genesis as dermal ossicles extending out from every rib tip, but with several bunched from the anterior two (or perhaps four) ribs. The manus and pes appear to be webbed, which would have added to the parachute effect. In the Permian there were few other tetrapods in trees, other than possibly early insectivores, like the diapsid Petrolacosaurus and the herbivorous synapsid Suminia, so gliding was not an adaptation to avoid predators, as in the living Draco.
Coelurosauravus may have been a decent glider, but the many subsequent phylogenetic modifications and improvements suggest that pseudoribs and their membranes may have originally had a display function, as indicated by the crest-like modifications to the skull that are absent in Icarosaurus and Kuehneosaurus. Prior to their use as gliding supports, the much shorter ossicles would have been decorative. In the phylogenetic race to compete to have the most showy lateral membranes as secondary sexual characteristics, a gliding ability was later coopted.
Senter (2003) considered the the bumpy upper bone (arrows point to it) a parietal, but it is actually the squamosal and supratemporal upside down. The red bone (rollover image) is a parietal and it is not bumpy. This was a key trait Senter (2003) used to establish a relationships between Coelurosauravus and Longisquama. |
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