Aerosaurus greenleeorum (Romer 1937, A. wellesi Langston and Reisz 1981) Earliest Permian, ~ 295 mya, ~1 m in length, has been recognized as a basal member of the Synapsida, a clade that ultimately produced mammals, but it is also a member of the Protodiapsida, a clade that ultimately produced diapsids, dinosaurs and birds. Derived from a sister to Protorothyris, Aerosaurus phylogenetically preceded Varanops and Archaeothyris in the synapsid line and also preceded Heleosaurus in the protodiapsid line.
Overall larger than and distinct from Protorothyris, the skull of Aerosaurus replaced a single canine with a short series of them. The temple area included a large fenestra formed by the central erosion of the jugal and postorbital, but principally the squamosal. The teeth were laterally compressed and strongly recurved. The teeth were so highly curved that they seem unable to penetrate flesh and the tooth row extends far behind the orbit. The lower teeth were relatively tiny. Such an arrangement suggests something other than a meat-eating diet, perhaps some sort of filtering or plant-eating. The parasphenoid is uniquely expanded laterally and posteriorly, and bears rows of teeth on ridges. The quadratojugal curved up posteriorly as did the slender mandible. A strange skull indeed, convergent with Casea.
Relative to the skull, the body was much larger and the ribs were deeper. The tail was unusually long for a synapsid.
The clavicles and forelimb were more robust. The manus was more robust. Metacarpal 4 was the longest. The manual digits were relatively shorter. The unguals were deeper.
The ilium was oriented posteriorly and was more robust. The ventral pelvis was deeper. The pes had shorter, more robust digits.
Aerosaurus was a terrestrial reptile, less well adapted to climb.
Chronologically the Synapsida (including mammals and humans) had their start with Archaeothyris 10 million years earlier, so Aerosaurus probably predated this.
All prior textbooks and studies included Casea, Eothyris and Odaleops as basal members of the Synapsida, but they are unrelated as shown by the present tree. The present study is the first to present this new nesting.