Askeptosaurus italicus (Nopcsa 1925) Middle Triassic ~225 mya, ~2 m long, was a derived from a sister to Stereosternum, Wumengosaurusand Xinpusaurus and phylogenetically preceded other thalattosauriformes, such as Miodentosaurus and Endennasaurus.
Distinct from Stereosternum, the skull of Askeptosaurus was more robust with a deeper, shorter, rostrum and mandible. The nasals were completely separated laterally as the premaxilla contacted the frontal. The postorbital was a large L-shaped bone extending to the back of the skull. The lateral temporal fenestra was greatly enlarged. The quadratojugal was absent. The upper temporal fenestra was greatly reduced. The supratemporals were reduced to narrow strips. The squamosal was reduced to a narrow angle atop the quadrate. The palatine and ectopterygoid are separated here and the pterygoid expands to a full triangular shape.
The vertebral column in Askeptosaurus was similar to Stereosternum, except the ribs were not robust and the neural spines were not inflated.
The scapula and coracoid were not coosified. The humerus was shorter by half. The third metacarpal and digit were the longest. Metacarpals 1-3 were aligned. Metacarpals 4-5 were aligned.
The femur was also shorter and again the third digit was the longest. Only four phalanges appear on digit IV. Pedal digits 3-5 were virtually identical. Metatarsals 1-3 were aligned. Metatarsals 4-5 were aligned.
A family tree of the Enaliosauria is here. A more complete tree of the Reptilia is here.