Ptyonius olisthmonaias (Thayer 1985) UALP 5264 Westphalian, 310 mya is a classic nectridean, with its distinctive tail made up of inverted triangle neural spines and extended ridges (fused hemal arches?) ventral to each centrum. Derived from a sister to Microbrachis and Scincosaurus, Ptyonius was a sister to Sauropleura.
Distinct from Scincosaurus, the skull of Ptyonius had a sharper snout, a larger naris and a larger orbit. The supratemporal and tabular were not co-ossified. The parietal and postparietal were not coossified. The postorbital portion of the skull was shorter.
The cervical series was longer. The caudals developed ventral ridges (Scincosaurus did not have hemal arches) that deepened the tail, possibly to improve sculling while swimming. The dorsal ribs were more gracile.
The clavicles were greatly enlarged extending nearly to the skull. The scapulocoracoid was poorly ossified. The pelvis was reduced. The limbs were reduced.