Updated July 21, with new skull images based on higher resolution pix.
Eudibamus cursoris (Berman et al. 2000) was a long-legged reptile from the Early Permian of Germany described as the earliest biped and originally considered a bolosaurid. The discovery of Eudibamus prompted a certain amount of excitement because previously bolosaurids (Bolosaurus and Belebey) were known principally by skulls only.
Berman et al. (2000) originally considered Eudibamus a bolosaurid based on the rounded shape of its very small teeth. They may have also seen the lateral temporal fenestra, but they never published a skull reconstruction, perhaps because it was badly crushed. In overall shape, the skull of Eudibamus does indeed resemble that of Bolosaurus.
Why Not Diapsids?
Even without the skull, Eudibamus nested here close to the diapsids, but the skull has a diapsid temporal region, quite similar in configuration to Spinoaequalis and Petrolacosaurus (Figure 2), both of which predated Eudibamus by less than 10 million years. Moreover, the rest of the anatomy also closely matches that of other diapsids. There is no large coronoid process, only a gentle rise in the posterior jaw. The teeth did not resemble the strange crushers of bolosaurids so much. Here they appear to be merely short and blunt.
If we take a look at the feet, for instance (Figure 3), there is a closer match to Petrolacosaurus and Spinoaequalis than to bolosaurid sister taxa, Casea and Eunotosaurus. Digit 1 is very tiny, unlike the robust digit 1 of bolosaurid sisters. Digit 4 included at least two phalanges beyond the ungual of digit 3, unlike digit 4 in bolosaurid sisters. Digit 5 did not extend beyond p4.1, unlike digit 5 in bolosaurid sisters.
Unfortunately few to no other sister taxa of bolosaurids are known that preserve the post-crania and feet. From the list of sister taxa, such as Milleretta, we can surmise that bolosaurids had a short neck, bulky body (perhaps with expanded dorsal ribs), short legs and short robust toes, like those of Casea and Eunotosaurus. The skull of Eudibamus was preserved curled back, facing posteriorly, which means the neck was long enough to do this. Some cervicals are missing, but the elongated ones that remain resemble those in Petrolacosaurus more the shorter ones in Milleretta.
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.