Eudibamus in detail: the wait is over, but the convergence remains

Bipedalism
There are two extremes of bipedal locomotion in tetrapods: 1) with the hind limbs held in a vertical posture (e.g. mammals, dinosaurs); and 2) with the hind limbs sprawling (e.g. Chlamydosaurus, pterosaurs and kin). Berman et al. 2021 report for Eudibamus, “its postcranial anatomy documents the earliest occurrence of a vertebrate capable of employing a cursorial quadrupedal or bipedal digitigrade locomotion with the limbs held in a vertical erect posture and swung pendulum-like in a parasagittal plane.”

Eudibamus was preserved with knees tucked in close to the torso
(Fig. 6), but only two sacrals are present, and the ilium has the tiniest anterior process. Figure 2 from Berman et al. 2021 (Fig. 6 frame 1) shows a large ilium, a good bipedal indicator. However, when scale bars are applied (Fig. 6 frame 2) the ilium is much smaller in life.

Note the ventral orientation of the acetabulum
relative to the ilium and the inturned femoral head. Typically strongly asymmetric pedes, like those found in Eudibamus, are found in arboreal taxa, like iguanas and anoles, but also in the bipedal sprinting lizard, Basiliscus. The stiff, attenuated tail of Eudibamus can also be found in related taxa (Fig. 3) and unrelated bipedal lepidosaurs like Langobardisaurus, Cosesaurus, Sharovipteryx and basal pterosaurs like Bergamodactylus. In summary, the case for bipedal locomotion in Eudibamus certainly appears possible.

I was happy to finally see
the flat teeth of Eudibamus (Fig. 1), so unlike those of basal diapsids. So like those of bolosaurids. So, that’s what all the fuss was about!

It is unfortunate
that Bolosaurus and Belebey are known with little to no post-crania. If they were robust with a short tail, like Diadectes, then there would be no question. The blunt teeth are better for herbivory and herbivores often have a large belly for plant digestion. Eudibamus breaks that rule (Fig. 3).

It is also unfortunate
that Berman et al. 2021 did not expand their taxon list to 1) demonstrate their clade ‘Parareptilia‘ becomes polyphyletic with additional taxa; 2) show bolosaurids nest close to diadectids, 3) show Eudibamus nests with basal diapsids, despite the bolosaurid-like teeth. Don’t “Pull a Larry Martin” and use only dental traits. Minimize your taxon exclusion. Be ready for massive convergence. Let the software decide.

Given the many scoring changes based on the new data,
I really thought Eudibamus would join the bolosaurids, or the bolosaurids would join Eudibamus, but neither came to pass. To force bolosaurids into the basal diapsid clade close to Eudibamus adds 18 steps to the most parsimonious tree.

References
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.
Bermian DS, Sumida SS, Henrici AC, Scott D, Reisz RR and Martens T 2021. The Early Permian bolosaurid Eudibamus cursoris: Earliest reptiel to combine parasagittal stride and digitigrade posture during quadrupedal and bipedal locomotion.