Eudibamus cursoris (Berman et al. 2000) Early Permian ~290 mya, ~25 cm in length (Figs. 1, 2), was originally considered a relative of Bolosaurus, but here it nests at the base of the Diapsida, as old as the other basal diapsid, Petrolacosaurus. Araeoscelis has similar blunt teeth.
The forelimbs were quite a bit shorter than the hind limbs in Eudibamus. The carpals were elongated, as in Petrolacosaurus. The medial digits were greatly reduced on the manus.
The hind limbs were robust and much larger than the forelimbs. The tibia+fibula was longer than the femur. Metatarsal 1 and pedal digit 1 were pariticularly short while metatarsal 4 and digit 4 were robust and elongated.
Eudibamus was originally considered to be the first bipdal vertebrate due to the disparity in the lengths of its limbs. If so, it may have left impressions of only the big fourth toe when running, which was large enough to have acted independently of the others. This would be unusual in a tetrapod experimenting with bipedality, but maybe there were lots of them, most more typical. Typically when the limbs begin to swing beneath the knees, as in Biarmosuchus and Sharovipteryx, more toes tend to become more even in their lengths. The same does not hold for Eudibamus, in which the femur appears to have an offset head.
The other possibility is that Eudibamus used all of its toes to walk with an extremely wide splay. Gephyrostegus in anterior view (Fig. 3) provides the reason why the digits were increasingly longer laterally in splay-legged tetrapods. All the toes have to clear the substrate on the recovery stroke. And all the toes contribute to the propulsive phase. This is taken to the extreme in Eudibamus. Perhaps it clung to tree trunks with splayed limbs, or foraged between rock cracks instead of running bipedally.
PILs (parallel interphalangeal lines) have been demonstrated to show how digits extend and flex in unison. Your own hand has PILs. In Eudibamus pedal PILs indicate an extremely wide sprawl, not a bipedal configuration. The manus was likely similar, but the medial digits of the manus are not well preserved if at all.
In Eudibamus the ankle shows no tendency to rotate on a simple hinge the way the ankle of Cosesaurus does. No sister taxa of Eudibamus are bipeds. Even so, the tibia is longer than the femur, which usually signals a bipedal gait and a rapid one at that. The torso is short. The tail is attenuate. The sacrals are in no way derived (a subject we will tackle tomorrow). Nothing about this reptile indicates a particular flattening nor deepening of the skull or torso. The unguals are short, not hook-like like the arboreal Iguana.
A Eudibamus sister, Milleropsis (Fig. 4), does have larger unguals, at least on the pes and considering the shape of that pelvis, the length of that attenuated tail and the limb and phalangeal proportions, this taxon might have made the better biped.
Confused? So am I.
Some aspects of Eudibamus point to a bipedal configuration. Others do not. Whenever the subject of bipedal lizards comes up, it’s always good to take a refresher course with Dr. Jayne and his treadmill wonders seen here. This video dispels most biases concerning bipedal running in reptiles.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.