Eudibamus cursoris (Berman et al. 2000) Early Permian ~290 mya, ~25 cm in length, 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. Derived from a sister to Milleropsis, Eudibamus phylogenetically preceded Petrolacosaurus and Araeoscelis. The skeleton is essentially complete, lacking only a few cervicals and the anterior mandible, which lies beneath the skull, which is turned 180 degrees to the posterior in situ.
Distinct from Milleropsis, the skull of Eudibamus had a slightly shorter rostrum. The teeth were smaller. The jugal, squamosal and probably the quadratojugal developed connections with one another resulting in a diapsid skull configuration. The parietal was narrower, opening up the upper temporal fenestra. The postorbital and postfrontal were smaller. The jugal developed a quadratojugal process. The squamosal developed a more robust jugal/postorbital process.
Only two large cervicals were preserved. Three others were added in accord with Araeoscelis, which also had 29 presacral vertebrae. The torso was relatively short with short dorsal ribs. Two sacrals were present. The anterior caudal transverse processes bent posteriorly. The attenuated tail preserves no chevrons or neural spines. Such an attenutated tail is also found in Triassic bipedal fenestrasaurs, such as Sharovipteryx.
The forelimbs were shorter. The carpals were elongated, as in Petrolacosaurus. The medial digits were greatly reduced.
The hind limbs were robust and much larger than the forelimbs. The tibia+fibula was longer than the femur. Pedal digit I was pariticularly short while digit IV was 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, which was large enough to have acted independently of the others. This would be an autapomorphy for this clade and unusual in a tetrapod experimenting with bipedality. 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 other possibility is that Eudibamus used all of its toes to walk with an extremely wide splay. Gephyrostegus in anterior view provides the reason why the digits were increasingly longer laterally in splay-legged tetrapods. This is taken to the extreme in Eudibamus. Perhaps it clung to tree trunks or foraged between rock cracks instead. |
