Eudibamus new specimen – svp abstracts 2013

From the abstract:
Sumida et al. 2013 wrote, “The Early Permian bolosaurid reptile Eudibamus cursoris is known from the 
Bromacker locality of central Germany. Initial description based on a single, well preserved specimen suggested it is the earliest known facultative biped predating archosaurian bipeds by nearly 60 million years. Its hind limbs are approximately equivalent to snout-vent length and nearly twice the length of the forelimbs. A new specimen that includes the caudal portion of the presacral column, pelvic girdle, and complete left hind limb sheds new light on the hind limb structure of Eudibamus. Although all elements of the limb are well ossified, the pubis and ischium are fused neither to each other nor their contralateral mates, yet medial and lateral centralia are fused. Distal tarsals 1-3 are unusually elongated proximodistally. In ventral view the astragalus shows a strongly developed ventrally projecting lip at the margin where the tibia articulates. Features supporting the interpretation that Eudibamus was a facultative biped include: a moderately developed lip along the dorsal margin of the acetabulum for reception of a near-vertically oriented femur; digits 1 and 2 are reduced relative to extremely elongate digits 3-5, indicative of potentially digitigrade foot posture; and penultimate phalangeal elements are not significantly elongate, suggesting it was not a vertical clinger and leaper. Three-dimensional laser surface scanning allowed a digital model of hind limb elements to be transferred into the animation and modeling software package Maya. Differing positions of the femoral head relative to acetabular surface were tested. The range of potential postures does include a parasagittal orientation of the hind
limbs.”

Figure 1. Click to enlarge. Eudibamus in situ (above), traced (middle) and reconstructed (below). The revised skull retains a large orbit and has a shorter rostrum.

Figure 1. Click to enlarge. Eudibamus in situ (above), traced (middle) and reconstructed (below). The revised skull retains a large orbit and has a shorter rostrum.

Notes
Rather than as a bolosaurid (and their short-legged, fat-belly, plant-eater kin), the large reptile tree nests Eudibamus (Fig. 1) as a basal diapsid, derived from a sister to Heleosaurus, Archaeovenator, Milleropsis, Millerosaurus and Tangasaurus. Eudibamus is also close to Spinoaequalis. This nesting is far from Bolosaurus and Belebey, two bolosaurids which are known principally from skull only material. The new nesting puts slender Eudibamus with other slender, long-legged insectivores with greatly reduced medial toes.

Given the length of the cervicals (Fig. 1), the hind limb length is not snout-to-vent length, but occiput to vent length as in Spinoaequalis. In any case, a hind limb nearly twice that of the fore limb is another trait shared with Spinoaequalis and not shared with bolosaurid sisters.

The Milleropsis pelvis is likewise not coossified and the lateral + medial centralia are fused. The Milleropsis ilium also includes a moderately developed lip along the dorsal rim, but not the dorsal rim of the acetabulum. The ilium also produces a slight anterior process. The Milleropsid femur appears to allow parasagittal movement, as in Eudibamus. As in the new Eudibamus specimen, distal tarsals 1-3 are also proximodistally longer in Milleropsis.

Bipedal lizard video marker

Figure 2. Click to play video. Just how fast can quadrupedal/bipedal lizards run? This video documents 11 meters/second in a Callisaurus at the Bruce Jayne lab.

Sumida et al. suggest the extreme asymmetries of the pedal digits were indicators of bipedality, but that’s the opposite of the situation in archosaur and synapsid parasagittal configuration. I’m not arguing against it. In any case, that extreme reduction is also known in all basal diapsids, including Petrolacosaurus, but is not present in bolosaurid sisters. The penultimate pedal phalanges are likewise short in basal diapsids.

By Analogy, not Homology
The range of motion studies appear to be similar to those of modern lizards capable of bipedal locomotion, with an example video here (Fig. 3). Note the heel never touches the ground and the gait is largely parasagittal. This is what Eudibamus looked like a full speed, but I understand I missed the 3D animation at the convention.

References
Sumida S, Berman D, Jefcoat B, Henrici A and Martens T 2013. New information on the hindlimb structure of the early Permian bolosaurid reptile Eudibamus cursors, the earliest known facultative biped. Journal of Vertebrate Paleontology abstracts 2013.

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