SVP abstracts 2017: Eudibamus forelimb description

Sumida et al. 2017 bring us new information
on the pectoral region of Eudibamus, (Figs. 1,2) an early likely biped in the sprawling manner of the unrelated extant iguanian lizards, Chlamydosaurus and Basiliscus by convergence.

Unfortunately,
Sumida et al. continue to cling to the invalidated tradition that Eudibamus is a bolosaurid, largely based on convergent tooth shapes and taxon exclusion in their analyses.

Figure 1. Basal diasids and proto-diapsids. Largely ignored these putative synapsids actually split from other synapsids while retaining the temporal fenestra trait that serves as the basis for the addition of upper temporal fenestra in diapsids. Included here are Protorothyris, Archaeovenator, Mycterosaurus, Heleosaurus, Mesenosaurus, Broomia, Milleropsis, Eudibamus, Petrolacosaurus, Spinoaequalis, and Tangasaurus.

Figure 1. Basal diasids and proto-diapsids. Largely ignored these putative synapsids actually split from other synapsids while retaining the temporal fenestra trait that serves as the basis for the addition of upper temporal fenestra in diapsids. Included here are Protorothyris, Archaeovenator, Mycterosaurus, Heleosaurus, Mesenosaurus, Broomia, Milleropsis, Eudibamus, Petrolacosaurus, Spinoaequalis, and Tangasaurus.

From the Sumida et al. abstract
Eudibamus cursoris, a bolosaurid parareptile, from the Early Permian Tambach Formation (approximately 290 mybp), Thüringer Wald (Thuringian Forest), of central Germany, has been interpreted as the earliest known facultative biped. This was initially proposed based on the postcranial limb proportions in the type specimen (MNG [Museum der Natur, Gotha, Germany] 8852), but the forelimb itself has never been formally described. A nearly complete left, and partial right forelimb are preserved in the type specimen. The forelimb is less than 60% the length of the hindlimb. Only a thin, blade-like scapula is visible. Brachial, antebrachial, and manual elements are slender and elongate compared to those of other basal amniotes. The humerus has two well developed distal condyles with terminally facing articular facets. Delto-pectoral attachments were along a narrow ridge. The radius and ulna are nearly subequal in length. Conspicuously, the ulna lacks a well developed olecranon process. Carpals are proximodistally elongate compared to other basal amniotes. The intermedium and lateral centrale and the radiale and medial centrale articulate end-to-end, and their combined lengths equal that of the ulnare; the intermedium and radiale, and the medial and lateral centralia are equal in length. Four distal carpals are visible, it is unclear whether whether the fifth is truly absent or simply unossified. The distal carpal associated with digit two is reduced to a tiny pebble of bone, whereas that associated with digit four is largest and somewhat wedge shaped. Four metacarpals, likely equivalent to digits two-five, are present. The proximal portion of metacarpal two is present but length of the entire element cannot be determined. No elements of digit one can be seen, though its absence could be an artifact of preservation; however, the presence of only four distal carpals suggests Eudibamus may have had only four manual digits. Three phalanges are preserved in digits three and four. Both come to blunt tips and neither exhibits a significantly elongate penultimate element. The overall limb proportions seen in Eudibamus could suggest facultative bipedality or vertical clinging and leaping. However, vertical clingers and leapers normally have at least one is proportionately elongate manual digit and well-developed manual claws. Neither phalangeal proportions, nor the two well-developed terminal phalanges show such adaptations in Eudibamus and its interpretation as a facultative biped remains the most plausible interpretation of its postcranial anatomy.”

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.

First of all,
Parareptilia has been invalidated as a monophyletic clade since 2012. 

Figure 2. Eudibamus skull revised here with new data compared to bolosaurids, on the left, and basal diapsids, on the right. Post crania for bolosaurids is very fragmentary. Bolosaurids are related to pareiasaurs and turtles, all derived from millerettids. Can you see why Eudibamus was confused with bolosaurids?

Figure 2. Eudibamus skull revised here with new data compared to bolosaurids, on the left, and basal diapsids, on the right. Post crania for bolosaurids is very fragmentary. Bolosaurids are related to pareiasaurs and turtles, all derived from millerettids. Can you see why Eudibamus was confused with bolosaurids?

Since 2011
Eudibamus has nested with other slender, speedy, basalmost archoauromorph diapsids (Araeoscelis, Petrolacosaurus and kin) (Fig. 1) in the large reptile tree, far from the squat, slow, bolosaurids, like Bolosaurus and Belebey that nest with diadectids and pareiasaurs.

Let’s look again
at the pectoral region and forelimb of Eudibamus as listed by Sumida et al. above. Note how many of these traits are also present in basal archosauromorph diapsid taxa and their outgroups shown in figure 1 above. Bolosaurids, by contrast, are known chiefly by skull material, so direct comparisons to forelimbs cannot be made.

Imagine the co-authors, grad students 
who disagree with Dr. Sumida on the phylogenetic position of Eudibamus, perhaps after testing a larger gamut of taxa or by reading this blog. All co-authors sign that they agree with what is in the abstract. This is how paleontology puts on blinders, clings to traditions and generally avoids rocking the hypotheses of senior professors.

Fortunately
non-academic renegades and independent researchers have no such restrictions, but are free to explore and experiment.

References
Sumida SS et al. 2017. Structure of the pectoral limb of the early Permian bolosaurid reptile Eudibamus cursors: further evidence supporting it as the earliest known facultative biped. SVP abstracts 2017.

Sumida 2009 Ted Talk video
What is Eudibamus?

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