YPM VP057103: neither Dromicosuchus nor Poposaurus

Revised March 31, 2020
with the realization that the postfrontal and postorbital of the YPM specimen were fused, the quadrate was dislodged from its anterior lean and the restoring of several other traits that now nest this taxon with wider-skulled Orthosuchus (Fig. 2), both of which had binocular vision due to somewhat forward-facing orbits.

Figure 1. YPM VP 057 103 skull in situ, traced with colors using DGS methodology and reconstructed.

Figure 1. YPM VP 057 103 skull in situ, traced with colors using DGS methodology and reconstructed.

Figure 1. The well-known skull of tiny Orthosuchus. Note the concave maxilla and dentary, resulting in a large gap.

Figure 1. The well-known skull of tiny Orthosuchus. Note the concave maxilla and dentary, resulting in a large gap.

Figure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

Figure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

Identified online
by Brian Switek in 2016 on Twitter Fossil Friday as Poposaurus (Fig. 6), and published by the Yale Peabody Museum as cf. Dromicosuchus (Fig. 4) by Sterling Nesbitt 2018, specimen YPM VP 057 103 (Figs. 1–3) nests in the large reptile tree (LRT, then 1342 taxa, now 1660+ taxa) as a member of the Crocodylomorpha, close to Dromicosuchus, but closer to Orthosuchus.

FIgure 1. YPM VP 057 103 in situ with bones colored and reconstructed skull shown alongside.

FIgure 2. YPM VP 057 103 in situ with bones colored and reconstructed skull shown alongside.

Notable traits in the YPM specimen:
The premaxilla was elevated and pointed anteriorly forming a shark-like nose. The rostrum was elongate. The cervicals are longer than in sister taxa. The pubis may have curved posteriorly, as in another quadruped, Trialestes (Fig. 5), which led to earlier confusion. Distinct from sister taxa (and most tetrapods), the humerus was much longer than the femur in the YPM specimen. This basal crocodylomorph with long limbs and a short torso appears to have been able to gallop rapidly, something a few extant crocs are able to do.

Figure 3. YPM VP 057 103 reconstructed using color tracings from figures 1 and 2 in two scales. The smaller one shows the tail attached.

Figure 3. YPM VP 057 103 reconstructed using color tracings from figures 1 and 2 in two scales. The smaller one shows the tail attached.

The skull of the YPM specimen
does indeed remind one of Dromicosuchus (Fig. 4), but the skull of the YPM specimen all by itself can nest it with basal crocs in the LRT, 20 steps apart from Dromicosuchus.

Figure 4. Dromicosuchus makes a first appearance here at PH.WP.com. Note the similarities to the YPM specimen. Phylogenetic analysis nests the YPM specimen apart from Dromicosuchus by 20 steps.

Figure 4. Dromicosuchus makes a first appearance here at PH.WP.com. Note the similarities to the YPM specimen. Phylogenetic analysis nests the YPM specimen apart from Dromicosuchus by 20 steps.

Poposaurus (Fig. 6) has distinctly different proportions. Likely the identification of this specimen changed behind the scenes between 2016 and 2018. Someone should mention this to Brian Switek so he can make an edit to his Twitter account.

Figure 1. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on its hypercarnivory.

Figure 6. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on its hypercarnivory.

References:
http://collections.peabody.yale.edu/search/Record/YPM-VP-057103

 

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