Avicranium: a 3D drepanosaur skull

Pritchard and Nesbitt 2017
bring us a new Late Triassic drepanosaur, Avicranium renestoiAMNH FARB 30834, based on CT scanning a crushed skull and reconstructing it digitally (Fig. 1). I added a little distance between the anterior and posterior elements in order to get a rounder orbit. I also restored the missing ascending process of the premaxilla and nasal.

Figure 1. Avicranium from Pritchard and Nesbitt 2017, in situ, original reconstruction and revised with rostral restoration.

Figure 1. Avicranium from Pritchard and Nesbitt 2017, in situ, original reconstruction and revised with rostral restoration in accord with the plesiomorphic drepanosaur, Vallesaurus. Distinct from Vallesaurus, Avicranium has a concave maxilla and lacks teeth. That long convex squamosal is unique.

The new reconstructed skull
looks like a little oviraptorid (Fig. 2)— strictly by convergence.

Figure 3. Khaan, an oviraptorid that nests with Limusaurus in the large reptile tree AND the repaired Cau, Brougham and Naish tree.

Figure 2. Khaan, an oviraptorid, has a skull similar to that of Avicranium.

The closest sister taxon of Avicranium
in the large reptile tree (LRT, 1087 taxa) is the drepanosaur, Vallesaurus based on the skull alone. Pritchard and Nesbitt nested Avicranium as a drepanosaur based on the cervical vertebrae (Fig. 1) and noticed ‘striking similarities’ to birds. The difference includes the crooked jaw line, perhaps related to the absence of teeth in Avicranium.

Figure 2. Vallesaurus is a sister to Avicranium in the LRT.

Figure 2. Vallesaurus is a sister to Avicranium in the LRT.

Pritchard and Nesbitt nested
drepanosaurs uncertainly, reporting “A phylogenetic analysis of Permo-Triassic diapsids supports the hypothesis that drepanosaurs are an archaic lineage that originated in the Permian, far removed from crown group Reptilia.” They did not realize their clade ‘Diapsida’ was actually diphyletic, with lepidosauriformes arising convergently with archosauriformes. The authors did not include taxa that nested basal to drepanosaurs in the LRT including Jesairosaurus, Palaegama and Saurosternon even those these have been on the Internet for several years. In fact, their figure 4 cladogram shows no outgroups for the Drepanosauromorpha, a very dangerous phylogenetic proposition. By contrast the LRT provides certain and verified outgroups back to Devonian tetrapods.

Figure 4. Subset of the LRT focusing on drepanosaurs and Avicranium.

Figure 4. Subset of the LRT focusing on drepanosaurs and Avicranium.

So once again taxon exclusion obscures relationships.
All Pritchard and Nesbitt had to do was to go online for some taxonomic suggestions and their unsolved problem would have been quickly remedied using their own character list. Are paleo-workers trying to avoid taxa offered for testing by the LRT? It would seem so given the present circumstances. Would it tear down the walls if someone knowingly confirmed the present hypothesis of interrelationships?

Well, that’s not going to happen,
except, as we’ve seen, without citation.

References
Pritchard AC and Nesbitt SJ 2017. A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida. Royal Society Open Science DOI: 10.1098/rsos.170499

wiki/Avicranium

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