Drepanosaur skull: Pritchard and Nesbitt 2014 JVP abstracts

Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

Figure 1. Megalancosaurus including the palate, the only palate ever figured for a drepanosaur. This is not the specimen described by Pritchard and Nesbitt 2014.

Pritchard and Nesbitt (2014) present new skull data based on a 3D drepanosaur skull (posterior elements only) from the Triassic of Arizona. Comments follow.

From the abstract:
“Drepanosaurs are an enigmatic clade of Late Triassic diapsids from Europe and North
America with superficially chameleon-like bauplans. The phylogenetic position of the
group among diapsids is contentious. (1) Most hypotheses suggest that drepanosaurs are
basal archosauromorphs closely related to ‘protorosaurs’ (e.g., Protorosaurus,
Tanystropheus). (2) Other phylogenies place drepanosaurs as non-saurian diapsids,
suggesting a substantially older origin for the lineage. Clarifying the phylogenetic
position of drepanosaurs is important to understanding the degree of taxonomic
diversification among diapsids prior to the Permo-Triassic Extinction (PTE).
The poor quality of the drepanosaur fossil record has hampered an understanding of
their position. (3) Nearly all drepanosaur skeletal material is badly distorted (4), and all
described skulls are crushed such that phylogenetically important characters are
obscured. (5) A new drepanosaur specimen from the Late Triassic Coelophysis Quarry of
New Mexico includes a partial, three-dimensionally preserved skull. The postorbital
region of the skull, atlas-axis complex, and anterior cervical vertebrae are preserved in
near-articulation. (6) 3D reconstruction of micro computed tomography (CT) data allows the first detailed description of most drepanosaur skull bones. Many are surprisingly
plesiomorphic (e.g., squamosal with massive descending process, quadrate lacking
posterior concavity, occipital condyle with notochordal pit), sharing more in common
with non-saurian diapsids than early archosauromorphs. (7).

A phylogenetic analysis of 300 characters and 40 early diapsids supports the
hypothesis that drepanosaurs fall outside of Sauria. (8). This suggests a very long ghost
lineage (~35 million years), extending well into the Late Permian. The results of this
phylogeny suggest that both drepanosaurs and a number of early saurian lineages must 
have originated by the Late Permian. Although the fossil record suggests an enormous
morphological diversification among saurians following the PTE, a great deal of
taxonomic diversification among diapsids must also have occurred prior to the extinction.”

(1) not at all contentious. Drepanosaurs are derived from Jesairosaurus in the Tritosauria. This has been known for several years.

(2) Protorosaurus and Tanystropheus are not related to one another. This has been known for many years.

(3) Repeating a false allegation.

(4) crushed flat, but not otherwise distorted (see Fig. 2).

(5) Not so, IMHO.  Use DGS to retrieve data. Works every time.

(6) good news, but the key traits are found in the preorbital region. The big question is: did they have an antorbital fenestra? I see one on several specimens.

(7) These traits were first identified in Megalancosaurus. The occiput data is news. Non-saurian diapsids could include sauropterygians, ichthyosaurians, rib gliders and basal younginiforms according to traditional trees, which are outdated at best. Saurians include lepidosaurs and archosaurs. In this regard, drepanosaurs are saurians, tritosaur lepidosaurs.

(8) 40 is way too few taxa if you don’t know where drepanosaurs nest, especially if Jesairosaurus and Huehuecuetzpalli are excluded (I haven’t seen the inclusion set). Using 420 taxa drepanosaurs firmly nest within the Tritosauria and Lepidosauria, thus within the traditional definition of Sauria, which is a junior synonym of Amniota/Reptilia. Actually there is no long ghost lineage. Drepanosaurs originated in the Triassic following Jesairosaurus in the Early to Middle Triassic.

This is my take (Figs. 1, 2) on the skull of the drepanosaur Megalancosaurus. Note the occiput is not exposed in this 2D crushed specimen. It’s a fragile construction with a large naris, an antorbital fenestra, large orbit, diapsid temporal architecture (like that of a pterosaur) and a Y-shaped hyoid.

Interpretation of figure 6, the skull of Megalancosaurus.

Figure 7. Interpretation of figure 6, the skull of Megalancosaurus. Struts of bone surround antorbital fenestra here.

Pritchard A and Nesbiitt S 2014. The cranial morphology of drepanosaurs and the PermoTriassic diversification of diapsid reptiles. JVP abstracts 2014.

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