Eunotosaurus not a stem turtle – SVP abstracts 2016

Yet another confirmation
made 5 years later than this Oct 01, 2011 blog post.
Figure 2. The nesting of Microleter with Delorhynchus, Acleistorhinus and Eunotosaurus.

Figure 1. The nesting of Microleter with Delorhynchus, Acleistorhinus and Eunotosaurus.

From the Hirayama R and Nakajima abstract:
Eunotosaurus africanus has been often hypothesized as an ancestral form for turtles, as they share the wider dorsal ribs and highly reduced number of axial skeletons of body portion (1). Recent research points out further synapomorphies such as the T-shaped cross section of dorsal ribs shared with Triassic stem turtles such as Pappochelys, (2) Odontochelys, and Proganochelys. Nonetheless, this theory has a serious weak point—in E. africanus, the gastralia were absent, or very poorly developed as reported in just a few specimens, if any. (3) Ontogenetic studies of extant turtles strongly suggest that the chelonian shell was formed as a result of association of both carapacial (dorsal vertebrae and ribs) and plastral (pectoral girdle and gastralia) elements, with reduction of distal portion of dorsal ribs. This scenario seems to be supported by the body plan of Triassic stem turtles (4). E. africanus, however, has uniquely developed distal part of dorsal ribs. Dorsal vertebrae of E. africanus indicate a horizontal movement of the body and the presence of intercostal muscles. E. africanus has a unique overlapping of dorsal ribs, unknown in any stem turtles. (5) A T-shaped cross-section of dorsal ribs is observed in several tetrapods such as ankylosaurs (Ornithisuchia (sic)), armadillos, and anteaters (Xenarthra), not unique to E. africanus and stem turtles. In conclusion, E. africanus has its own uniquely derived defensive body structure, exclusively depending on dorsal ribs, not probable as an ancestral form to the turtle body plan. (6)
Fiigure 1. The turtle mimic Eunotosaurus from the Middle Permian was actually closer to Acleistorhinus.

Fiigure 2. The turtle mimic Eunotosaurus from the Middle Permian was actually closer to Acleistorhinus.

Notes
  1. In the LRT Eunotosaurus nests with Acleistorhinus and Microleter, far from turtles, which arise twice from phylogenetically miniaturized pareiasaurs like Elginia and Sclerosaurus.
  2. In the LRT Pappochelys nests with basal placodonts like Palatodonta, even further from turtles.
  3. This is a false presumption. Gastralia do not become the plastron in turtles, which arises in both clades of turtles.
  4. A large gamut phylogenetic analysis clears this problem up. Sclerosaurus is Middle Triassic. Elginia is Late Permian, both with origins much earlier.
  5. Excellent point. Too bad we have no post-crania for LRT sister taxa.
  6. Too bad that the authors do not tell us what Eunotosaurus is in their analysis (if they produced one). The nesting of Eunotosaurus with Acleistorhinus occurs when both are present in the inclusion set and bones are correctly identified. That does not happen very often in academic publications.
References
Hirayama R and Nakajima Y 2016. Is Eunotosaurus africanus really ancestral to turtles? Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.

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