Stephanospondylus pugnax (Geinitz and Deichmuller 1882) Early Permian ~290 mya was a rarely studied diadectid/pareisaur without wide cheek flanges or "horns" and the closest sister taxon to turtles. No prior turtle phylogeny has included and tested this taxon.

Distinct from Anthodon, the skull of Stephanospondylus had no flaring quadratojugals, but instead had a skull architecture more similar to Diadectes and Orobates with a gently embayed posterior and tabular "horns". The teeth were elongated, as in Orobates. The nares faced anteriorly.

The neural spines were inflated.

The scapula had an anterior process, matching the anterior lean in turtles. The pelvis had a tall ilium and a separated pubis and ischium. The limbs were robust and short.

Williston (1917) classified reptiles based on their temporal fenestra or openings and so placed turtles with primitive reptiles without temporal openings.

Broom (1924) reported, "Those who regard the structure of the temporal region of the skull as the safest guide to affinity will naturally place the chelonianans either with the primitive mammal-like reptiles or the cotylosaurs (primitive reptiles without temporal fenestration); those who hold that more reliance can be placed on the structure of the girdles and limbs will be more impressed with the affinities to the primitive diapsids such as Sphenodon."

Rieppel and DeBraga (1996) considered turtles to be diapsids close to Placodus in which the temporal fenestra had resealed. Unfortunately they also considered the squamosal to be the quadratojugal and the supratemporal to be the squamosal in the turtle Proganochelys. Further confusing the issue, they nested Placodus within the Lepidosauriformes following Younginiforms.

Gregory (1946) compared
living and fossil turtles with placodonts, "cotylosaurs" (captorhinids, pareiasaurs,
procolophonoids, and diadectomorphs, all considered amniotes at the time), and
with seyrnouriamorphs. He concluded that Testudines were derived from Paleozoic
"cotylosaurs," and that among those, pareiasaurs approached Triassic turtles more
closely than the geologically older diadectids. Although placodonts. especially
Henodus, had evolved an amazingly turtle-like appearance, Gregory concluded
that they were not related to turtles and that convergent evolution, especially related
to dermal armor, causes a serious problem in recognizing testudine relationships.

Commenting on the Li et al. (2008) paper on Odontochelys, Reisz and Head reported, "Its osteology contradicts the view that turtles have pareiasaurian affinities, and, along with molecular data, supports evolutionary hypotheses that they are closely related to another group, the diapsid reptiles.”

In the present, more inclusive study, turtles do have their origins with pareiasaurs AND they are somewhat close to the purported diapsids Reisz and Head were referring to: the lepidosaurs. Turtles are NOT related to placodonts as Rieppel and DeBraga (1996) suggest.

New micro RNA studies by Lyson et al. (2011) support a lizard/turtle relationship.

Broom R 1924. On the classification of the reptiles. Bulletin of the American Museum of Natural History 51:39-45.

Geinitz HB and Deichmüller JV 1882. Die Saurier der unteren Dyas von Sachsen. Paleontographica, N. F. 9:1-46.

Gregory WK 1946. Pareiasaurs versus placodonts as near ancestors to turtles. Bulletin of the American Museum of Natural History 86:275-326

Kissel R 2010. Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Toronto: University of Toronto Press. pp. 185. online pdf

Li C, Wu X-C, Rieppel O, Wang L-T, Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.

Lyson TR, Sperling EA, Heimberg AM, GauthierJA, King BL, and Peterson KJ 2011. MicroRNAs support a turtle + lizard clade. Biol Lett 2011 : rsbl.2011.0477v1-rsbl20110477. abstract - online news story

Reisz RR and Head JJ 2008. Turtle origins out to sea. Nature 456, 450–451.

Rieppel O and deBraga M 1996. Turtles as diapsid reptiles. Nature 384:453-454.

Rieppel O and Reisz RR 1999. The Origin and Early Evolution of Turtles. Annual Review of Ecology and Systematics 30: 1-22.

Romer AS 1925. Permian amphibian and reptilian remains described as Stephanospondylus. Journal of Geololgy 33: 447-463.

Stappenbeck R 1905. Uber Stephanospondylus n. g. und Phanerosaurus H. v. Meyer: Zeitschrift der Deutschen Geologischen Gesellschaft, v. 57, p. 380-437.

Williston SW 1917. The phylogeny and classification of Reptilies. Journal of Geology 28: 41-421.

wiki/Stephanospondylus