Stephanospondylus pugnax (Geinitz and Deichmuller 1882) Early Permian ~290 mya was originally considered a rarely studied diadectid, and therefore a pre-amniote, not part of the reptile clade. Here it nests as a pareiasaur cousin without wide cheek flanges or "horns." Stephanospondylus is the closest sister taxon to turtles. No prior turtle phylogeny has included and tested this taxon. The last common ancestor of pareiasaurs and Stephanospondylus + turtles was the millerettid, Milleretta RC70, which is the sister toOdontochelys, a turtle-like millerettid by convergence.
Distinct from Anthodon, the skull of Stephanospondylus had no flaring quadratojugals, but instead had a robust skull architecture more similar to Millleretta with a gently embayed posterior and nascent tabular "horns." The teeth were elongated, as in Milleretta. The nares faced anteriorly.
The neural spines were inflated. The dorsal ribs had expanded costal plates, as in Odontochelys. The interclavicle was likely incorporated into a plastron.
The scapula had an acromion process, as in turtles.
The pelvis had a tall ilium and a separated pubis and ischium. The pubis included a distal attachment to a presumed ventral plastron.
The limbs were robust and short. The femur had a distintly angled neck, as in turtles.
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.
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