The search for the origin of turtles has produced some odd and divergent nestings, but it’s easy to see the concept in each case. Turtles have a long list of unusual characters led by their carapace and plastron, the “shell” that protects them. They have fewer ribs than most other reptiles. For most researchers the search for turtle sisters has focused on taxa with a carapace and/or fewer and broader ribs.
The Starting Line – The Most Primitive Known Turtles
Of course we have to start with (by universal consensus) the two most primitive turtles, Proganochelys (Baur 1887, Gaffney 1990, Fig. 1, above) and Odontochelys, both from the Late Triassic. In today’s blog we’ll take a look at the “also-rans” and “close but no cigars” that were not related to turtles, yet resembled them by convergence.
Lyson et al. (2010) added Eunotosaurus africanus (Seeley 1892) to a tree created by Li et al. (2009) in their earlier study of the basal turtle Odontochelys. Lyson et al. (2010) employed only 30 taxa. Unfortunately they specified “Diadectomorpha” as the outgroup, along with Seymouria. That was an inappropriate decision because Diadectes and Limnoscelis are reptiles nesting deep within the Lepidosauromorpha, as determined by the large study. Lyson et al. (2010) considered Eunotosaurus a turtle sister candidate because it reduced its dorsal rib count to just ten and each one was expanded in width producing a sort of internal carapace. Here, in a larger study that specified Ichthyostega as a basal outgroup Eunotosaurus nested as a sister to Acleistorhinus and Milleretta, which had similarly wide ribs and shared more traits with Eunotosaurus than either did with turtles.
Placodonts with a Carapace
Members of the Placochelyidae (Fig. 3) were placodonts that developed a turtle-like carapace, but not in the pattern of genuine turtles. Placochelys went so far as to evolve paddles like a sea turtle. Placodonts were slow-moving plant-eaters (or shellfish crushers) that descended from a sister to Palatodonta. de Braga and Rieppel (1997) and Rieppel and Reisz (1999) nested turtles with sauropterygians (which includes placodonts). They considered turtles to be diapsids that had lost their temporal openings. Unfortunately their study also recovered a turtle sisterhood with the Permo/Triassic rib-gliders. So, on the face of it, the likelihood of the slender arboreal gliders as sister taxa to bulky terrestrial turtles and marine herbivores should have raised a red flag. Here rib gliders and sauropterygians (including placodonts) are not related to each other so turtles cannot nest between them as sister taxa. Too few taxa, once again, is the single source of most of the problems with the studies of de Braga and Rieppel (1997) and Rieppel and Reisz (1999).
A Basal Enaliosaurian
Sinosaurosphargis yunguiensis (Li, Rieppel, Wu, Zhao and Wang 2011) Anisian, Middle Triassic, ~230 mya, ~60 cm in length was originally considered a possible turtle ancestor because the dorsal ribs were broadened to such an extent that the each one touched its neighbors and the torso was covered in bony osteoderms. Here Sinosaurosphargis nested outside the base of the Sauropterygia, as a sister to Claudiosaurus and some distance from the placodonts. Sinosaurosphargis had many more ribs than turtles do, the carapace was not produced the same way as in turtles and the skull was distinct in morphology.
Earlier we looked at the most parsimonious ancestor as determined by the large reptile family tree. Stephanospondylus did not have a carapace but was a Diadectes-like turtle sister taxon that preceded turtles by 65 million years. So, there was still a big time window in which to find closer turtle ancestors. No carapace or ribs are known for this taxon, but the rest of the anatomy was closer to turtles than any other tested taxon.
Earlier I wrote a short blog on turtle/pterosaur relations, but that was done by eliminating half the taxon list to prove a point and should not be considered the final word due to the mass exclusion experiment. Still, turtles and pterosaurs nested close to each other than to placodonts or dinosaurs.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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