They Look Like Turtles. They’re Not Turtles.

Strange Bedfellows
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.

Proganochelys. Formerly the most primitive turtle.

Figure 1. Proganochelys. One of  the most primitive known turtles.

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.

Eunotosaurus

Figure 2. Eunotosaurus, a milleretid not related to turtles.

Eunotosaurus
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.

Members of the Placochelyidae

Figure 3. Members of the Placochelyidae, Placochelys, Henodus and Cyamodus in clockwise order.

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).

Sinosaurosphargis.

Figure 1. Sinosaurosphargis, a shelled sister to Claudiosaurus, not turtles.

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.

Stephanospondylus
Earlier we looked at the most parsimonious ancestor as determined by the large reptile family treeStephanospondylus 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.

References
Baur G 1887. On the phylogenetic arrangement of the Sauropsida: Journal of Morphology, v. 1, n. 1:93-104.
Cisneros JC, Rubidge BS, Mason R and Dube C 2008. Analysis of millerettid parareptile relationships in the light of new material of Broomia perplexa Watson, 1914, from the Permian of South Africa. Journal of Systematic Palaeontology 2008 (6): 453–462. doi:10.1017/S147720190800254X
Cox CB 1969. The problematic Permian reptile Eunotosaurus. Bulletin of the British Museum of Natural History 18: 167–196.
deBraga M and Rieppel 0 1997. Reptile phylogeny and the interrelationships of turtles. Zoological Journal of the Linnean Society 120:281-354.
Gauthier J Kluge AG and Rowe T 1988. Amniote phylogeny and the importance of fossils. Cladistics 4: 105-209
Gaffney ES 1990. The comparative osteology of the Triassic turtle Proganochelys, Bull. Am. Mus. Nat. Hist. 194: 1–263.
Gregory WK 1946. Pareiasaurs versus placodonts as near ancestors to turtles. Bulletin of the American Museum of Natural History 86:275-326
Gow CE 1997. A reassessment of Eunotosaurus africanus Seeley (Amniota: Parareptilia). Palaeontologia Africana, 34:33–42.
Gow CE and de Klerk B 1997. First record of Eunotosaurus (Amniota: Parareptilia) from the Eastern Cape. Palaeontologia Africana, 34: 27–31.
Keyser AW and Gow CE 1981. First complete skull of the Permian reptile
Eunotosaurus africanus Seeley. South African Journal of Science 77: 417–420.
Li C, Rieppel O, Wu X-C, Zhao L-J and Wang LT 2011. A new Triassic marine reptile from southwestern China. Journal of Vertebrate Paleontology 31 (2): 303-312. doi:10.1080/02724634.2011.550368.
Layson TR, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA 2010. Transitional fossils and the origin of turtles. Biology Letters June 9 2010. doi: 10.1098/rsbl.2010.0371
Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
Lyson TR, Bever GS, Bhullar B-AS, Joyce WG, Gauthier JA 2010. Transitional fossils and the origin of turtles. Biology Letters 6:830-833. online PDF
Modesto SP 2000. Eunotosaurus africanus and the Gondwanan ancestry of anapsid reptiles. Palaeontologia Africana, 36:15–20.
Rieppel O and Reisz RR 1999. The Origin and Early Evolution of Turtles. Annual Review of Ecology and Systematics 30: 1-22.
Rubidge BS, Modesto, S, Sidor C and Welman J. 1999. Eunotosaurus africanus from the Ecca–Beaufort contact in Northern Cape Province, South Africa — implications for Karoo Basin development. South African Journal of Science 95: 553–555. online pdf
Watson DMS 1914. Eunotosaurus africanus (Seeley) and the ancestors of the Chelonia, Proceedings of the Zooogical Society of London 11:1011–1020.

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