Earlier the large reptile tree recovered several turtle ancestors among Sclerosaurus, various pareiasaurs, Stephanospondylus and, more distantly, bolosaurs. Today we’ll add a few more pareiasaurs to see what the updated tree (now 565 taxa) recovers (Fig. 1).
we see small taxa at the origin of major clades. That also happens in stem pareiasaurs (Bolosaurus, 2.5 cm skull length, Stephanospondylus,11 cm skull length, Fig. 3). Odontochelys (4cm skull length) is a small soft-shelled turtle. Sclerosaurus, the smallest of the pre-turtle candidates (Fig. 3), has a skull length of 8 cm, exclusive of the horns.
The added pareiasaur taxa
(Fig. 1) clarify the list of more distantly related turtle ancestors. Smaller horned forms without a shell, like Sclerosaurus, ultimately evolved to become shelled turtles with horns, like Meiolania, and then without horns, like Proganochelys. The loss of teeth may have happened at least twice (Fig. 1) and perhaps thrice.
The clade of pareiasaurs
is understudied and, to my eye, currently in need of a good phylogenetic analysis based on every known bone. I am not able to do that with available data on the Internet, but the above cladogram is a good starting point. The latest work on Deltavjatia (Tsuji 2013) does not cover the entire clade, but does show an unusual variety for a single genus (Fig. 3). Perhaps a bit too much lumping here.
“Pareiasaurs appear very suddenly in the fossil record. It is clear that these animals evolved from Nycteroleterids, perhaps a Rhipaeosaur-like form.” The large reptile tree does not confirm that assessment. Rather it finds pareiasaurs evolved from bolosaurs and millerettids. Wikpedia considered nycteroleterids to be procolophonids. They are not related according to the large reptile tree. Macroleter does nest as a sister to Stephanospondylus, which is basal to pareiasaurs + turtles.
Wikipedia also reports
“Some paleontologists have argued that pareiasaurs include the direct ancestors of modern turtles. Pareiasaur skulls have several turtle-like features, and in some species the scutes have developed into bony plates, possibly the precursors of a turtle shell. Jalil and Janvier, in a large analysis of pareiasaur relationships, also found turtles to be close relatives of the “dwarf” pareiasaurs, such as Pumiliopareia. However, the exact relationships of turtles remains controversial, and pareiasaur scutes may not be homologous with the shells of turtles.“
Hallucicrania (Lee 1995), The clade Hallucicrania was coined by MSY Lee, for Lanthanosuchidae + (Pareiasauridae + Testudines). Lee’s pareiasaur hypothesis is looking rather less likely following the discovery of Odontochelys, a transitional aquatic turtle with very non-pareiasaur-like teeth and whose half shell matches embryonic development in modern testudines. Recent cladistic analyses reveal that lanthanosuchids to have a much more basal position in the Procolophonomorpha, and that the nearest sister taxon to the pareiasaurs are the rather unexceptional and conventional looking nycteroleterids (Müller & Tsuji 2007, Lyson et al. 2010) the two being united in the clade Pareiasauromorpha (Tsuji et al. 2012).
The large reptile tree notes that these clade members are related to each other, but the clade is not monophyletic.
Pareiasauroidea (Nopcsa, 1928), The clade Pareiasauroidea (as opposed to the superfamily or suborder Pareiasauroidea) was used by Lee 1995 for Pareiasauridae + Sclerosaurus. More recent cladistic studies place Sclerosaurus in the procolophonid subfamily Leptopleuroninae (Cisneros 2006, Sues & Reisz 2008) which means the similarities with pareiasaurs are the result of convergences.
The large reptile tree notes that these taxa are related to each other, but the clade is not monophyletic and recovers Sclerosaurus with pareiasaurs, not procolophonids.
Pareiasauria (Seeley, 1988), If neither Lanthanosuchids or Testudines are included in the clade, the Pareiasauria only contains the monophyletic family Pareiasauridae. It’s a traditional linnaean term.
Correct. But it also contains Sclerosaurus and turtles if you want it to be monophyletic.
Lee, M. S. Y. 1993. The origin of the turtle body plan: bridging a famous morphological gap. Science 261: 1716-1720.
Lee MSY 1997. Pareiasaur phylogeny and the origin of turtles. Zoological Journal of the Linnean Society, 120(3): 197-280. doi:10.1111/j.1096-3642.1997.tb01279.x
Jalil N-E and Janvier P 2005. Les pareiasaures (Amniota, Parareptilia) du Permien supérieur du Bassin d’Argana, Maroc. Geodiversitas, 27(1) : 35-132.
deBraga M and Rieppel O 1997. Reptile phylogeny and the interrelationships of turtles. Zoological Journal of the Linnean Society, 120: 281-354.
Tsuji L. 2013. Anatomy, cranial ontogeny and phylogenetic relationships of the pareiasaur Deltavjatia rossicus from the Late Permian of central Russia. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 104(2):1-42. DOI: 10.1017/S1755691013000492