Earlier the large reptile tree recovered a dual origin for soft shell and hard shell turtles. Here (Figs. 1-3) we’ll put the pertinent taxa to scale as animated GIF files. These help demonstrate evolution in a crude sort of way. Unfortunately, this is the best we can do at present with known taxa and published data. More discoveries will fill in the gaps.
Figure 1. Hard shell turtle evolution with Bunostegos, Elginia, Meiolania and Proganochelys to scale. Basal hard shell turtles had horns and club tails. The anterior rotation of the forelimbs is a derived trait.
It would be nice to find some Elginia postcrania
A reduction in size and loss of teeth coincided with the appearance of the carapace and plastron in hard shell turtles. Unfortunately, this critical stage is represented at present by a skull-only taxon, Elginia. Basal turtle taxa, like Meiolania, had horns and the limbs remained oriented laterally. A club tail trailed basal turtles. Did that develop earlier? We have not seen the ribs of Bunostegos published yet. One wonders if they were different than those of other pareiasaurs. Probably not if they were unremarkable.
Figure 2. Hard shell turtle evolution featuring Bunostegos, Elgenia, Meiolania and Proganochelys – NOT to scale. Even the palate of Bunostegos is very close to a turtle palate.
The skull of hard shell turtles
demonstrates the appearance and reduction of knobs/horns along with the elimination of teeth, the reduction and anterior rotation of the naris, reduction of the preorbital region relative to the postorbital region and the gradual appearance of the quadrate in lateral view. The reduction of the horns likewise reduced the dorsal exposure of the post parietals and tabulars. but the supratemporal remained a large element. Unfortunately it has been traditionally interpreted as a squamosal.
Figure 3. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx.
The evolution of soft shell turtles
also begins with a size reduction from Arganaceras to Sclerosaurus. Thereafter the skull continued to shrink, as the plastron and carapace developed in Odontochelys. Teeth disappeared thereafter, as in Trionyx. Convergent with hard shell turtles the enlargement of jaw muscles in derived turtles included the enlargement of post temporal fenestra anteriorly. embaying the posterior skull. So, not listed yesterday, soft shell turtles converge (or rather developed in parallel) with hard shell turtles, given present data.
Figure 4. New cladogram of turtle systematics. Note the separation of soft shell turtles with orbits visible in dorsal view from domed hard shell turtles with laterally oriented orbits here.
Small pareiasaurs from China
Since size is an issue in turtle origins, when you find a small pareiasaur, it is worthy of notice. Here (Fig. 5) are two and maybe three humeri from small pareiasaurs, smaller than Sclerosaurus. None are slenderized nor do they develop spherical proximal articulations as seen in turtles. Apparently they just belong to small or young pareiasaurs.
Figure 5. Small pareiasaur humeri from Benton 2016. Note the scale bars. Some of these are smaller than Sclerosaurus (diagram), yet none are slenderized as in turtles.
Lee 1993 was correct
in putting pareiasaurs in the ancestry of turtles. That agrees with a large gamut reptile cladogram (subset Fig. 4).
However
Benton (2016) summed up current thinking when he reported, “An unusual aspect of pareiasaurs is that they were identified as an out-group, even the sister group, of turtles by Lee (1993, 1995, 1996, 1997), based on their shared characters of a rigid covering of dermal armour over the entire dorsal region, expanded flattened ribs, a cylindrical scapula blade, great reduction in humeral torsion (to 25°), a greatly developed trochanter major, an offset femoral head, and a reduced cnemial crest of the tibia.
“This was disputed by other morphological phylogenetic analyses (e.g. Rieppel & deBraga, 1996; DeBraga & Rieppel, 1997; Rieppel & Reisz, 1999; Li et al., 2009) that indicated a pairing of turtles and lepidosauromorphs among the diapsids, and by molecular phylogenetic studies of modern reptiles that repeatedly placed turtles among the Diapsida, and the Archosauromorpha in particular (e.g. Hedges & Poling, 1999; Field et al., 2014). New finds of the Triassic proto-turtles Pappochelys and Odontochelys show close links to the Middle Permian Eunotosaurus, and turtles are confirmed as archosauromorphs on the basis of fossil and molecular data, and not related to pareiasaurs (Joyce, 2015; Schoch & Sues, 2015).”
It is interesting to note what Benton does not report…
…a long list of turtle synapomorphies for Pappochelys and or diapsids and or archosauromorphs. He doesn’t because he can’t. A long list of turtle synapomorphies with these clades has not been compiled because it cannot be compiled. Unfortunately, Benton is following the latest literature, not testing it and not seeing the red flags. (Remember Benton was part of the Hone and Benton (2007, 2009) fiasco that attempted to test two origin of pterosaurs hypotheses by eliminating one of them only partly due to self-inflicted typos. The rest was a hatchet job as you can read again here).
Figure 5. Odontochelys pectoral elements reconstructed. Here the acromion process originates along the lower rim of the scapula. Pelociscus is an extant soft shell turtle. The coracoid of Odontochelys has been cracked at the glenoid. The green area is a hypothetical restoration. The glenoid of the scapula still had a thin veneer of matrix on it when photographed. The ? could be an acromion process. Or it could be a rib. The procoracoid of Sclerosaurus is absent here.
Morphology must trump DNA in prehistoric taxa
In the large reptile tree Pappochelys nests with basal sauropterygians, like Palatodonta, a skull-only basal placodont taxon. Several taxa near this node, including Henodus, Placochelys and Sinosaurosphargis independently developed turtle-like shells. So there was selective pressure to do so in that clade and niche at that time, convergent with extant turtles. No one knows yet why turtle DNA does not nest turtles with lizards more often or why mammal DNA does not nest mammals more closely with archosaurs in concert with the topology of the large reptile tree.
References
Benton MJ 2016. The Chinese pareiasaurs. Zoological Journal of the Linnean Society, doi: 10.1111/zoj.12389
The Pterosaur Heresies 