Another Triassic turtle enters the LRT

This turned out to be a somewhat ‘ho-hum’ event,
but a good opportunity to review turtle origins, still suffering from taxon exclusion and inappropriate taxon inclusion, as determined by the LRT (subset Fig. 4), which tests all contenders.

Sterli, de la Fuente and Rougier 2007 described
“a complete cranial and postcranial anatomy and a phylogenetic analysis of Palaeochersis talampayensis (Rougier, de la Fuente and Arcucci 1995), the oldest turtle from South America, is presented here.”

Unfortunately
taxon exclusion and bone misinterpretation mar this description, published before the genesis of ReptileEvolution.com and a distinctly different take on turtle origins based on including more taxa. Palaeochersis (Figs. 1, 2; PULR 68; Late Triassic, Norian-Rhaetian) is the third Triassic turtle to enter the LRT after Proganochelys and Proterochersis (Fig. 3).

Sterli, de la Fuente and Rougier 2007 report,
“Palaeochersis talampayensis has primitive character states in the skulllike the presence of lacrimal and supratemporal bones, the presence of a quadrate pocket, a foramen trigemini partially enclosed, middle ear limits partially developed, presence of an interpterygoid vacuity, presence of a cultriform process and a high dotsum sellae. However, Palaeochersis talampayensis also has derived character states like the absence of vomerine teeth, basipterygoid articulation sutured, processus paraoccipitalis tightly articulated to quadrate and squamosal, and cranioquadrate space partially enclosed in a canal. The postcranial description was based on Palaeochersis holotype (PULR 68) and on specimen PULR 69 represented by an almost complete skeleton and a hindfoot, respectively.” 

According to the LRT
Proganochelys, Proterochersis and Palaeochersis were terminal taxa, leaving no descendants. Instead, according to the large reptile tree (LRT, 1740+ taxa, subset Fig. 4), a Late Permian/Early Triassic sister to Niolamia gave rise to a Late Permian/Early Triassic sister to Meiolania (Fig. 5) gave rise to a Triassic hornless Kallokibotion, otherwise known only from the Latest Cretaceous and later. More derived taxa include Kayentachelys (Early Jurassic) filling in the temporal gap. In turtles the cranial horns are primitive. The loss of cranial horns is a derived trait. That’s something turtle workers have yet to appreciate.

Taxon exclusion mars all prior basal turtle studies.
In the LRT Palaeochersis nests with another Triassic turtle, Proganochelys. Given their overall and detailed morphology, that was expected and comes as no surprise.

Not much else to report here,
other than the identities of cranial bones in hard-shell turtles need to be reconsidered in light of their ancestry from the late-surviving, giant-horned turtles, Niolamia and Meiolania and before that, ancestral pareiasaurs like Elginia and Bunostegos (Fig. 4). These are taxa traditionally omitted from recent turtle ancestry studies. The LRT omits no putative ancestral turtle candidates. Rather the LRT tests them all. And for new readers, note (Fig. 4) soft-shell turtles had their own parallel origin and ancestry from different small horned pareiasaurs.

Longtime readers may notice:
Several of the older turtle skull illustrations (Fig. 5) were recolored to match an emerging standard pattern (e.g. bright green for supratemporals). My fault that I did not do this from the very beginning ten years ago. More work for me, but lookout for different colors in older blogposts.

References
Rougier GW, de la Fuente and Arcucci AB 1995. Late Triassic turtles form South America. Science 268:855–858.
Sterli J, de la Fuente MS and Rougier GW 2007. Anatomy and relationships of Palaeochersis talampayensis, a Late Triassic turtle from Argentina. Palaeontographica Abt. A 281:1–61.

wiki/Palaeochersis (Spanish)