Earlier we nested the knobby-faced pareiasaur, Bunostegos (Sidor et al. 2003, Tsuji et al. 2013, Turner et al. 2015; Fig. 1), with spiky Elginia at the base of all hard-shelled turtles, like Meiolania and Proganochelys. Soft-shelled turtles, like Odontchelys, as you might remember, were derived from a distinct, but closely related pareiasaur clade arising from Arganceras and Sclerosaurus, indicating that living turtles are diphyletic with two clades going back to shell-less ancestors among the smaller pareiasaurs.
was reported (Tsuji et al. 2013) to also have a parasagittal (erect, upright) gait, which is not only odd, but unique for both pareiasaurs and turtles. That put up a red flag. Sorry it took so long to get to. I think I see a mistake here in the humerus identification. Tsuji et al. might have mistaken a left humerus for a right one, based on the cartoon illustration of a complete specimen (Fig. 1). It might have been an easy mistake to make because Tsuji et al. report at least 9 individuals, several sizes, each and all represented by a short list of disarticulated bones.
Let’s start with what we know:
Everyone knows that Proganochelys (Fig. 2) nests as a basal turtle. It is the basalmost turtle in which the elbows were anterior to the shoulders in a normal configuration (in the more basal Meiolania they are primitively lateral). That rotation turns the traditional lateral condyles into medial condyles in practice. I want you to note the slight indentation lateral to the ball-like proximal humerus that fits into the socket-like shoulder glenoid in figure 2. You’ll see that again in Bunostegos (Fig. 3), but much larger.
Meiolania is an even more primitive hard-shell turtle
though this is still not the working hypothesis among traditional paleontologists. Here (Fig. 3) we’ll look at the humerus of Meiolania and other parts (Figs. 4-7) that will match what few bones were recovered from the Bunostegos site.
That little dip
in the medial condyle of Proganochelys (Fig. 2) is much larger here (Fig. 3) in Bunostegos and small again in the basal turtle Meiolania. Look again at figure 1 and you’ll see the big basin in Bunostegos was incorrectly flipped in the Brown University illustration.
We’ve been looking for the ancestors of turtles for some time now
And unfortunately these three papers on Bunostegos completely overlooked the possibility of a close relationship to Meiolania and other basal hard-shell turtles. You can see the evolution of the pectoral girdle and other bones provides the most gradual accumulation of derived traits known at present. At present, this blog and ReptileEvolution.com are the only studies that have recovered this heretical relationship.
It is interesting to see
the change in the orientation of the shoulder glenoid in the Bunostegos growth series (Fig. 5). Interestingly, the smaller specimens have more laterally directed glenoids, as in basal turtles (Fig. 4), which are also smaller.
The evolution of the turtle pelvis
is best seen in a series of pre-turtle and basal turtle pelves (Fig. 6). The acetabulum in all cases is lateral, but hard-shell turtles develop an acetabular crest that roofs over the joint and altogether form a socket shape for the ball-like femoral head (Fig. 7). This occurs concurrent with the appearance of the carapace and plastron.
The evolution of the turtle femur
can be seen in this series of pre-turtle and basal turtle femora (Fig. 7). Note the gradual development of the ball joint on the proximal femur along with the development of the sigmoid (=’S’) shape of the femur. These developments coincide with the appearance of the carapace and plastron.
I was not able to find
comparable pareiasaur humeri. They are not online and I don’t think anyone has done a large comparative study replete with a rich trove of illustrations yet. Basal turtles are smaller than most pareiasaurs. The hind limbs sprawl more.
I’d like to see
if any osteoderms or turtle-like ribs were found at the Bunostegos site. None have been reported so far. Hopefully this report will spur further studies with an eye toward gathering more pre-turtle data in Bunostegos. At present the many authors don’t know how really special their fossils are. There is a better story here than the false report of parasagittal limbs.
Sidor CA, Blackburn DC and Gado B 2003. The vertebrate fauna of the Upper Permian of Niger — II, Preliminary description of a new pareiasaur. Palaeontologica Africana 39: 45–52.
Turner ML, Tsuji LA, Ide O, Sidor CA 2015. The vertebrate fauna of the upper Permian of Niger—IX. The appendicular skeleton of Bunostegos akokanensis (Parareptilia: Pareiasauria). Journal of Vertebrate Paleontology: e994746. doi:10.1080/02724634.2014.994746.
Tsuji LA, Sidor CA, Steyer JSB, Smith RMH, Tabor NJ and Ide O 2013. The vertebrate fauna of the Upper Permian of Niger—VII. Cranial anatomy and relationships of Bunostegos akokanensis (Pareiasauria). Journal of Vertebrate Paleontology 33 (4): 747. doi:10.1080/02724634.2013.739537
Brown University website with news on Bunostegos