Bunostegos: maybe not so oddly erect in stance after all…
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.
Figure 1. The originally published cartoon of Bunostegos with skeletal elements laid on top of it. As you can see, the right humerus was mistakenly illustrated in the right hand position. And this may have led to the error of proposing that this pareiasaur was uniquely erect in gait. Image from Brown University website (see below)
Bunostegos
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.
Figure 2. Here’s Proganochelys in dorsal view. Note the humerus. If you look closely you’ll see a small depression lateral to the proximal articulation with the shoulder glenoid. And note the larger of the two proximal processes is lateral here, medial when the elbow is oriented posteriorly as in most other tetrapods.
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.
Figure 3. The left humerus of Bunostegos and the basal turtle Meiolania for comparison, both in dorsal view.. Colors denote homologous areas. That little dip in the medial condyle of Proganochelys (Fig. 2) is much larger here in Bunostegos and small in Meiolania.
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.
Figure 4. Pre-turtle pectoral girdle evolution. Here homologous areas are colorized. The acromion process is broken on all specimens of Bunostegos. Pink arrow points anteriorly. Note the lowering of the acromion process in Bunostegos. We don’t know how long it was. Also note the narrowing of the scapula. Note the maturation (ontogenetic) changes to the glenoid in Bunostegos. The more lateral orientation is on the smaller/younger specimens, as in basal turtles.
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.
Figure 5 Once again, and this time to scale, the pectoral girdles of Bunostegos. Note the more lateral orientation of the glenoid in young specimens, as in turtles (Fig. 3).
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.
Figure 6. Turtle pelvis evolution. Here are the changes in the pelvis of pre-turtles and basal hard-shelled turtles. We don’t know how long the pubis was in Bunostegos. The ischium is narrower in the last three taxa here. Meiolania has a tall, pareiasaur-like ilium. Bunostegos has a pointed posterior ilium, as in Proganochelys.
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.
Figure 7. Turtle femur evolution. Here the femoral head is interned in Bunostegos and assumes a spherical shape in the turtles, Meiolania and Proganochelys. We know the turtles held the femur horizontally, not parasagittaly.Pink arrro points anteriorly in these left femurs.
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.
Figure 8. Even though the femur has an offset and spherical head in this basal turtle, Proganochelys, still it does not indicate a parasagittal gait, but a horizontal, sprawling one.
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.
References
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
Source: https://pterosaurheresies.wordpress.com/2016/02/24/bunostegos-maybe-not-so-oddly-erect-in-stance-after-all/
