Thadeosaurus renested

Earlier Thadeosaurus (Carroll 1981, Figs. 1-3) nested close to protorosaurs in the large reptile tree (still needs to be updated), as a sister to Tangasaurus at the base of the Enaliosauria. Another look at Currie’s (1984) tracings (Fig. 3), rather than Carroll’s 1981, 1993 reconstruction (Fig. 1), inspired a new reconstruction (Fig. 2) and nested it within the Enaliosauria, on the other side of Tangasaurus, between Thadeosaurus and Acerosodontosaurus. So this new nesting shifts Thadeosaurus a few nodes. Thadeosaurus usually nests with tangasaurs. So everyone is in agreement here.

Figure 1. Original reconstruction of Thadeosaurus from Carroll 1981, 1993.

Figure 1. Original reconstruction of Thadeosaurus from Carroll 1981, 1993. I didn’t find the fused scapulocoracoid. All the specimens separated these elements, but they were all subadults to juveniles. The new reconstruction found more variability in the vertebra and a shallower torso.

At this grade, these basal enaliosaurs show no obvious aquatic adaptations. Rather, Thadeosaurus appears to have been a long-legged sprinter. The skull remains very much like those of basal diapsids. No special features there.

Figure 1. Thadeosaurus reconstructed from bits and pieces over large and small specimens scaled up to the large specimen. Only one of the smallest juveniles preserves any skull bones.

Figure 2. Thadeosaurus reconstructed from bits and pieces over large and small specimens scaled up to the large specimen. Only one of the smallest juveniles preserves any skull bones. Are those sternal plates or posterior coracoids? You have to know the phylogenetic nesting to be sure.

Sometimes you just have to employ more than one specimen to create a chimaera. In this case, Thadeosaurus was reconstructed from several specimens of various sizes (Fig. 3). Hopefully these are all congeneric and conspecific specimens, as reported by Currie (1984). I didn’t see any red flags here. Nothing about the reconstruction is at odds with Currie’s observations.

Figure 2. The various specimens in various sizes, all to scale attributed to Thadeosaurus. If the largest specimen did not have a bone, it was scaled up from the smaller specimens.

Figure 3. The various specimens in various sizes, all to scale attributed to Thadeosaurus. If the largest specimen did not have a bone, it was scaled up from the smaller specimens.

When genera look so much alike, as basal enaliosaurs do, it is paramount to get the details right. Try to get back to the original material. If not, try to get back to the in situ tracing. If not, use the reconstruction and hope it is accurate.

References
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383
Currie PJ 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology 4(1 ): 68-84.

wiki/Thadeosaurus

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s