Update on the Rossman 2002 “Brazilosaurus”

Figure 1. The nesting of Rossman's "Brazilosaurus" (the PIMUZ AIII 0192 specimen) at the base of the Thalattosauria is confirmed with the addition of post-cranial and new cranial data.

Figure 1. Click to enlarge. The nesting of Rossman’s “Brazilosaurus” (the PIMUZ AIII 0192 specimen) at the base of the Thalattosauria is confirmed with the addition of post-cranial and new cranial data.

Today: Some new data and a new reconstruction of the skull and postcrania of the intriguing and potentially important postcrania of PIMUZ A/III 0192. This mesosaur-ish reptile was attributed by Rossman 2002 to Brazilosaurus sanpauloensis. 

Earlier the only data I had on this specimen was a line drawing of a skull from Rossman 2002. This data resulted in a nesting outside of Brazilosaurus + Stererosternum + Mesosaurus, at the base of the Thalattosauria. Notably, with the additional data, the nesting did not change.

Crushed skulls are often the best Because all the parts are crushed into a single plane and you can rebuild that “house of cards” or “crushed eggshell” in many views. Some parts are visible through the orbit. The occiput often flips to the side.

Mesosaurs are key
Workers have attempted to nest mesosaurs based on its lack of temporal fenestration — and they (Modesto 2006) end up with pareiasaurs and millerettids and procolophonoids. But with it’s hyper-long teeth, Mesosaurus is clearly a derived form. What we’re looking for is a basal taxon that looks like Mesosaurus with small plesiomorphic teeth. Then perhaps we’ll see more evidence for the diapsid skull morphology. And that’s exactly what we find.

Figure 2. Click to enlarge. The Rossman "Brazilosaurus" PIMUZ AIII 0192. This is a basal thalattosaur and a derived mesosaur. Due to severe crushing elements from the other side of the skull made it appear that the skull lacked fenestration following the generally accepted but mistaken hypothesis that all mesosaurs lacked temporal fenestra.

Figure 2. Click to enlarge. The Rossman “Brazilosaurus” PIMUZ AIII 0192. This turns out to be THE  basal thalattosaur as well as a basal mesosaur. Due to severe crushing elements from the other side of the skull made it appear that the skull lacked fenestration following the generally accepted but mistaken hypothesis that all mesosaurs lacked temporal fenestra. Note the tiny forelimbs and deep tail chevrons. Low dorsal spines and gracile ribs are also noteworthy.

Here (Fig. 1) the Rossman (2002) “Brazilosaurus” nests at the base of the Thalattosauria, whether using the Rossman data as is, or pulling slightly different traits out (Fig. 2).

Rossman (2002) presented photos of several interesting mesosaurs. Among them was the SMF-R-4710 specimen attributed to Stereosternum (Fig. 3). This one does nest with Stereosternum, but with more open temporal fenestrae, its nests at the base of the mesosaurs, close to the base of the ichthyosaurs and thalattosaurs.

Figure 3. The SMF-R-4710 specimen attributed to Stereosternum, but with larger temporal fenestrae, smaller teeth and other distinct traits.

Figure 3. Click to enlarge. The SMF-R-4710 specimen attributed to Stereosternum, but with larger temporal fenestrae, smaller teeth and other distinct traits. The high unfused dorsal processes are also seen on Hupehsuchus and Utatsusaurus.

If you think this Stereosternum specimen is starting to look a lot like Wumengosaurus, you’d be right. And Wumengosaurus now nests at the base of Hupesuchus + Ichthyosaurs, so we’re getting closer and closer to the common origin of both. One keeps its teeth, the other does not.

Figure 3. Stereosternum SMF-R-4710 reconstructed from traced image (in color below). Here temporal fenestration is clearly diapsid.

Figure 3. Stereosternum SMF-R-4710 reconstructed from traced image (in color below). Here temporal fenestration is clearly diapsid.

Lest you doubt
Running the large reptile tree with all temporal fenestration traits deleted recovers a single tree unchanged from the full character list tree. If anyone wants to come up with better data or a better tracing, please use specimen numbers.

There are so many specimens of mesosaurs
and so many variations and only three names for them (Mesosaurus, Brazilosaurus, Stereosternum). Someone needs to put it all together and catalog some of the important specimens (e.g. the holotypes). We’ll need new generic names for the specimens above.

If someone has a good photo of the Brazilosaurus holotype (Shikama and Ozaki 1966), please let me know.

References
Cope ED 1886. A contribution to the vertebrate paleontology of Brazil. Stereosternum tumidum, gen. et sp. nov. Proceedings of the American Philosophical Society 23(121):1-21.
Modesto S 2006.
 The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society, 2006, 146, 345–368.
Shikama T and Ozaki T 1966. On a Reptilian Skeleton from the Palaeozoic Formation of San Paulo, Brazil” Transactions and Proceedings of the Palaeontological Society of Japan, New Series 64: 351–358.

 

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