Brazilosaurus and something like it at the base of the Thalattosauria

Brazilosaurus holotype – Shikama (1966)
Brazilosaurus (Fig. 1) was described by Shikama (1966) as a sort of mesosaur with a longer neck, shorter teeth and other minor differences. From the available online pdf (created from photocopies), I can only read the line drawing, but photos were published originally (if anyone has them, I’d like to see them, they’re too dark on the pdf).

Figure 2. The holotype Brazilosaurus from Shikama (1966, above) and bones colorized (below) based on sister taxa. The apparent giant cheek region may represent the occiput crushed into the bedding plane as it is the right size and shape and no sister taxa have anything similar. The gracile jugal indicates the presences of a lateral temporal fenestra, as in mesosaurus.

Figure 2. The holotype Brazilosaurus from Shikama (1966, above) and bones colorized (below) based on sister taxa. The apparent giant cheek region may represent the occiput crushed into the bedding plane as it is the right size and shape and no sister taxa have anything similar. The gracile jugal indicates the presences of a lateral temporal fenestra, as in mesosaurus. These are the problems working with line drawings rather than fossils or photos. No giant teeth here, as in Mesosaurus.

The illustrated cheek region is much too large compared to all sister taxa, so it may represent the occiput, which it is the right size to be. The jugal looks similar to those in Stereosternum and Mesosaurus, so likewise probably framed a lateral temporal fenestra. Guesswork at this point.

Brazilosaurus referred specimen – Rossman (2002) 
published this image (Fig. 1) of a specimen he attributed to Brazilosaurus. While not too far off from each other phylogenetically, no one would ever confuse the two. They’re just too different. Note the scale bars. Those tells us the Rossman specimen is not just a short-rostrum juvenile. Overall it’s the same size but has a shorter/taller rostrum and larger orbit.

My re-interpretation of the Rossman bones is in color. The reinterpretation is closer to the interpretation of sister taxa. The parietal goes on top of the head, not posterior to the orbit, for instance. And an unidentified bone sticks up from the cranium and that bone has what appears to be half of a pineal / parietal opening. When I see hirez photos I’ll revisit this if necessary.

Figure 1. Specimen referred to Brazilosaurus by Rossman (2002, above), colorized with bones reidentified below. Although sharing many traits with Brazilosaurus (Fig. 2), it also shares many traits with Xinpusaurus suni (Fig. 3).

Figure 2. Specimen referred to Brazilosaurus by Rossman (2002, above), colorized with bones reidentified below. Although sharing many traits with Brazilosaurus (Fig. 2), it also shares many traits with Xinpusaurus suni (Fig. 3). Apparently this diapsid resealed its temporal fenestrae, but a good look at the fossil itself is needed to confirm.

The referred specimen is likewise known to me only as a drawing with flaws (Fig. 2). Again, I wish I had access to the original specimen, or to a good photo. Or to the post-crania. All attempts at contacting Rossman have failed. I understand he is in poor health.

Xinpusaurus suni (Fig. 3) is a basal thalattosaur (close to ichthyosaurs like Utatsusaurus) sharing many traits with the Rossman specimen. It has no upper temporal fenestra, but it has a large lateral temporal fenestra. So these fenestra appear to come and go, as they do in basal diapsids like Araeoscelis.

Figure 3. Xinpusaurus suni, a basal thalattosaur sharing many traits with the Rossman specimen.

Figure 3. Xinpusaurus suni, a basal thalattosaur sharing many traits with the Rossman specimen. This specimen suggests the parietal of Rossman might be the postorbital. Here the septomaxilla are no longer indicated. The tiny bones above the nares are reduced nasals. It is important to see the original materials to solve these problems.

Adding Brazilosaurus to the large reptile tree nests it with Mesosaurus. Adding the Rossman specimen nests it basal to all thalattosaurs and therefore close to Xinpusaurus. Hupehsuchus + Utatsusaurus are outgroups.

So, one more mesosaur – thalattosaur connection.

References
Rossmann T 2002. Studien an Mesosauriern (Amniota inc. sed., Mesosauridae): 3. Neue Aspekte zur Anatmie, Erhaltung und Paläoökologie aufgrund der Exemplare im Paläontologischen Institut der Universität Zurich. Neues Jahrbuch fur Geologie und PaläontologieAbhandlungen 224, 197-221.
Shikama 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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