Phytosaur Skull Preparation IV

It has been almost a year since I last posted on the preparation of this phytosaur skull from Petrified Forest National Park.  Worked has progressed during this time and most of the upper surface of the skull has been exposed.  As can be seen the skull is very nicely preserved and barely distorted.  Interestingly it appears to be fully crested (i.e., the premaxillary crest extends down the entire length of the snout) giving the snout a ramp-like profile.  We still need to clean off the rock at the back of the skull to make a better identification, but clearly this skull belongs to a pseudopalatine phytosaur.

Beautiful New Aetosaur Skull Material of Stagonolepis from Poland

This is absolutely beautiful material, which allows for a detailed description of the skull of the European genus Stagonolepis.  Sulej differentiates this material as a new taxon, S. olenkae.  This material is believed to from an equivalent of the Lehrberg beds of Germany, thus from the Middle Keuper (Carnian).  Thus it is older than material from the Southwest U. S. (Calyptosuchus wellesi) that has often been assigned to Stagonolepis on the basis on radial armor patterning. The same quarry that produced these skulls provided the holotype material of the non-dinosaurian dinosauriform Silesaurus opolensis. Be advised that the evolutionary hypothesis presented here for aetosaurs is based on hypothetical stratigraphic position (which can not be precisely determined globally) and not a phylogenetic analysis. However, although I don't agree with all of the details of this paper, the specimen is perfectly preserved and provides lots of new information, especially the skull roof and braincase shown below.

Sulej, T. 2010. The skull of an early Late Triassic aetosaur and the evolution of the stagonolepidid archosaurian reptiles. Zoological Journal of the Linnaean Society 158, 860–881.

Abstract - Disarticulated bones of several individuals recovered from the Late Triassic fluvial and lacustrine deposits at Krasiejów, Poland, are here described, allowing the restoration of the skull structure of a new aetosaurian archosaur: Stagonolepis olenkae sp. nov. The Krasiejów deposits probably correspond in age to the Lehrberg Beds (late Carnian) of Baden-Württemberg, Germany. The stratigraphical position of the new taxon combined with other available evidence is used to propose a model of aetosaurian evolution. The proposed phylogenetic position of Aetosaurus ferratus (Norian, Germany) as the basal aetosaurid is refuted and this species is instead proposed to be the most derived member of the Stagonolepis–Aetosaurus evolutionary lineage. Gradual change in several morphological characters can be observed from Stagonolepis robertsoni, through the new species from Krasiejów, to the stratigraphically youngest Aetosaurus ferratus. These changes include a decrease in the number of teeth and a decrease in the convexity of the ventral profile of the maxilla. The anterior elongation of the maxilla is associated with the expansion of the anterior tip of the maxilla towards the naris. In S. robertsoni and S. olenkae, the maxilla extends to middle of the naris, whereas in Aetosaurus, it reaches the anterior half of the naris.

The First Morphological Evidence for Turtles as Archosauromorphs

Bhullar, B.S., and G.S. Bever. 2009. An archosaur-like laterosphenoid in early turtles (Reptilia:Pantestudines). Breviora 518:1-11. doi: 10.3099/0006-9698-518.1.1

Abstract: Turtles are placed with increasing consistency by molecular phylogenetic studies within Diapsida as sister to Archosauria, but published gross morphology–based phylogenetic analyses do not recover this position. Here, we present a previously unrecognized unique morphological character offering support for this hypothesis: the presence in stem turtles of a laterosphenoid ossification identical to that in Archosauriformes. The laterosphenoid is a tripartite chondrocranial ossification, consisting of an ossified pila antotica, pila metoptica, and taenia medialis + planum supraseptale. It forms the anterior border of the exit for the trigeminal nerve (V) and partially encloses the exits for cranial nerves III, IV, and II. This ossification is unique to turtles and Archosauriformes within Vertebrata. It has been mistakenly dismissed as anatomically dissimilar in these two groups in the past, so we provide a complete description and detailed analysis of correspondence between turtles and Archosauriformes in each of its embryologically distinct components. A preliminary phylogenetic analysis suggests other potential synapomorphies of turtles and archosaurs, including a row or rows of mid-dorsal dermal ossifications.

Thanks to Randall Irmis for bringing this to my attention.

Late Triassic Mystery Fossil #1 is...

...the posterior portion of the right squamosal bone of a phytosaur. Because of the robust nature of these elements they are often preserved even when the rest of the skull is not. In fact, they are a very common fossil in the Chinle Formation. This is actually fortunate because the morphology of the posterior process of the squamosal is diagnostic and therefore the entire skull does not need to be recovered to determine the presence of a taxon. The specimen to the upper left belongs to Pseudopalatus pristinus and is characterized by being extremely thickened and knoblike (down in the photo is posterior), with many surfaces for muscle attachment. Phytosaur evolution is characterized by changes in the "post temporal" arcade, including depression and narrowing of the supratemporal fenestrae as well as a posterior elongation and mediolateral widening of the squamosal process. The common preservation and recovery of isolated squamosals in concert with their diagnostic nature make them excellent biostratigraphic index fossils and thus they should be collected whenever found. The picture below shows a complete upper portion of a skull of P. pristinus with the position of the squamosal fragment above (from a different individual) outlined.