Field of Science

Bone Histology of Phytosaur, Aetosaur, and Other Archosauriform Osteoderms

Just in time for Christmas...

Scheyer, T. M., Desojo, J. B., and I. A. Cerda. 2013. Bone histology of phytosaur, aetosaur, and other archosauriform osteoderms (Eureptilia, Archosauromorpha). Anatomical Record (early view) DOI: 10.1002/ar.22849

Abstract -
As in other archosauriforms, phytosaurs and aetosaurs are characterized by the presence of well-developed osteoderms. Here we provide a comparative study on the microstructure of phytosaur (five taxa) and aetosaur (thirteen taxa) osteoderms. For outgroup comparison, we sampled osteoderms of the sister taxon to Aetosauria, Revueltosaurus callenderi, and the doswelliid Jaxtasuchus salomoni. Phytosaur, aetosaur, and Jaxtasuchus osteoderms are composed of a diploe structure, whereas the Revueltosaurus osteoderm microanatomy is more compact. The external cortex of phytosaurs, Revueltosaurus and Jaxtasuchus osteoderms is mainly composed of parallel-fibered bone. In aetosaurs, the external cortex mainly consists of lamellar bone, with lines of resorption within the primary bone indicating successive cycles of bone erosion and deposition. The basal cortex in all the specimens is composed of parallel-fibered bone, with the cancellous internal core being more strongly developed in aetosaurs than in phytosaurs. Woven or fibro-lamellar bone was recorded in both phytosaurian and aetosaurian taxa, as well as in Jaxtasuchus. Structural fibers, which at least partly suggest metaplastic origin, were only recorded in the internal core of two phytosaurs and in the
basal cortex of one aetosaur. Osteoderm thickness and cancellous to compact bone ratios appear to be subject to ontogenetic change. Minimum growth mark counts in osteoderms sampled indicate that some aetosaurs and phytosaurs lived for at least two decades. Bone microstructures are more uniform in phytosaur osteoderms and show a higher level of disparity among aetosaur osteoderms, and at least in the latter, histological features are potentially apomorphic for species/genus level.

Standardizing Triassic Stratigraphic Nomenclature in New Mexico

This is a new paper written by a group of geologists who are largely responsible for conducting much of the current geological mapping in New Mexico, and is an attempt to standardize the nomenclature used for Phanerozoic rocks especially the Triassic. Key recommendations regarding the Triassic rocks are abandonment of the Chinle as a Group and keeping it at the formation level, removal of the Dockum from the "Chinle Group" and reinstatement as the Dockum Group as traditionally used, and a suggestion where to divide strata between the Chinle and Dockum.  Consideration of the Chinle as a group and subsuming the Dockum has been controversial and never fully accepted since it was first proposed in the early 1990s. Thus, this paper suggests abandonment of much of the nomenclature proposed by Spencer Lucas and colleagues over the last couple of decades.

Cather, S. M., Zeigler, K. E., Mack, G. H., and S. A. Kelley. 2013. Toward standardization of Phanerozoic stratigraphic nomenclature in New Mexico. Rocky Mountain Geology 48:101-124. doi:10.2113/gsrocky.48.2.101
 
Abstract - Nomenclature for Phanerozoic strata in New Mexico has been rapidly evolving, but not all proposed changes have been widely accepted. From a perspective of geologic mapping, we evaluate some recent nomenclatural proposals for Pennsylvanian, Permian, Triassic, Jurassic, and Paleogene units. Because of the long shelf-life of geologic quadrangle maps and the desirability of minimizing nomenclatural diversity among them, we present guidelines with which we argue for a conservative approach to changes in stratigraphic nomenclature.

Redescription of "Paleorhinus" (Phytosauria) Specimens from Germany

Butler, R. J., Rauhut, O. W. M., Stocker, M. R., and R. Bronowicz. 2013. Redescription of the phytosaurs Paleorhinus (‘Francosuchus’) angustifrons and Ebrachosuchus neukami from Germany, with implications for Late Triassic biochronology. Zoological Journal of the Linnean Society Early View. DOI: 10.1111/zoj.12094

 
Abstract - Phytosaurs are a diverse and morphologically distinctive clade of superficially crocodile-like archosauriforms that had a near global distribution during the Late Triassic. Because their remains are among the most abundant vertebrate remains recovered in many Upper Triassic terrestrial formations, phytosaurs are used extensively in long-range biochronological and biostratigraphic correlations. The biochronologically oldest and earliest branching known phytosaurs include an array of nominal species from the early Late Triassic of the United States, Germany, Poland, Morocco, and India that have been synonymized within the genus Paleorhinus, and subsequently used to define a global ‘Paleorhinus biochron’. However, recent phylogenetic work suggested that the North American species previously referred to Paleorhinus are paraphyletic. Here, we reassess the systematics and anatomy of putative specimens of Paleorhinus from southern Germany. Two well-preserved basal phytosaur skulls from the Blasensandstein (Carnian) of Bavaria form the holotypes of Francosuchus angustifrons and Ebrachosuchus neukami, both of which were synonymized with Paleorhinus by previous workers. We demonstrate that Francosuchus angustifrons shares unique synapomorphies with specimens referred to Paleorhinus bransoni from the Late Triassic of Texas, and thus refer the species to Paleorhinus. By contrast, the longirostrine Ebrachosuchus is highly distinctive in morphology, and our new cladistic analysis of Phytosauria demonstrates that it represents a valid taxon that is more closely related to Phytosauridae than to Paleorhinus. We provide the first autapomorphy-based support for a monophyletic but restricted Paleorhinus (supported by a nodal row on the jugal, and low paired ridges on the squamosal) and confirm that previous broader conceptions of Paleorhinus are likely to be paraphyletic.

Mammal-like Tooth from the Upper Triassic of Poland

 Świło, M., Niedźwiedzki, G., and T. Sulej. 2013. Mammal-like tooth from the Upper Triassic of Poland. Acta Palaeontologica Polonica Accepted Manuscript doi:http://dx.doi.org/10.4202/app.00016.2013.

Abstract - Triassic discoveries have extended the record of near-mammals (Mammaliaformes) back to the Norian, about 215 Ma, and reveal a significant diversity of Late Triassic (Norian-Rhaetian) forms. We now add to this Late Triassic diversity a nearly complete double-rooted right lower molariform tooth (ZPAL V.33/734) from the Polish Upper Triassic that is significant because it comes from uppermost Norian–lower Rhaetian rocks and is the first discovery of a mammal-like tooth in the Mesozoic of Poland. The described tooth shows transitional dental morphology between advanced cynodonts and mammaliaforms and it appears to represent a basal mammaliaform (Hallautherium genus), probably belonging to Morganucodonta.