Expanding the Late Triassic Record of the Dinosaur Precursor Dromomeron romeri: A New Record from the Chinle Formation of Arizona.

A new open access paper from my co-worker, friend, and colleague Adam Marsh documenting a new record of the dinosaur precursor Dromomeron romeri from the Chinle Formation Arizona. This further demonstrates the importance of museum collections and apomorphy based identification work to identify stratigraphic, chronologic, and bibliographical extensions, improving our understanding of early dinosaur distributions.  

Marsh, A. D. 2018. A new record of Dromomeron romeri Irmis et al., 2007 (Lagerpetidae) from the Chinle Formation of Arizona, U.S.A. PaleoBios, 35. Retrieved from https://escholarship.org/uc/item/8w5755sg

Abstract - The relatively recent discovery and contextualization of silesaurid and lagerpetid dinosauromorphs has led to a revolution in understanding the early evolutionary history of the dinosaurian lineage. Lagerpetids are known from North America and South America in Middle and Upper Triassic rocks, especially the Chinle Formation of New Mexico and the Dockum Group of Texas. Until now, only a single specimen of Dromomeron gregorii was known from the Upper Triassic Chinle Formation of Arizona. However, a new lagerpetid astragalus specimen (MNA V7237) from the Owl Rock Member of the Chinle Formation found on Ward Terrace in the Navajo Nation of Arizona is referred to Dromomeron romeri. MNA V7237 represents the youngest radioisotopically-dated record of Lagerpetidae, indicating that D. romeri persisted throughout the entire Norian (Otischalkian into the Apachean) in North America.

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.

There are no Known Aetosaur Fossils from Madagascar!

I happened to click on the Wikipedia page for aetosaurs today. It is really shaping up as someone (or maybe a few people) is putting a lot of work into it. A few errors here and there and a couple taxonomic issues that will be addressed in some future publications (not all by me). However, the most glaring thing that caught my eye was location column for Desmatosuchus lists Madagascar (Isalo Group) as a unit containing fossils of Desmatosuchus. This is an occurrence I addressed in my M.S. thesis (Parker, 2003) and in my 2008 paper on the genus Desmatosuchus. It all stems from a problem in assigning a geological age to a fossil bearing horizon in the upper part of the Isalo II (part of the Isalo Group; Burmeister, 2000; Burmeister et al., 2006). This horizon includes fish fossils, as well as the remains of dinosaurs (sauropod, theropod) and other archosaurs, including teeth that are superficially similar to those of phytosaurs and a handful of osteoderms that belong to some type of pseudosuchian (see photo below from Burmeister, 2000).

Kurtis Burmeister first approached me in the late 1990s asking my opinion if these osteoderms could be from aetosaurs. I though the resemblance was purely superficial and despite he presence of an anterior bar and pitted ornamantation, the overall morphology and the lateral sutures were not typical of aetosaurs.  In his thesis, he suggested they could be crocodyliform (pers. comm. from Mike Parrish), but preliminarily assigned them to the Aetosauria and noted a possible assignment to Desmatosuchus. This identification was based on showing the osteoderms to another aetosaur "expert" (Burmeister, pers. comm.).

A few years later I was approached again by another member of the research team who showed me the specimens again.  This time I was with a small group of Triassic workers and coincidently we had a crocodyliform specialist with us as well.  We all agreed that they were definitely not Desmatosuchus, not aetosaurian, and most likely a crocodyliform. In a subsequent publication (Burmeister et al., 2006) they are refered to an indeterminate crocodylotarsian (pseudosuchian) and the superficial resemblance to aetosaurs is discussed, although the authors note the osteoderms probably represent a goniopholidid crocodyliform and that the horizon is probably Early Jurassic in age. Parker (2008) argued that they were not aetosaurian and possessed characters found in mesoeucrocodylians.

Despite all of this ambiguity, these specimens were explicitly assigned to the aetosaur Dematosuchus haplocerus by Lucas et al. (2003) and used to provide an Adamanian (late Carnian) age for these beds. This identification and correlation was followed by Lucas (2010).  Discussions with colleagues and the current Wikipedia entry for aetosaurs demonstrates that the identification of these specimens as aetosaurian is still misunderstood.  For the 4th time (and hopefully the last) I would like to propose my opinion (based on two personal observations of the material and the figure above) that these are not aetosaur plates and most certainly not referable to Desmatosuchus. The ornament is too deep and irregular, furhtermore, if these are fragments of paramedian plates then the ornament would be too large.  Finally, the sutural edges are completely different than anything found in aetosaurs, and certainly does not represent the "tongue-and-groove" articular surface found in desmatosuchines. There currently is no evidence for aetosaurs in the Isalo II and the age of the upper beds is most likely Jurassic (Burmeister et al., 2006; Parker, 2008) and not Adamanian in age (contra Lucas, 2010).

REFERENCES

Burmeister, K.C., 2000, Paleogeographic and biostratigraphic implications of new early Mesozoic terrestrial vertebrate fossils from the Poamay site: central Morondava Basin, Madagascar [M.A. thesis]: Santa Barbara, University of California, 109 p.

Burmeister, K.C., J.J. Flynn, J.M. Parrish, and A.R. Wyss. 2006. Paleogeographic and biostratigraphic implications of new early Mesozoic vertebrates from Poamay, central Morondava Basin, Madagascar. New Mexico Museum of Natural History Science Bulletin 37:457–475.

Lucas, S.G. 2010. The Triassic timescale based on nonmarine tetrapod biostratigraphy and biochronology; pp. 447-500 in  Lucas, S. G. (ed.) The Triassic Timescale. Geological Society, London, Special Publications, 334.

Lucas, S.G., K.E. Zeigler, A.B. Heckert, and A.P. Hunt. 2003. Upper Triassic stratigraphy and biostratigraphy, Chama Basin, north-central New Mexico. New Mexico Museum of Natural History & Science Bulletin 24:15–39.

Parker, W.G. 2003. Description of a new specimen of Desmatosuchus haplocerus from the Late Triassic of Northern Arizona. M.S. thesis. Northern Arizona University, Flagstaff, AZ.

Parker, W.G. 2008. Description of new material of the aetosaur Desmatosuchus spurensis (Archosauria: Suchia) from the Chinle Formation of Arizona and a revision of the genus Desmatosuchus. PaleoBios 28:1-40.