Introducing Sierritasuchus macalpini

The new issue of the Journal of Vertebrate Paleontology [28(3)] contains a new paper by me (and coauthors Michelle Stocker and Randy Irmis) describing a new taxon of aetosaur. If you have never heard of aetosaurs they are heavily armored, probably omnivorous, crocodile-line archosaurs that are extremely common fossils in Late Triassic terrestrial deposits. Aetosaurs are characterized by their armor, which consists of row after row of rectangular armor plates (osteoderms). Interestingly the dorsal (upper) surface of these plates is ornamented and this armor pattern is diagnostic of taxa. Even more informative is the morphology of what are termed lateral plates, or plates protecting the flanks of the animal. All aetosaurs can roughly be divided into three groups (clades) based on lateral plate morphology (Parker, 2007).

This is the fourth (and last) paper in a series which reanalyzes the genus Desmatosuchus (see also Parker, 2005, 2007, 2008). In 2000 I was studying the collections at the University of Michigan Museum of Paleontology (UMMP) when I came across the partial skeleton of small aetosaur that had been assigned to the genus Desmatosuchus. This specimen immediately caught my attention because although it represented a Desmatosuchine, it was quite different from Desmatosuchus (the holotype was on display upstairs). However, this specimen was was a bit of an enigma because of its incompleteness, crushing (it was also overprepared), and the fact that it appeared to be a juvenile. I gave a presentation on this specimen back in 2001 at the Western Association of Vertebrate Paleontologist’s meeting where I concluded that it either represented a new taxon or was possibly a juvenile form of Longosuchus (another Desmatosuchine) (Parker, 2001). Despite moving on to other projects, this fossil was always in the back of my mind and in 2004 enlisted Michelle Stocker to work with me on the description. The specimen was distinct enough from both Desmatosuchus and Longosuchus to warrant erection as a new taxon; however, we were still bothered by the small size of the specimen because very little is known regarding ontogentic changes to aetosaur armor morphology. Then two things happened; 1) we discovered juvenile Typothorax (another aetosaur) material at Petrified Forest National Park that clearly showed that little if any change occurred through ontogeny; 2) Bill Mueller at Texas Tech University turned us on to a lateral plate from a larger individual that was clearly referable to the same taxon, and possessed the same characters as the UMMP specimen. Still, we needed to be sure so we recruited Randy Irmis to conduct histological work to determine an ontogentic stage for the material. Randy was able to conclude that although the specimen was not fully grown, it was not exactly a very young juvenile either. Thus we felt confident enough to complete the study and erect a new taxon.

This is the first published study that attempts ontogentic stage determination in an aetosaur using the histology of osteoderms, and we hope that this will become a very important tool for future studies. For those who are wondering, the name Sierritasuchus is from Sierrita de la Cruz Creek near where the specimen was found. The species name S. macalpini honors the late Archie MacAlpin who collected the specimen in 1939. MacAlpin was a student of Ermine Cowles Case who published much on the Late Triassic of Texas. MacAlpin was later a geology professor at the University of Notre Dame. Sierritasuchus is currently only known from two specimens from the Tecovas Formation (Dockum Group) of Texas. It differs from both Desmatosuchus and Longosuchus (the two best known desmatosuchines) by various characters of the osteoderms and vertebrae. The picture above shows the majority of the holotype material (UMMP V60817). The reconstruction for this post was generously completed by Jeff Martz.

REFERENCES

Parker, W.G. 2001. An enigmatic aetosaur specimen from the Upper Triassic Dockum Formation of Texas. Western Association of Vertebrate Paleontologists with Mesa Southwest Museum and Southwest Paleontological Society Abstracts 2001:23.

Parker, W.G. 2005. A new species of the Late Triassic aetosaur Desmatosuchus (Archosauria: Pseudosuchia). Compte Rendus Palevol 4:327-340.

Parker, W.G. 2007. Reassessment of the aetosaur “Desmatosuchus” chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology 5:41-68.

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.

Parker, W.G., Stocker, M.R., and R.B. Irmis. 2008. A new desmatosuchine aetosaur (Archosauria: Suchia) from the Upper Triassic Tecovas Formation (Dockum Group) of Texas. Journal of Vertebrate Paleontology 28:692-701.