New Mexico Geological Society Annual Spring Meeting — Abstracts

Based on newly prepared material from the Upper Triassic (Revueltian) Snyder Quarry in north-central New Mexico in the collection of the New Mexico Museum of Natural History (NMMNH) we attempt to model the shape of the carapace of Rioarribasuchus chamensis. We believe this animal was approximately 640 mm wide at its broadest point. The well preserved left medial plate (NMMNH P-33820) used in this reconstruction is approximately 1.2 times the length and 1.55 times the width of that of the largest scute of the previous reconstruction of Desmatosuchus haplocerus (Long and Murry, 1995, NMMNH C-2433). The Desmatosuchus reconstruction is approximately 330 cm long; multiplying this by the ratio of the lengths of Rioarrabasuchus scutes to those of Desmatosuchus we estimate  approximately 390 cm for the length of R. chamensis. As a result we conclude that Rioarribasuchus was a substantially larger and thicker bodied animal than its relative Desmatosuchus. By comparison with D. haplocerus we infer that R. chamensis had approximately 23 rows of dermal armor between the end of the cervical plates and the beginning of the caudal plates. The collection of the NMMNH contains parts of seven rows totaling nine medial plates. This leads to the conclusion that many of the medial plates of R. chamensis remain unknown. We would expect an equivalent number of lateral plates, but at this point we only have six of R. chamensis in the NMMNH collection. Also of note is the fact that no large shoulder spikes have been found at the Snyder Quarry other than one fragmentary spike only 9 cm long with an estimated 2 cm missing at the tip.  Conversely, Desmatosuchus spikes measures up to 22.5 cm in NMMNH C-2433. A previous reconstruction of the carapace of R. chamaensis was attempted by Parker (2007) but shows several questionable features; the most visible of these is the forward facing spikes on the medial plates. These should be facing caudally based on the imbrication of the medial plates. This change moves the spikes on these plates to the outside edge of the carapace rather than along the midline. Further, several of the plates referred to in Parker (2007) as anterior caudal lateral plates display signs of imbrication and widening that would place them in the cervical region of the carapace. Another issue is the line of dorsal spines running the length of the tail. We see no evidence of this in the fossils in the NMMNH collection.
 

References:

1. Long, R. A. and Murry, P. A., 1995, Late Triassic (Carnian and Norian) tetrapods from the southwestern United States: New Mexico Museum of Natural History and Science, Bulletin 4, 91 p.
    
2. Parker, W. G., 2007, Reassessment of the aetosaur ‘Desmatosuchus’ chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia): Journal of Systematic Paleontology, v.e 5, p. 41-68