Chindesaurus bryansmalli

Long et Murry 1995

Evidence: a few partial skeletons and isolated bones.

Carnian to Norian

Bull Canyon, Chinle, Petrified Forest, and Tecovas Formations

Arizona, New Mexico, and Texas; USA

Biology: 2.4 meters long – 15 kilograms

It is possible that at least one adult Chindesaurus stayed with its young in a small family group, if a larger femur associated with smaller remains can be considered parental (Downs 2005).

Evolution

Traditionally, Chindesaurus was considered most similar to herrerasaurids (Long et Murry 1995). However, because of its broad range of features similar to Eoraptor, herrerasaurids, and guaibasaurids, Chindesaurus cannot be confidently placed any more specifically than Saurischia (Langer 2004) and new remains support this conclusion (Irmis et al. 2007). Doubt has been cast on this species status as a herrerasaurid because all characteristics thought to define that group are common to most dinosaurs (Parker et Irmis 2005). It is sometimes considered distinctly outside the Herrerasauridae (Langer 2004 and Parker et al. 2006). Adding more uncertainty to the status of Chindesaurus, many of the specimens potentially assigned to this species have been pulled into question (Heckert et Lucas 1998). Sues et al. (2011) noted that Chindesaurus is somewhat similar but distinct from newly described basal saurischians such as Tawa and Daimonosaurus (Sues et al. 2011). However, those authors still placed it in the Herrerasauridae, albeit as the most advanced member (Sues et al. 2011). Superficially, Chindesaurus certainly resembles the Herrerasauridae and, as Bettencourt et Kellner (2009) suggested, Chindesaurus may be the northern representatives of the Herrerasauridae (Bettencourt et Kellner 2009). Because of similarities to Eoraptor, Guaibasauridae, and Herrerasauridae, it is almost definitely part of that early basal saurischian group. Its overall similarity to herrerasaurids prompts me to place it within that family.

Originally considered to belong to Chindesaurus, a partial ilium was reassigned its own genus and species, Caseosaurus crosbyensis (Hunt et al. 1998). However, no family was assigned to the bone. It shares many characteristics with a variety of basal dinosaurs including ornithischians, herrerasaurids, guaibasaurids, and protodinosaurs (Langer 2003) but is usually considered synonymous with Chindesaurus because both species share certain key features (Langer 2004). In the past its status as a herrerasaurid has been generally accepted (Heckert et Lucas 1999 and Heckert et Lucas 1998). According to more recent research, it seems especially similar to Staurikosaurus and herrerasaurids (Grillo et Azevedo 2011) but Nesbitt et al. (2007) also pointed out some similarities to Efraasia and Saturnalia. However, Chindesaurus lacks the portions of the ilium comparable to the Caseosaurus specimen so union of the two species is elusive. Additionally, the incomplete nature of the original Caseosaurus ilium obscures any features that might grant it a position as a dinosaur at all (Nesbitt et al. 2007). New discoveries from the Hayden Quarry of the famous Ghost Ranch in New Mexico have provided at least two more partial skeletons of Caseosaurus (Downs 2005). A fourth specimen, a single femur, from the same area of the Hayden Quarry may belong to a different dinosaur or, because it is much larger than the other specimens, it may be an adult. A possible second specimen, from the Snyder Quarry, may help resolve the issue (Nesbitt et al. 2007). This ilium, originally assigned to Eucoelophysis, is complete enough to encompass both Chindesaurus and Caseosaurus and is indistinguishable from both, strongly indicating the two animals are synonymous.

Chindesaurus appears to be a herrerasaurid with some guaibasaurid and Daemonosaurus/Tawa characteristics. Among the Herrerasauridae, it is most similar to Staurikosaurus.

References:

Bittencourt, J.D., A.W.A. Kellner. 2009. “The anatomy and phylogenetic position of Staurikosaurus pricei Colbert 1970.” Zootaxa 2079: 1-56.

Downs, A. 2005. “The Hayden Quarry, a new Upper Triassic fossil locality at Ghost Ranch, New Mexico.” New Mexico Geological Society, 56^th Field Conference Guidebook, Geology of the Chama Basin: 339-340.

Grillo, O.N., and S.A.K. Azevedo. 2011. “Pelvic and hind limb musculature of Staurikosaurus pricei (Dinosauria: Saurischia).” Anais da Academia Brasileira de Ciências 83(2): 73-98.

Heckert, A.B., and S.G. Lucas. 1998. “Global correlation and chronology of Triassic theropods (Archosauria: Dinosauria).” Gaia N^o 15: 63-74.

Heckert, A.B., and S.G. Lucas. 1999. “Global correlation and chronology of Triassic theropods (Archosauria: Dinosauria).” Albertiana 23: 22-35.

Hunt, A.P., S.G. Lucas, A.B. Heckert, R.M. Sullivan, and M.G. Lockley. 1998. “Late Triassic dinosaurs from the western United States.” Geobios 31(4): 511-531.

Irmis, R.B., S.J. Nesbitt, K. Padian, N.D. Smith, A.H. Turner, D. Woody, and A. Downs. 2007. “A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs.” Science 317: 358-361.

Langer, M.C. 2003. “The pelvic and hind limb anatomy of the stem-sauropodomorph Saturnalia tupiniquim (Late Triassic, Brazil).” PaleoBios 23(2): 1-30.

Langer, M.C. 2007. “Basal Saurischia.” In: Weishampel, D.B., P. Dobson, H. Osmólska (eds). The Dinosauria: Second Edition. Berkley, CA: University of California Press. 25-46.

Long, R.A., and P.A. Murry. 1995. “Late Triassic (Carnian and Norian) tetrapods from the Southwestern United States.” New Mexico Museum of Natural History and Science Bulletin 4.

Nesbitt, S. J., R. B. Irmis, et W. G. Parker. 2007. “A critical re-evaluation of the Late Triassic dinosaur taxa of North America.” Journal of Systematic Palaeontology 5(2): 209-243.

           

Parker, W.G., and R.B. Irmis. 2005. “Advances in Late Triassic vertebrate paleontology based on new material from Petrified Forest National Park, Arizona.” In: Heckert, A.B., and S.G. Lucas. Vertebrate Paleontology in Arizona. New Mexico Museum of Natural History and Science Bulletin 29: 45-58.

Parker, W.G., R.B. Irmis, and S.J. Nesbitt. 2006. “Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona.” In: Parker, W.G., S.R. Ash, and R.B. Irmis (eds). A Century of Research at Petrified Forest National Park: Geology and Paleontology. Museum of Northern Arizona Bulletin 62: 160-161.

Sues, H., S.J. Nesbitt, D.S. Berman, and A.C. Henrici. 2011. “A late-surviving basal theropod dinosaur from the Late Triassic of North America.” Proceedings of the Royal Society B. doi:10.1098/rspb.2011.0410.