Hesperosuchus agilis Colbert, 1952

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 38-39

Hesperosuchus agilis Colbert, 1952

AGE:?Early Norian, Late Triassic ( Lucas, 1998a).

OCCURRENCE: Blue Mesa Member, Chinle Formation near Cameron, Arizona ( Colbert, 1952).

HOLOTYPE: AMNH FR 6758, portions of the skull including the quadrate, maxillae, dentaries, portion of the premaxilla, part of the nasal, part of the jugal, part of the squamosal, partial braincase (opisthotic, basioccipital), cervical, dorsal, and caudal vertebrae, osteoderms, humerus, ulna, radius, partial radiale, parts of the manus, femora, tibiae, fibulae, partially articulated pes.

REMARKS: Colbert (1952) named Hesperosuchus for a partially eroded, articulated specimen from the base of the Chinle Formation that was collected by Barnum Brown. The well-preserved specimen is threedimensionally preserved, but many of the delicate elements are missing or unidentifiable. Colbert (1952) made a few errors in the identification of elements in his description, but Walker (1970) corrected these mistakes. For example, the ‘‘pterygoid’’ ( Colbert, 1952: fig. 9) is actually a sacral rib from the first primordial sacral. Bonaparte (1971) suggest- ed that there are two individuals in the holotype. However, there are no apparent duplications of any of the elements.

Most crocodylomorph-like bones and associated skeletons from the Chinle Formation and Dockum Group have been assigned to Hesperosuchus without specific justification ( Parrish, 1991; Long and Murry, 1995; Clark et al., 2000). The better preserved specimens from the Coelophysis Quarry from the top of Chinle Formation have been separated out as a separate terminal taxon (see below). Here, I score only the holotype for this terminal taxon.

Much of the skeleton of Hesperosuchus was eroded before Barnum Brown recovered it in the 1930s. Brown and the AMNH preparators screen-washed thousands of pounds of matrix and recovered bone fragments from the resultant concentrate. Hundreds of bone fragments, teeth, and pieces of the holotype of Hesperosuchus were separat- ed. Colbert’s (1952) description focused on the material recovered in situ and the obvious bones collected on the surface. However, some of the bones described by Colbert belong to a dinosauromorph (e.g., the elongated metatarsals, one of the humeri), the sacral vertebra belongs to Vancleavea ( Nesbitt et al., 2009a) , and other material collected at the locality represents the remains of fishes, phytosaurs, amphibians, or other archosaurs. Fortunately, the preservation of the holotype of Hesperosuchus is unique among the other bones; the weakly weathered or in situ bones are a dark chocolate brown and the weathered bones are orange to yellow, whereas the other fragments are blue, black, tan, or dark grey. Furthermore, the outer surfaces of the bones of the holotype of Hesperosuchus are exquisitely preserved. These two factors allow the fragments of Hesperosuchus to be separated. As a result, parts of the skull (squamosal, nasal), osteoderms, pelvis, and manus were recovered. Furthermore, the screen-washed material was distributed throughout the fossil vertebrate collections at AMNH. I found parts of the holotype of Hesperosuchus with the aetosaurs and parareptiles.

KEY REFERENCES: Colbert, 1952; Walker, 1970; Parrish, 1991; Clark et al., 2000.