Hesperosuchus

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 : 39-40

Hesperosuchus ‘‘ agilis ’’ Clark et al., 2000

AGE: Late Norian–?Rhaetian, Late Triassic ( Heckert et al., 2008).

OCCURRENCE: Coelophysis Quarry , ‘‘siltstone member’’ of the Chinle Formation, Ghost Ranch, northern New Mexico.

REFERRED MATERIAL: CM 29894, skull and anterior portion of the skeleton; YPM 41198, partially disarticulated skull, pubis, hind limb .

REMARKS: Clark et al. (2000) described a well-prepared skull and partial skeleton from the Coelophysis Quarry at the top of the Chinle Formation and referred the specimen to Hesperosuchus agilis . However, the holotype of Hesperosuchus , from the Blue Mesa Member, near the base of the Chinle Formation and the specimen from the Coelophysis Quarry , from the top of the Chinle sequence, may be separated by as much as 20 million years. Clark et al. (2000) used the following two characters to refer CM 29894 to Hesperosuchus agilis : (1) deep anterior end of the dentary; and (2) the configuration of the maxillary tooth row with a rapid increase in size of the anterior teeth from the small, slender first to the very large fourth tooth. The first character is also in Postosuchus kirkpatricki (TTU-P 9000) and Polonosuchus (ZPAL Ab III/543), and I see little difference between the anterior portions of the dentaries of CM 29894, Dromicosuchus (UNC 15574), and Sphenosuchus (SAM 3014). The second character does not seem to be unique among suchians. Therefore, CM 29894 cannot be unambiguously assigned to Hesperosuchus agilis . CM 29894 and another identical crocodylomorph skull and partial skeleton, YPM 41198, are treated as a separate terminal taxon. Although there are no apparent differences in the holotype of Hesperosuchus and CM 29894, no unique characters link the two taxa exclusive of other crocodylomorphs. Therefore, they are treated as separate taxa.

KEY REFERENCES: Clark et al., 2000.