Lewisuchus admixtus Romer, 1972d

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 : 46

Lewisuchus admixtus Romer, 1972d

AGE: Ladinian, Middle Triassic ( Rogers et al., 2001).

OCCURRENCE: Chañares Formation, Argentina.

HOLOTYPE: UNLR 1, posterior portion of the skull, maxilla, dentary, articular (now apparently lost), cervical and dorsal vertebrae, scapulocoracoid, and humerus.

REMARKS: Lewisuchus was named by Romer (1972d) based on a partial skull and articulated anterior half of a skeleton. The posterior half of the skull, maxilla, dentary, and postcranium were found in the same nodule, but not articulated ( Romer, 1972d). Romer (1972d) argued that the cranial material and postcrania belong to the same individual based on agreement of size and the ‘‘thecodont nature’’ of the material. I agree with Romer’s argument and argue that none of the elements is duplicated in the specimen. The maxilla and dentary agree in size and the maxilla differs from those of any other archosauriform from the Chañares assemblage. Furthermore, the maxilla bears a large antorbital fossa that is present on the dorsal process of the maxilla, which is a character present only in archosaurs. The femur described by Romer (1972d) is actually a tibia as observed by Arcucci (1998). The size of the hind limb agrees with the rest of the specimen. Romer (1972d) illustrated and described the posterior portion of a mandible. At the time of this study, these elements seem to be lost.

Romer (1972d) considered Lewisuchus a pseudosuchian (at that time, Pseudosuchia was a wastebasket taxon) and made comparisons to ‘‘coelurosaurs’’ (1972 usage), Hesperosuchus , and ‘‘ Teleocrater ’’ (a taxon never formally described). Little else was said about the taxon until Parrish (1993) included it in his phylogeny of pseudosuchians. Parrish found it more closely related to crocodylomorphs than to ‘‘prestosuchids.’’ This result was a direct result of Parrish (1993) having scored a ‘‘crocodile-normal’’ astragalus for Lewisuchus . Arcucci (1998) declared that this astragalus belongs to a much smaller proterochampsian, and her assessment is followed here. Moreover, Arcucci (1997, 1998) stated that Pseudolagosuchus and Lewisuchus are the same taxon (see below). Hutchinson (2001a) accepted that the two were synonymous.

KEY REFERENCES: Romer, 1972d; Parrish, 1993; Arcucci, 1997, 1998.