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The Late Triassic Canjilon quarry (upper Chinle Group, New Mexico) phytosaur skulls: Evidence of sexual dimorphism in phytosaurs.

ASU Author/Contributor (non-ASU co-authors, if there are any, appear on document)
Andrew B. Heckert Ph.D., Assistant Professor of Geology (Creator)
Appalachian State University (ASU )
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Abstract: The Canjilon quarry, located in north-central New Mexico near Ghost Ranch, is a death assemblage of phytosaurs stratigraphically high in the Petrified Forest Formation of the Chinle Group (Revueltian = early-mid Norian). The site yields numerous fossils of Pseudopalatus-grade phytosaurs, with at least 11 skulls collected from the locality by Charles Camp in 1928 and 1933 and another one more recently by Ghost Ranch. A re-examination of these skulls reveals two morphotypes that differ only in the lengths and relative robustness of their premaxillae and septomaxillae. In these two morphotypes, the premaxillae define the shape and length of the rostral crest, the dimensions of which are independent of skull size. In one morphotype, the premaxillae are long, thin bones that lead to an abrupt, volcano-like narial crest. In the second morphotype, the premaxillae are of nearly the same length, but expand dorso-ventrally halfway along their lengths, creating a longer and more robust crest. The most parsimonious explanation of these two variants in rostral crest morphology in a single death assemblage is that Pseudopalatus-grade phytosaurs were sexually dimorphic. Thus, the larger, more robust crest of the first morphotype is a display feature, most likely of the male animal. The more gracile snout and crest characterize the female morpho In the sample of phytosaurs examined, there are three individuals of the more robust morphotype, six individuals that are the more gracile morphotype, and one juvenile that cannot be assigned to either morphotype because the skull anterior to the nares is missing. This is the first clear evidence of sexual dimorphism in phytosaurs, and has important implications for phytosaur species-level taxonomy.

Additional Information

Zeigler, K.E., Lucas, S.G., and Heckert, A.B., (2002) The Late Triassic Canjilon quarry (upper Chinle Group, New Mexico) phytosaur skulls: Evidence of sexual dimorphism in phytosaurs. New Mexico Museum of Natural History and Science Bulletin 21, p. 279-284. (ISSN 1524-4156) Archived in NC DOCKS with permission of the editor. The version of record is available at:
Language: English
Date: 2002