Smith_merrill_2006_main

Facial variation in Coelophysis bauri and the status of Megapnosaurus (Syntarsus)

Suggests that variations in the facial anatomy of Coelophysis may represent differences between males and females.

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Abstract

A considerable amount of variation is observed within the theropod sample from the Ghost Ranch Quarry within the Upper Triassic upper Petrified Forest Member of the Chinle Formation in north-central New Mexico. The meaning of this variation was unclear, but has been generally considered a result of ontogenetic, sexual, or systematic differences. Historically, these specimens have been referred to a single species, Coelophysis bauri. Twenty-three Coelophysis bauri skulls from Ghost Ranch and one of Megapnosaurus (Syntarsus) kayentakatae, from the Lower Jurassic Kayenta Formation of northern Arizona, were measured to create a morphometric database in order to conduct statistical analyses. Megapnosaurus (Syntarsus) kayentakatae data were included in the C. bauri matrix for comparative purposes. The systematic position of the Kayenta material is in flux and may change in the future (R. Tykoski, personal commun., 2005). Despite the considerable taphonomic distortion, a meaningful biological signal is present in the multivariate data. PC I is interpreted as a size vector. PC II is a contrast between the height and length of the skull. PC III is a contrast between the maxilla and premaxilla. The results indicate that a single variable species is present at Ghost Ranch that is distinct at the generic level from Megapnosaurus (Syntarsus) kayentakatae. Two morphs were identified within the Ghost Ranch sample. They might be regarded as male and female, but a specific designation is not warranted.