Technical_article

Variation in Coelophysis bauri

Discusses variation in the bones of Coelophysis from Ghost Ranch and its significance for interpreting growth, sexual dimorphism and taxonomy.

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Abstract

The large series of skeletons and partial skeletons of Coelophysis bauri collected from the Upper Triassic Chinle Formation at Ghost Ranch, New Mexico, affords an opportunity to study the ontogeny, individual variation, and possible sexual dimorphism in this species. Although truly small specimens have not been found in the deposit, it is possible to follow an incomplete growth series in this dinosaurian species involving a threefold increase in linear dimensions. By working back from this partial sequence, it is estimated that there was probably between a ten to fifteenfold increase in size from hatchling to adult, during which there were proportional changes in the size of the orbit, length of the neck, and length of the hind limb and some changes in skull proportions. Substansive differences are to be seen in individual variations, which are especially marked between two adult specimens of approximately the same size. One of these specimens has a larger skull, a longer neck, and a smaller forelimb than the other, and is further differentiated by fusion of the sacral vertebrae, not seen in its counterpart. These differences may be an expression of sexual dimorphism. A variation of fusion within the skeleton is especially marked in the ankle of Coelophysis, particularly as seen in the size and solidity of the ascending process of the astragalocalcaneum, the fusion of this bone to the tibia, and the fusion of the second and third distal tarsals to their respective metatarsals. Fusion or the lack of fusion in the ankle seems to be independent of size, and seemingly is a mark of individual variation.