Sex in the Coelophysis

Just so everyone knows, thinking up fun titles is half the enjoyment I get out of writing these everyday; the other half is knowing that someone somewhere is learning or at least questioning what they know and critically thinking thanks to things I publish. Regardless, today I really wanted to mention that Coelophysis remains have exhibited evidence that suggests that these animals were sexually dimorphic. Discovering evidence of sexually dimorphic characters in fossil animals is always an intriguing and important part of being able to interpret potential interactions within populations. Certain sexually dimorphic characters can even lend themselves to interpretations of mating rituals; these are typically based on modern examples of similar characters in extant taxa. The difference in Coelophysis is not based on any interesting crests or additions to the skeletal frame. Instead, the dimorphism is suggested by differences in overall size of specimens; the forms are noted as being "gracile" and "robust". The two specimens initially used to describe these conditions are housed at the American Museum of Natural History and are AMNH 7223 (gracile) and AMNH 7224 (robust). As with many other sexually dimorphic species, the gracile form is thought to be female while the robust form is thought to represent male specimens. These forms are present in all ages of Coelophysis, meaning that the size difference is not only attributable to a difference in species and that sexually dimorphic characters begin to appear early in the life cycle. Gracile forms had longer skulls and necks, short forelimbs, and fused sacral vertebrae whereas robust forms had opposite characters in these areas. Some parental care has been noted in these animals and they have been considered gregarious due to the mass death assemblages at Ghost Ranch in New Mexico; though these may represent coincidental incidents in which food or water was limited and drew in large numbers of animals. Regardless, the sharing of food between adult and juvenile depicted in this image, is very neat and, though it does not show sexual dimorphism explicitly, we can assume that this is the female form with a baby and that they are gregarious animals here at least.

Denver Museum of Nature and Science