THE ARCHOSAURIAN MORPHOSPECIES CONCEPT WITH APPLICATION TO THE GHOST RANCH POSTOSUCHUS

The amount of skeletal difference required to justify a new fossil vertebrate species is subjective and has therefore varied considerably among taxonomists. In order to address this question, the amount of skeletal difference historically regarded as valid in the erection of new dinosaurian species within a previously named genus has been quantified. The survey considers 28 dinosaurian genera and 68 species from both the Saurischia and Ornithischia. Calculation using the number of differences and the total number of bones in the dinosaurian skeleton yields an average skeletal variation of 2.2%. Considering 338 distinct ossifications, cogeneric species differ by approximately three skeletal differences. Fifty-eight percent of the skeletal distinctions occur in the skull.

The Ghost Ranch specimen of Postosuchus and Postosuchus kirkpatricki are virtually identical in most respects yet differ in certain skeletal features that may be variously attributed to sexual variation, ontogeny, teratology, or subspecific if not specific differences. With regards to the morphospecies calculations, the Ghost Ranch specimen is considered Postosuchus kirkpatricki despite a morphological difference in the ilium distinct from that of the paratype Postosuchus kirkpatricki as well as lack of bone sculpture attributed to a juvenile state.