Belgrade teeth of Psysogaleus sp. show transition from primitive to more modern species. The Eocene version of this shark is Physogaleus secundus; the Miocene version is Physogaleus contortus. P. secundus is usually finely but incompletely serrated and is about half the size of P. contortus. Specimens from the Belgrade formation may be completely or incompletely serrated. While larger than P. secundus, they are smaller than typical P. contortus. This illustrates the difficulty of pinning a species name to a given specimen, particularly when it represents a species in transition.

Fig. 1 Physogaleus teeth Belgrade Formation: Late Oligocene, North Carolina