OCCURRENCE AND SYSTEMATICS OF THE EOCENE ALLIGATORID PROCAIMANOIDEA

New specimens of the small Eocene alligator Procaimanoidea have been recovered from the Wasatch and Bridger formations of the northern Green River Basin of Wyoming, allowing for a reexamination of the geologic range, paleoecology, and systematic position of this poorly understood taxon.

Two species of Procaimanoidea have been reported previously: P. kayi from the early Bridgerian (Br1b); and P. utahensis from the Uintan (Ui3). The validity of and relationship between these species and the systematic position of Procaimanoidea within alligatorids is not well known. This study analyzes the previously described specimens and new material collected from the latest Wasatchian (Wa7, Lostcabinian) through earliest Bridgerian (Br1a, Gardnerbuttean).

Procaimanoidea is distinct from closely related alligatorids by at least four autapomorphies of the skull alone. Skulls of the two Procaimanoidea forms show uniform differences in their boney characteristics. The most dramatic of these are that P. kayi is characterized by having a frontal with a broad interorbital plate and a jugal that vertically thins posteriorly, while P. utahensis has a frontal with a narrow interorbital plate and a vertically subequal jugal. The most likely interpretation of these differences is that the two Procaimanoidea forms represent distinct species. Specimens from this study show that the two species overlap temporally, so the differences cannot be ascribed to within-lineage evolution, and since non-size related sexual dimorphism is rare in crocodylians, it is improbable that the difference represent males and females of the same species. Since all specimens are clearly assignable to one of the two forms, it is unlikely that the differences represent variation within a single species. Finally, the two morphologies can be identified regardless of the size of the individual, eliminating ontogentic change as an explanation.

Procaimanoidea kayi is now known to range from the Lostcabinian (Wa7) through the early Blacksforkian (Br1b) and P. utahensis from the Lostcabinian through the Uintan (Ui3). All specimens recovered from lacustrine facies of the Green River Formation represent P. utahensis, while all those from fluvial deposits are referable to P. kayi, suggesting that the two species preferred different aquatic habitats.