A NEW MICROVERTEBRATE ASSEMBLAGE FROM THE WILLIAMS FORK FORMATION (CAMPANIAN - MAASTRICHTIAN) OF COLORADO AND EVIDENCE FOR DISCRETE CLIMATE/FAUNAL ZONES ACROSS LARAMIDIA DURING THE ‘EDMONTONIAN’

The Williams Fork Formation (WFF; Mesaverde Group) of northwestern Colorado’s Douglas Creek Arch preserves an understudied yet diverse terrestrial and aquatic biota. The ReBecca’s Hollow locality has yielded a diverse actinopterygian-dominated fauna from a freshwater deposit. Vertebrates from this site include the hydodontid cf. Meristodonoides, bony fish like Beleonostomus, cf. Melvius, Cyclurus, Atractosteus, Paralbula casei, cf. Coriops, Estesesox foxi, “Palaeolabrus montanensis”, acipenserids(?), otophysans, and hiodontids. Amphibians from the site include albanerpetontids, scapherpetontids, Prodesmodon, Opisthotriton, Theatonius, and a toothed frog. Diapsids are represented by anguids, helodermatids, serpentes, baenids, chelydrids, indeterminate trionychids, plastomenines, Basilemys, Adocus, alligatoroids, Paronychodon, cf. Saurornitholestes, cf. Richardoestesia, tyrannosaurids, hadrosaurids, thescelosaurids, and ceratopsids. This locality yields over 37 fish and tetrapod taxa, many of which have yet to be described from the WFF. The WFF is temporally correlative with the lower Horseshoe Canyon Formation of Alberta and the Prince Creek Formation of Alaska. This assemblage is markedly different from its northern contemporaries, with the presence of warm-climate taxa such as cf. Melvius, Atractosteus, Paralbula, Theatonius, squamates, turtles, and alligatoroids, and the absence of higher latitude ‘Edmontonian’ taxa like Holostean A. Many of the vertebrates from ReBecca’s Hollow are known from Judithian localities in UT and NM, and Lancian localities in MT and WY. This discrepancy in faunal composition implies that Laramidian actinopterygians, lissamphibians, and non-dinosaurian reptiles experienced a high degree of endemism at the Campanian-Maastrichtian transition. Additionally, the Lancian affinities of the WFF Fauna suggest that many key non-mammalian species from well-known Hell Creek and Lance Formation deposits originated in the Southwestern US.