Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper Cretaceous: Maastrichtian), northwestern Wyoming

Abstract

The Harebell Formation is a syntectonic sequence of conglomeratic sediments deposited in a narrow, rapidly subsiding trough that formed in the latest Cretaceous along the eastern margin of the ancestral uplift of what are today the Teton and Gros Ventre Mountains of northwestern Wyoming. On at least two occasions subsidence temporarily exceeded the rate of sediment supply and the area was flooded by a brackish or marine incursion from the Western Interior Seaway that lay to the east. The age of the Harebell Formation is Maastrichtian, corroborated by 4 0K/ 4 0Ar isotopic ages, vertebrate and palynomorph biostratigraphy, and a preliminary magnetostratigraphic analysis which correlates it to the geomagnetic reversal time scale from the upper part of C31R to the base of C30N. Sandstone slabs collected from the lower Whetstone Falls Member contain nine partial and complete footprints attributable to a theropod (Dinosauria: Saurischia). The footprints were formed as surface tracks in the tabular-bedded sandstone by dinosaurs that roamed the burrowed and leaf-littered sand flats and shallow waters along the margins of a low-energy, brackish-water embayment. Eight of the nine footprints represent a hitherto unknown ichnogenus, representing a four-toed pedal morphology for a theropod dinosaur which is unprecedented in the Late Cretaceous. The theropod nature of the tracks is implied by the length and narrowness of the digits and the sharp claw impressions. The tracks have clearly defined impressions of four toes, none of which appears to be a hallux in the traditional theropod sense of a small, retroverted hallux. The metapodial impression is also unlike that of other known theropod tracks: greater in relief than the digits but quite small in area. The tracks represent at least two individuals, although no clear trackways are available. Exallopus lovei, gen. et sp. nov., represents a type of theropod not currently recognized from body fossils.