A DINOSAUR-DOMINATED ICHNOFAUNA FROM THE LOWER CRETACEOUS (APTIAN) PATUXENT FORMATION OF VIRGINIA

Lower Cretaceous (Aptian) Patuxent-equivalent strata within the Potomac Formation of Virginia are well known for their abundant land-plant fossils. In sharp contrast, the only vertebrate fossil reported from these beds prior to 1995 was an impression of the posterior portion of a single fish (cf. Paraelops sp.). This paucity of vertebrate remains also contrasts with the record of moderately abundant and diverse vertebrate body fossils and footprints that have been found in the Patuxent Formation in Maryland. Recently, this disparity in abundance has been partially offset by the discovery of abundant fossil footprints at several localities in the vicinity of Fredericksburg, Virginia. These footprints have added eleven previously unrecognized animals to the vertebrate fauna of the Virginia Patuxent Formation: a frog, a turtle, a crocodile, and eight kinds of dinosaurs. The dinosaur footprints represent an ornithomimid (cf. Megalosauropus), two kinds of sauropods (Brontopodus and possibly a new ichnogenus), an ankylosaur (Tetrapodosaurus), a hypsilophodontid (new ichnogenus), two kinds of iguanodontids (Caririchnium and Gypsichnites), and a primitive hadrosaurid (Amblydactylus). One of the footprints was made by a sauropod estimated to have been about seventy feet in length, which makes it by far the largest dinosaur known from Virginia. Most of the dinosaur footprints appear to have been made by animals similar or identical to those known elsewhere from the Aptian of North America. The Tetrapodosaurus tracks, however, are distinctive and were made by an ankylosaur different from any of the forms known from western North America. This Virginia dinosaur assemblage so far includes about half the number of species that typically are found in the better known Early Cretaceous dinosaur communities of North America, including Maryland. Dinosaurs dominate the Fredericksburg footprint assemblages because the sediments in that area mostly consist of coarse-grained sandstones that were not suited to preserving footprints of small animals. The newly discovered footprints clearly show that vertebrate life was diverse and abundant in Virginia during the Early Cretaceous, even though osteological traces of that fauna are exceedingly rare.