Jack's birthday site, a diverse dinosaur bonebed from the Cretaceous Two Medicine Formation of Montana

Abstract

Jack's Birthday Site, a diverse vertebrate assemblage from the Upper Cretaceous Two Medidne Formation of western Montana, was taphonomically investigated and compared with other predominantly iguanodontoid bonebeds from the area. The large bone sample at Jack's Birthday Site allowed statistical evaluation of the preservational and compositional variation within the site. Evidence, including sedimentary fades, plant and invertebrate fossils, and bone orientation and condition, indicates Jack's Birthday Site represents part of a small, shallow floodplain lake. Lithologies and fossil preservation vary from northwest to southeast over a distance of 50 m, representing a transition from lake through shoreline to marginal shoreline/floodplain environments. The vertebrate assemblage contains ten dinosaur taxa and a variety of non-dinosaurs and indudes two taphonomic fractions. The first, consisting of attritional, predominantly isolated and allochthonous elements, represents a time-averaged assemblage. The other consists of assodated, parautochthonous remains restricted to a single horizon. Taxa represented by associated remains indude three iguanodontoids, Hypacrosaurus, Prosaurolophus, and Gryposaurus, and the theropod Troodon. Associated individuals of these taxa have non-random distributions within the site and observed taxonomic dustering may reflect group behavior and/or event mortality. The four or more Troodon represent the first described multiindividual troodontid occurrence. Other predominantly iguanodontoid assemblages, like Jack's Birthday Site, are single highly concentrated bone horizons occurring in silty mudstones. Most are primarily parautochthonous with some degree of skeletal assodation and likely represent mass-mortality. The size-frequency profile of the Camposaur bonebed supports a catastrophic origin. Jack's Birthday Site differs in its diversity, the other localities being nearly monospecific, and its size-frequency profile for iguanodontoids which suggests strongly selective mortality and/or preservation. These plus the site's variable preservation indicate that Jack's Birthday Site is a much more time-averaged assemblage. Both hadrosaurids and lambeosaurids appear to have been gregarious. Lack of association between small (total length <3 m) and larger individuals suggests that juvenile growth rates may have been as rapid as large ungulates. Size-frequency profiles for Maiasaura peeblesorum suggest seasonally synchronous reproduction and high juvenile mortality.