Taphonomy of Jack's Birthday Site, a diverse dinosaur bonebed from the Upper Cretaceous Two Medicine Formation of Montana

Abstract

Jack's Birthday Site is a diverse vertebrate assemblage from the Upper Two Medicine Formation of western Montana. Age is roughly 74 Ma. The site covers roughly 3000 m 2 , and excavations over 140 m 2 . A large bone sample (> 1600 skeletal elements) allowed statistical evaluation of the preservational and compositional variation within the site. Evidence, including sedimentary facies, 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. This variation represents a transition from lake through shoreline to marginal shoreline/floodplain environments. Containing ten dinosaur taxa and a variety of non-dinosaurs, Jack's Birthday Site provides one of the best single faunal samples of the region. The vertebrate assemblage includes two taphonomic fractions. The first, consisting of attritional, predominantly isolated and allochthonous elements, represents a time-averaged assemblage. The other consists of associated, parautochthonous remains restricted to a single horizon. Taxa represented by associated remains include three iguanodontoids, Hypacrosaurus, Prosaurolophus , and Gryposaurus , and the theropod Troodon . Associated individuals of these taxa have non-random distributions within the site (Fisher's Exact Test, p < 0.01) and observed taxonomic clustering may reflect group behavior and/or event mortality. The four or more Troodon represent the first described multi-individual troodontid occurrence. The diversity and spatial complexity of the parautochthonous fraction of the Birthday Site assemblage is difficult to explain. Discussion of possible event mortality mechanisms focuses on three: drought, botulism and cyanobacterial toxicosis. These share an ability to act over an ecologically significant period of time, affect a variety of species and concentrate mortality along water sources, aspects that may have been important in generating the Birthday Site assemblage. Evidence at the site favors drought or perhaps a drought/botulism hypothesis. However, taxonomic clusters could represent completely separate events with a variety of causes.