Analysis of hadrosaur teeth from Egg Mountain Quarry, a diffuse microsite locality, upper Cretaceous, Two Medicine Formation, northwest Montana

Abstract

Egg Mountain locality as part of the Willow Creek Anticline in the Upper Cretaceous Two Medicine Formation of northwest Montana has become well known for its preservation of dinosaur eggs, eggshell, nesting traces, insect traces and more recently described mammals and lizards. The diffuse micro-vertebrate locality also turns up an abundance of hadrosaur and theropod teeth. Population dynamics of dinosaurian taxa are known though analysis of long bone material and histology. Here we test the utility of hadrosaur teeth for the same means. Field excavations of a new Egg Mountain Quarry from 2010-2015 resulted in the collection of 564 hadrosaur tooth fragments presumed to belong to Maiasaura peeblesorum in addition to a variety of theropod teeth. The most complete Maiasaura teeth were measured and compared to Maiasaura specimens from museum collections. A subset of the Egg Mt. assemblage was subjected to histological analysis as means to understand tooth formation and shedding rates. Unique factors were developed and applied to the assemblage translating tooth abundance into an abundance of individuals and ultimately mortality and survivorship. Analysis of museum collections and Egg Mt. hadrosaur teeth revealed high proportions of juvenile individuals as part of the abraded and largely incomplete Egg Mt. specimens. High mortality rates occur within the first year of life followed by a marked decrease until adult age and size is attained. Seasonal nesting grounds and a seasonal and semi-arid environment proposed for the Egg Mt. area is supported by high proportions of young individuals. Individuals gathering at ephemeral ponds and lakes, including identified theropod taxa, promote a concentration of shed and disarticulated teeth in conjunction with autochthonous mammal, lizard, and dinosaur nesting material. Ontogenetic changes in hadrosaur teeth and the conditions of the Egg Mt. assemblage complicate what an individual tooth represents; however, shed and disarticulated hadrosaur teeth appear to show effectiveness in paleoecological population studies. Continued work on abundant dinosaur tooth assemblages would hone these new techniques and potentially provide insight into similar microsite assemblages around the world.