AN EXPERIMENTAL AND FIELD-BASED APPROACH TO MICROVERTEBRATE BONEBED TAPHONOMY IN THE JUDITH RIVER FORMATION OF NORTH-CENTRAL MONTANA

The Campanian Judith River Formation of north-central Montana preserves abundant concentrations of small compact bones and bone fragments, teeth, and scales of aquatic and terrestrial vertebrates, including dinosaurs, crocodiles, turtles, mammals, amphibians, and fish. These types of fossil localities, which are commonly referred to as “microvertebrate bonebeds” or “microsites,” are often used to reconstruct aspects of vertebrate paleoecology. However, their origins are in many cases relatively poorly understood. What physical, chemical, and/or biological factors promote the recurrent accumulation of small physicochemically resistant vertebrate elements, and what environments in the terrestrial and marine realms are most conducive to their preservation?

This study explores the origins of vertebrate microfossil bonebeds in the Judith River Formation using a combination of field and experimental data. Two types of fossil bone concentrations were studied in the field: low-energy pond/lake deposits (herein referred to as Type I deposits) and channel-lag deposits (herein referred to as Type II deposits). Fossils are generally dispersed throughout Type I deposits (which preserve <1% bone by volume), but are usually confined to discrete lenses and layers in Type II deposits (which can yield up to 3% bone by volume). Bones recovered from both types of locality are similar in shape, rounding, and surface features, but differ in degree of sorting (Type II concentrations are better sorted). In flume experiments, an active flow was directed through sediment beds with known background bone concentrations (0.1%, 1%, 10% by volume). Secondary concentrations formed in all three experiments. The coarse bone fraction was minimally transported, while the fine fraction was winnowed to distal parts of the flume. Preliminary analyses indicate Type II deposits may be derived from pre-existing Type I deposits based on: (1) overall similarity of fossil material, (2) trends in sorting (which are consistent with fluvial reworking), and (3) formation and downstream fining of secondary concentrations in flume experiments.