THE STRATIGRAPHY OF VERTEBRATE SKELETAL CONCENTRATIONS: INSIGHTS GAINED FROM THE UPPER CRETACEOUS OF MONTANA

The astounding variety and abundance of vertebrate skeletal concentrations in the Campanian Two Medicine (TMF) and Judith River (JRF) Formations of Montana provide an exceptional opportunity to explore an array of paleobiological and geological questions. For example, to what extent do bonebeds in their many expressions accurately record biological signals - exactly how does vertebrate paleoecology and behavior translate into bone-rich deposits? What are the genetic links between bonebed formation and sedimentary dynamics - can sedimentologic processes alone generate relative concentrations of vertebrate hardparts? And to what degree are bonebeds associated with significant stratigraphic intervals and surfaces, such as well-developed paleosols, marine flooding surfaces, and sequence boundaries?

Concentrations of bones and teeth in the TMF–JRF record show a striking trend in preservation that presumably reflects differential rates of subsidence and net sedimentation in a foreland basin setting. The proximal TMF is characterized by relatively higher rates of net sedimentation and "snapshot" taphonomy (brief time-averaging per assemblage is the norm), whereas the more distal JRF record exhibits abundant evidence of prolonged time-averaging. This renders the TMF record more suitable for the analysis of ecologic-scale phenomena and the JRF record relatively insensitive to short-term biologic or ecologic events.

Concentrations of bones and teeth in the TMF–JRF record also show a non-random distribution with regard to transgressive and regressive phases, with the vast majority of occurrences (41 of 49 that can be placed in section with confidence) situated in the transgressive record. This trend holds for both terrestrial and shallow marine strata and may reflect the accessibility of productive facies, contrasts in burial rates, and/or differential diagenesis.

Finally, the distribution of vertebrate skeletal concentrations in the TMF-JRF interval is poorly correlated with the inferred durations of erosional and omissional hiatuses. Instead, vertebrate skeletal concentrations associated with stratigraphic discontinuities tend to be patchy and closely track the abundance of fossil material in underlying and lateral facies.