AN INSIDERS VIEW OF THE VERTEBRATE FOSSIL RECORD: X-RAY COMPUTED TOMOGRAPHY OF VERTEBRATE MICROFOSSIL BONEBEDS FROM THE UPPER CRETACEOUS JUDITH RIVER FORMATION, MONTANA

Vertebrate microfossil bonebeds (VMBs) are localized concentrations of small, disarticulated, and often taxonomically diverse vertebrate hardparts (e.g., teeth, scales, scutes, vertebrae, unidentifiable bone sand and bone pebbles). They are commonly studied in Mesozoic and Cenozoic records to recover otherwise rarely found small-bodied taxa, and to document relative taxonomic abundance and species richness in ancient vertebrate communities. We are using X-ray Computed Tomography (XRCT) to explore the taphonomy of a suite of richly fossiliferous VMBs in the Coal Ridge Member of the Upper Cretaceous (Campanian) Judith River Formation of north-central Montana. Vertebrate bioclasts in Judith River Formation VMBs are often found co-mingled with abundant invertebrate shell debris and plant fragments. Fortuitously, the bioclastic fraction of the site under investigation consists almost exclusively of vertebrate skeletal debris set in a silty claystone matrix—this particular site lacks invertebrate shell debris and preserves only a small amount of carbonaceous plant debris, making it somewhat less complicated than most of the other VMBs in our collection. Using an XRCT approach we are able to quickly observe in-situ hundreds of mm-scale vertebrate bioclasts in their original spatial orientations. Samples are scanned using a 225 kV microfocus system at the University of Minnesota XRCT lab, with 3-D volumes analyzed using Avizo Fire. This method allows us to directly observe the distribution, sorting, and orientation of bioclasts, along with other taphonomic attributes (such as breakage and potential associations) typically lost using traditional extraction methods such as soaking and sieving. This approach also allows characterization of the 3-D spatial orientation of vertebrate bioclasts in relation to associated molds (in this case, voids where mollusk shell debris has been dissolved). In addition to documenting various taphonomic attributes of VMBs in situ, this study tests the correlation between data derived from sieved collections and intact blocks of matrix scanned using XRCT.