TAPHONOMY OF VERTEBRATE MICROFOSSIL BONEBEDS IN THE UPPER CRETACEOUS (CAMPANIAN) JUDITH RIVER FORMATION OF CENTRAL MONTANA

Vertebrate microfossil bonebeds (VMBs) are concentrated deposits of predominately small, disarticulated, and taxonomically diverse vertebrate hard parts. Fairly common in Mesozoic and Cenozoic terrestrial records, VMBs have been exploited to recover otherwise rarely found small-bodied taxa (particularly Mesozoic mammals) as well as to estimate relative abundance and species richness in ancient vertebrate communities. Nevertheless, their taphonomic origins are poorly understood.

The Campanian Judith River Formation (JRF) of Montana preserves abundant VMBs in well-documented facies contexts, thereby offering an ideal opportunity to study VMB taphonomy. Three VMBs from the JRF were processed using a newly developed automated sieving system that minimizes damage and maintains any potential associations. All bioclasts (vertebrate, invertebrate, plant) >0.5 mm were separated by hand picking under light microscopy, producing 17,569 vertebrate bioclasts from approximately 80 kg of matrix. Yields ranged from 182–333 specimens/kg (raw counts are inflated to some degree by breakage during recovery). Most fossils are unidentifiable fragments in the 1-3 mm size range, but more than 20% of recovered specimens are identifiable. This identifiable fraction is dominated by teeth, jaw fragments, vertebrae, fish scales, and scutes.

Taphonomic attributes were recorded on a subset of recovered material that includes both identifiable and unidentifiable specimens. Rounded bioclasts are present in both mudstone- (pond/lake) and sandstone-hosted (fluvial) VMBs, but are more common in the latter (35% vs. 27%). Reflective polish is also developed in both settings, again more commonly in the fluvial VMB (21% vs. 10%), but regardless of facies context, all bones that exhibit polish also exhibit rounding. Taphonomic indications of predation/scavenging are rare but present in all sites, including minute bite marks and teeth/scales devoid of enamel/ganoine (consistent with gastric processing).

Our analyses of VMBs in the JRF are ongoing but suggest that previous models are inadequate to explain their origins. Only with a firm understanding of VMB origins can any biases be formally assessed, thereby permitting analytical comparisons of paleoecological signals.