EVIDENCE OF FEEDING ECOLOGY IN VERTEBRATE MICROFOSSIL BONEBEDS: A CASE STUDY FROM THE UPPER CRETACEOUS JUDITH RIVER FORMATION OF NORTH-CENTRAL MONTANA

Vertebrate microfossil bonebeds (VMBs) are concentrated deposits of predominately small, disarticulated, and often taxonomically diverse vertebrate hardparts. They have been collected for decades to recover otherwise rarely found small-bodied taxa, and are without question the richest source of Mesozoic mammalian fossils. Studies of VMBs have traditionally focused on taxonomic and faunal reconstructions, with little consideration of formative processes. Accordingly, the underlying causes for the accumulation of vertebrate skeletal material in VMBs remain poorly understood. Some previous studies have argued for hydraulic origins, with material from various sources accumulating in localized lenses in fluvial systems. Others have argued that predation plays a major formative role in VMB formation. Still others have argued for background mortality and the in situ accumulation of resilient skeletal components over time. In an effort to better understand processes linked to VMB formation, we studied a sample of over 21,000 surface-collected specimens from 24 VMB sites in the Upper Cretaceous Judith River Formation of Montana. We specifically focused on tooth marks and digestive traces to characterize and quantify evidence of feeding and potential predation. Digestion was inferred from the corrosion and loss of enamel on teeth and ganoine on gar scales. Previous studies have proposed that enamel and ganoine are stripped from teeth and scales upon passage through crocodilian digestive systems. Feeding traces were confirmed in all of the 24 sites under investigation, although elements showing modifications consistent with feeding still comprise a small fraction (typically less than 2%) of the total sample. Tooth marks of varying size and origin (theropod, crocodile, fish) were confirmed in 17 sites, but are relatively rare overall. The most common inferred feeding trace was the loss of ganoine on gar scales, with 18 sites exhibiting this modification. Interestingly, there is a correlation between the abundance of stripped gar scales and the abundance of crocodilian fossils in a site, with the four sites characterized by the highest occurrence of crocodilians also characterized by the greatest abundance of modified scales.