DEPOSITIONAL ENVIRONMENTS AND SPECIMEN PRESERVATION IN THE ARIKAREEAN-AGED CABBAGE PATCH BEDS OF WESTERN MONTANA (RENOVA FORMATION)

The Cabbage Patch beds of western Montana host a rich record of faunal change leading up to and across the Oligo-Miocene boundary. This suite of fossil assemblages offers the opportunity to study the response of mammalian communities to the climatic and environmental changes of the Arikareean, such as the spread of grasslands. Because fossil assemblages are influenced by taphonomic processes that can bias their taxonomic and paleoecological composition, failure to account for differences in taphonomy among fossil assemblages can confound patterns of faunal change coeval with climatic and environmental disturbances.

In my work on the Cabbage Patch beds, I have compared the taphonomy of well-sampled fossil localities of the lower and middle units of the Cabbage Patch beds. I used a sedimentology-based approach to infer the depositional environment. I also used a specimen-based analysis of taphonomic features to determine differences in the preservation of fossil specimens across the assemblages studied.

An analysis of the lithology, sedimentological features, and in-situ fossil remains supports the conclusion that ash-derived fluviatile overbank deposits are the predominant depositional environment in the Cabbage Patch beds. These fine-grained sediment overbank deposits are taphonomically uniform throughout the section. Lacustrine deposits are also present but, with few exceptions, they are not as fossiliferous. The lowermost horizons of the Cabbage Patch beds also include a richly fossiliferous channel-lag deposit.

The vertebrate fossil assemblages are composed of numerous, isolated and disarticulated bones and teeth of mammals, frogs, and rare turtles, salamanders, and fish. Articulated bones are rare and appear to be associated with burrows. Identifiable mammalian remains are dominated by teeth and jaws but more fragile elements, such as ribs, are also recovered. Most elements are small, columnar, and have little abrasion.

Specimen preservation is the same across sedimentologically similar fossil assemblages of the Cabbage Patch beds. This result suggests that differences in taxonomic or ecomorphological composition across the beds are not the product of taphonomic differences but represent the response of faunas to climatic and environmental challenges.