Mammals in Montana: the birth of modern mammal communities

An example of a fossil found this past summer: the partial lower jaw of a 
small mouse. Note the scale bar. These very small fossils require hours of 
crawling the outcrop with eyes wide open.
       30 to 20 million years ago in Montana, instead of cows or buffalo, herds of oreodonts munched the vegetation. They shared the landscape with rhinos, packs of dogs, and fierce pig-like entelodonts. This period of time, called the Arikareean, marks the beginning of the modern mammal communities we see today. More familiar animals, like raccoons and beavers, began to replace the stranger groups – entelodonts, hippo-like anthracotheres, and creodonts (an extinct order of carnivorous mammals).

       This changing ecosystem is what I try to understand. We have a pretty good idea of what this time period was like in the Great Plains of Nebraska, South Dakota and Wyoming. We also know much from the rich fossil record of Oregon; but the picture is much dimmer when it comes to the northern Rocky Mountains. Fortunately, in the 1970s, the work of Donald Rasmussen (and others before and after him) brought to paleontologists’ attention the Cabbage Patch beds of Montana. This series of fossil-bearing rocks located about one hour east of Missoula is an opportunity to explore a region geographically intermediate between the better known records of Oregon and Nebraska. Studying the Cabbage Patch beds and contrasting them to the Oregon and Nebraska records will help understand how varied mammalian faunas were across North America and provide insights into the assembly of modern mammalian communities.

We were not the only ones prospecting. At this same locality, 
almost under the foot of this fawn, we found oreodonts, 
which are distant relatives of deer.

       Although much fieldwork has been previously undertaken in the fossil-bearing layers of Cabbage Patch, a lot remains to be done to collect enough fossils (and data on their geological context) to paint a fuller picture of the ecosystem of the place around the Oligo-Miocene boundary (23 million years ago). With that in mind, I undertook a month of fieldwork last summer to collect additional fossils and consider their preservation and depositional environments.

       With the help of two undergraduate students from the UW Biology department and welcoming landowners, I was able to collect many fossil bones, teeth, and shells as well as hundreds of pounds of sediment containing fossils to screenwash on campus (over 1500 lbs.). The main goal of this year’s work was to relocate a number of localities and assess their productivity. The other goal of the field season was to collect some data on the nature of the rocks containing the fossils.

Thien-Y Le, graduate of the department of biology, wrapping some fossils 
near a very productive locality where she had just found some dog teeth.
       We found over 200 fossils. We were particularly successful at finding carnivores. These animals were previously poorly known from this area (a single fossil dog was found in the early 20th century). We found 10 more specimens demonstrating a diversity of small dogs and possibly mustelids (badger relatives) in the area, most likely preying on the abundant rodents. We also found many rodent teeth including gophers, birch mice, aplodontids, and beavers.

       We also were able to collect – with the help of Dr. Caroline Strömberg and PhD candidate Elisha Harris, their crews, and collaborators – some rock samples that will help us understand the habitats in Cabbage Patch at the time. These samples will be used to determine the kinds of plants present and the climatic conditions around Drummond (MT) 28 to 23 million years ago.

       Next summer, a second season of fieldwork will help determine the depositional environments of the fossils to validate the hypothesis that the signal observed in community composition is ecological.

-Jonathan Calede


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