Biogeography of Arikareean mammalian communities

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As part of my dissertation, I am interested in studying the biogeographic relationships between Arikareean deposits in the northern United States (Oregon, Montana, Nebraska, Wyoming, and South Dakota). As part of a statistics class at the University of Washington, I have started investigating taxonomic similarities across Arikareean faunas using (in part) ordination methods.

These initial results are published in an in-house journal:

Calede, J. 2012. Biogeography and endemism in Arikareean faunas (North America, 30-18.8 Ma). Electronic Journal of Applied Multivariate Statistics 4:12–22.

You can read the abstract below:

The rich and geographically widespread Arikareean fossil record (North America, 30 to 18.8 Ma) provides a unique opportunity to test hypothesis of geographic clusters and gradients in faunal composition. I use multivariate statistics (cluster analysis and ordinations) to test the hypotheses that early modern mammalian faunas of the Arikareean were significantly different across biogeographic regions, were arranged along geographic gradients and that this apparent biogeographic pattern cannot be accounted for by the incomplete nature of the fossil record. The results show that there is support for a biogeographic structure of Arikareean communities with some possibly endemic assemblages (Cabbage Patch Beds of western Montana, Delaho Formation of southwestern Texas). The fossil assemblages reflect a strong longitudinal gradient. No evidence for a latitudinal gradient is found. Neither age nor sampling seem to influence the observed pattern which instead is correlated with depositional environment. This suggests that much of the observed faunal differences across assemblages and regions can be explained by habitats constraining local and regional faunas. Further study exploring the functional diversity of these assemblages will further investigate the controls on community assembly during the late Oligocene-earlyMiocene.

Cabbage Patch Beds

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The Cabbage Patch beds of western Montana  span roughly 6.5 million years from about 29. 5 to 23 million years ago crossing the boundary between the Oligocene and the Miocene. This series of fossil-bearing horizons is located in Powell, Granite, Silver Bow, and Deer Lodge counties (Montana). The Cabbage Patch beds house a rich vertebrate (mostly mammals) and invertebrate (mostly land and freshwater snails) fauna as well as floral remains (mostly in the form of phytoliths). These beds have mostly been studied by Dr. Donald Rasmussen in the 1960s and 1970s.

I am continuing Dr. Rasmussen’s work with the goal of comparing the fauna from Cabbage Patch to faunas of the same age located in Oregon (John Day Formation) and Nebraska (Arikaree Group). I am collecting additional fossils and geological data from the field and  further analyzing the fossils collected by Dr. Rasmussen housed at the University of Montana and the University of Kansas (mostly).

The goals of my dissertation are to better understand:

Collaborators at the University of Washington and the University of Michigan are investigating these deposits using isotopes and phytoliths to better understand the environment at the time in Montana. You can read more about their research here.

Cabbage Patch carnivores

Photo courtesy of Judy Carlson

Photo courtesy of Judy Carlson

The  carnivore fauna from Cabbage Patch currently include a single specimen, a dog named Cynodesmus thooides. My crew and I recently came upon another 15+ specimens while doing field and collection work. I am currently working on the description of this previously unrecognized diversity of carnivorans with an undergraduate student. This is part of a larger project investigating the taxonomic affinities of the Cabbage Patch fauna with the faunas from the Great Plains (Arikaree Group) and Oregon (John Day Formation).

We have already identified 2 more species of dogs, expanded the local stratigraphic range of C. thooides, and found some fossil mustelids (weasels, badgers, wolverines, and their relatives).

Systematics of entoptychine gophers

 

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I am working with Jenny Glusman, a former undergraduate in Biology at the University of Washington, on the systematics of entoptychine gophers from Cabbage Patch. We are studying the shape of the last upper molar (and other diagnostic teeth) of this group of fossil gophers abundant in the Cabbage Patch beds and the John Day Formation using geometric morphometrics. We are specifically investigating how well the shape of the diagnostic cheek teeth can help differentiate fossil entoptychine taxa. We are using  modern geomyine gopher species to validate our approach. The end goal is to be able to determine the geographic and temporal range of entoptychine gophers in the northern United States around the Oligo-Miocene boundary.

Preliminary results of this work were presented at the Annual Meeting of the Society for Cenozoic Research and recently at the Annual Meeting of the Society of Vertebrate Paleontology.