I use the fossil record to get a deep time perspective on modern impacts affecting mammal communities.
I want to know: Can we use the fossil record to investigate ecological questions? What was the ecology of Cenozoic mammals? How did environmental change impact the tempo and mode of mammalian biodiversity 30 to 15 million years ago?
The dramatic rate and magnitude of contemporary extinctions have led to a growing body of research on the “6th mass extinction.” Because of their importance in our everyday lives, much of this research has focused on mammals, revealing the grim picture that over 20% of mammal species are currently threatened by extinction. Habitat loss, driven by anthropogenic activities, is the primary cause of this situation. Ecological work, incorporating observational and experimental studies, provides land managers and conservationists with solutions to forecast and mitigate such loss over the next century. However, these endeavors provide little empirical data on the trajectory of mammalian communities over timescales exceeding historical records, in the absence of human disturbance, or in the presence of climatic and environmental conditions beyond those endured by modern faunas leaving unanswered a critical question: How do mammalian faunas react to dramatic environmental changes over evolutionary timescales?
The Cabbage Patch Beds
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:
- The taphonomy of the Cabbage Patch deposits
- The systematics of an abundant group of mammals from Cabbage Patch
- The biogeographic relationships of Cabbage Patch with other regions at the time
- The advent of modern mammalian communities at the time of the spread of grasslands
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.
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.