Functional diversity of early modern mammalian communities 30 to 20 million years ago

Background/Question/Methods

The origin, maintenance, and regulation of community structure is an ongoing debate in ecology. The challenges of extinctions, selective losses in biodiversity, and changes in ecology of individual species remind us of the importance of understanding community assembly rules. The fossil record offers a unique opportunity to explore broad scale patterns in changes in community structure over evolutionary timescale. The end Oligocene – earliest Miocene (Arikareean, 30-18.8 Ma) is a critical transitional period in the evolution of mammals from the archaic Eocene faunas into the already-modern Miocene ones. The Arikareean fossil record includes deposits with long, continuous sequences of fossil-bearing rocks that allow the investigation of paleocommunity structure over an evolutionary timescale (i.e. several million years) in rich and stratigraphically well constrained faunas from central Oregon, western Montana, as well as western Nebraska. I use collection records from museums across the United States to assess the disparity and trajectory through time of mammalian paleocommunities from these three regions. Specifically, I use richness, evenness, and abundance records of ecological guilds to demonstrate biogeographic differences in community patterns across the first three time slices of the Arikareean (Ar1-Ar3, 30-19.5 Ma). 

Results/Conclusions

Preliminary results indicate that both Nebraska and Oregon faunas occupy the whole ecological space spanned by Arikareean communities whereas the Montana fauna appears confined to a small portion of this possible ecological space. Additionally, these results show that the geographically intermediate Montana fauna displayed stronger affinities to both the Great Plains’ and the Columbia Plateau’s faunas earlier in the Arikareean (Ar1) becoming further ecologically disparate later on during the Arikareean and finally isolated in the last time slice of this analysis (Ar3). This ecological change within the Montana fauna coincides with a change in the body size distribution of the fauna as indicated from cenograms. The later part of the Arikareean (Ar3) in Montana exhibits many small mammals and fewer large ones than earlier. This pattern is quite unlike that observed elsewhere in Nebraska and Oregon. This may be a consequence of the delayed spread of open vegetation in the Rocky Mountains suggested by phytoliths evidence. The study of changes in community composition and dynamics of these fossil mammalian faunas will provide context for and further the understanding of the threats to modern mammalian faunas.