Mapping the Origin of Terrestrial Faunal Assemblages Using Strontium Isotopes

ABSTRACT

The fossil record is being called upon to enhance our understanding of population responses to climate change and to serve as a guide for predicting ecosystem responses in the future. However, interpretations of the fossil record have heretofore been predicated on two untested hypotheses. First, that the spatial scale of the response seen in the fossil record, especially in terrestrial settings, is well understood, and second, that community change through time represents shifts in local populations rather than changes in sampling area. PIs Hadly and Paytan propose to test these hypotheses by analyzing the strontium isotopic composition a rich sequence of fossil bones from Yellowstone National Park. The Yellowstone sequences span the past 3000 years and are augmented by a comprehensive modern collection from the vicinity of the fossil sites. Because strontium isotopes are not fractionated by biological processes, the strontium content of the bedrock and overlying regolith, the vegetation that takes up strontium from the soil and the animals consuming the vegetation will be strongly related. The sampling localities are distributed across a heterogeneous geologic landscape, with each site underlain by bedrock with distinctly different 87Sr/86Sr isotopic signatures. If the fossil sites represent local collections, Hadly and Paytan predict that the two assemblages will record significantly different strontium isotope ratios. Conversely, they expect values much different from the local bedrock (and possibly more similar to each other) if the collecting radii of the sites are large and incorporate regional average strontium values. Preliminary data indicate that these fossil assemblages each have distinct Sr isotopic compositions representing the local bedrock. The PIs will determine whether it is possible both to rule out shifts in collection area through time and to constrain statistically the scale at which ecological, evolutionary, and paleoclimatic interpretations of a given site should be applied. Further, the PIs propose to test the generalities of their approach by investigation of samples from a middle Pleistocene faunal assemblage from the central Rocky Mountains, Colorado.

What is the intellectual merit of the proposed research?

The investigators propose to determine the range of animal use of the environment in a novel way. They plan to assess habitat use through time in a series of fossiliferous localities from three Rocky Mountain localities. In the absence of such analyses, the spatial component of sampling bias cannot be accounted for in most studies of either modern or paleontologic assemblages. This analysis will determine to what extent strontium isotopes can be used as a tool for assessing the ecological fidelity of the fossil record.

What are the broader impacts resulting from the proposed research?

The methods and results from this study will be of interest to paleontologists interested in reconstruction of past communities and to ecologists intent on understanding the spatial scale of ecosystem functioning. This research also has relevance to other specialists such as evolutionists, conservationists, geologists, and behaviorists. Through the synthesis of several fields and integration of diverse field, analytical and statistical methods, Hadly and Paytan emphasize the interdisciplinary training of graduate and undergraduate students.