Dr. Ross Secord

Museum of Paleontology
1109 Geddes Avenue
The University of Michigan
Ann Arbor, MI 48109-1079
Phone: 734-764-0489
rsecord_@_umich.edu (remove spaces)

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General Research Interests

My research interests are centered around mammalian paleobiology. In general, I am interested in the distribution of ancient mammals, both in space (biogeography) and time (biostratigraphy), and in the environmental and climatic conditions that influenced mammalian distribution and evolution. I am interested in how climate affects mammalian faunal properties, such as species diversity (richness) and faunal turnover rates, and in ways to study these phenomena quantitatively. A prerequisite to the study of faunal and environmental change is the need to establish the temporal order of faunas and floras through time and to place these biotas into a geochronologic context. For this reason, much of my work to date has focused on mammalian biostratigraphy and geochronology.

My paleoecological interests include the use of stable isotopes of carbon from tooth enamel to characterize ancient environments dominated by C3 vegetation. Stable carbon isotope ratios (δ¹³C) vary in C3 plants largely because of environmental factors. The lowest δ¹³C values in modern plants are found in the understories of closed-canopy forests, where only a small amount of light reaches the ground and photosynthetic rates are slow. Values increase gradationally from the understory into the canopy and into open areas. For these reasons, δ¹³C values across a diverse sampling of vegetation can be used to broadly characterize the structure of the forest (e.g. closed-canopy, open-canopy, savanna). The δ¹³C values of the vegetation are directly reflected in the teeth of mammalian herbivores, which provide an archival record of the mammal’s diet, and a diverse sampling of herbivores should reflect isotopic parameters of the floral habitat. I am in the process of applying these concepts to early Eocene faunas and plan to sample modern faunas from various C3 environments to strengthen interpretations and develop a more concise model for interpreting ancient environments.

My biogeographic interests include mammalian dispersals between continents. The largest immigration “event” of the Paleogene occurred during the Paleocene-Eocene boundary thermal maximum (PETM), when the first representatives of true primates appeared in North America, as well as those of two orders of ungulates (Perissodactyla and Artiodactyla). Several other important events occurred in the Paleogene, including the immigration of rodents into North America from Asia at about 57 Ma. I am interested in better documenting the timing of immigrations in order to test hypothetical mechanisms that facilitated immigration, such as global warming “spikes,” tectonic events, and drops in sea level. I am also interested in testing the regional synchroneity of immigrant first appearances which are often used as datums by biostratigraphers.

Previous Research

Research for my dissertation focused on late Paleocene mammalian faunas from the northern Bighorn Basin, Wyoming. It included revision of biostratigraphic zonation, quantification of species diversity, and refinement of taxonomic composition. An important part of my research was devoted to constructing a stable oxygen isotope stratigraphy from mammal teeth. Oxygen isotope ratios (δ¹⁸O) were used as a proxy for temperature change to test the hypotheses that the immigration of rodents into North America was facilitated by short-term global warming. δ¹⁸O values in mammalian tooth enamel reflect the composition of surface water, which is influenced by the composition of precipitation, which in turn is sensitive to temperature. For these reasons, it is possible to identify short-term warming and cooling trends, and “spikes” in temperature from the δ¹⁸O of mammal teeth from single lineages. My findings corroborate the warming hypothesis and provide the first evidence that a short-term warming event may have played a key role in the immigration of rodents into North America by opening a high latitude dispersal route. They are also consistent with elevated temperature in the Bighorn Basin during the PETM immigrations.

Another component of my research focused on mammalian taxonomic diversity (species richness) from the middle to the late Paleocene. I divided faunas into three groups based on differences in lithology, taphonomy, and collecting methods. The Paleocene record was divided into eleven bins based on mammalian biozones. I rarefied samples to correct for unequal sample size among bins. The resulting diversity curve shows a significant drop in diversity in the middle Paleocene followed by diversity increase in the late Paleocene. Because reconstruction of long-term temperature change using the isotopic composition of mammal teeth may not be reliable, I correlated marine temperatures based on δ¹⁸O data to the mammalian biozones. The marine temperature curve and the diversity curve follow the same long-term trends, suggesting that mammalian diversity in the Paleocene was influenced by temperature.

Research for my master’s thesis focused on determining the age of new Paleocene faunas from the Carbon Basin, Wyoming, in order to place age constraints on structurally complex rock units. I determined that the two principal faunas were latest Torrejonian or earliest Tiffanian in age (middle Paleocene) and may represent a previously unsampled biozone. This age determination, coupled with geologic mapping and the considerably younger age of a stratigraphically higher fauna, allowed me to place age constraints on two pulses of Laramide tectonism and to document the presence of a major unconformity in the Hanna Formation.

Future Research

My current research focuses on the floral structure of early Eocene environments with the use of δ¹³C values in mammalian faunas, as discussed above. I plan to expand this work, in collaboration with others, to include environments through the PETM. A rapid and extreme increase in global temperatures during the PETM coincided with dramatic changes in faunal and floral composition. Global temperatures increased by 5 to 10ºC, and paleobotanical data suggest that continental warming was great enough to cause some plants to migrate northward by as much as 1500 km. Other evidence suggests a 20% increase in relative humidity during the PETM. Such large changes in climate may have caused changes in forest structure through the PETM. By studying δ¹³C values in a diversity of mammals before, during, and after the PETM, changes in forest structure can be inferred. Understanding the effects of global warming during the PETM could be important for developing predictive climate models and ultimately could contribute to predicting the long-term consequences of climate change.

I am also interested in studying the influence of climate on the geographic range distributions of Paleogene taxa. This is best done over a wide latitudinal range on a regional scale. Faunas in Texas contain co-occurrences of species that have non-overlapping stratigraphic ranges in faunas in Wyoming and more northerly areas. This may be due to shifting geographic range boundaries in response to changing climate and environmental conditions, or to more localized differences in habitat structure. I plan to test these hypotheses and others by studying faunas that have independent age control distributed over a wide latitudinal gradient, and compare the shifting range boundaries of species to temperature trends inferred from the marine record.

My future work will also include the study of micro-mammals from richly fossiliferous late Paleocene freshwater limestones in the Bighorn Basin. Very little is known about the postcranial and cranial anatomy of many small mammals of this age. The extraction and study of new skeletal material holds great promise for the reconstruction of locomotor habits and ultimately will contribute to the reconstruction of ancient ecosystems. It will also provide new data for use in phylogenetic reconstruction of mammalian groups at a time just before many modern groups first appear in the fossil record.