I am researching the nature and timing of recovery from the end-Cretaceous mass extinction event, focusing on mammalian evolution. How did biogeographic variation in the Late Cretaceous contribute to recovery in the earliest Cenozoic? How related were the lineages that diversified after the extinction? Were there "biodiversity hotspots" from which colonizing taxa spread and emigrated? Among Mammalia, I work primarily with the now-extinct Order Multituberculata ("multis" to their friends) which are most informative with respect to my questions. In the Late Cretaceous multis lived on all the northern continents, and they are the most diverse and most commonly identified terrestrial vertebrates that survive the end-Cretaceous extinction and re-diversify afterward. In some respects, such as range of body sizes, their way of chewing food, and their great diversity, multis resemble living rodents. In other respects, their skeletons have more in common with those of primitive mammals.

The above questions can only be answered in a phylogenetic framework. I must know how species are related in order to distinguish those that have originated from local lineages from those that have immigrated from somewhere else. Earliest Cenozoic mammalian assemblages include a high percentage of immigrant species. However, there is remarkably little agreement among paleontologists about multituberculate relationships, partly because most multis are known only from their teeth.

In collaboration with Dr. Yuki Tomida, I am currently describing and incorporating into a phylogenetic matrix significant new multituberculate fossils. My phylogenetic analyses indicate that many "new" species must have been diversifying in North America during the Late Cretaceous. The recognition of these ghost lineages implies that different regions within North America had different mammalian faunas.

In the Late Campanian (78-74 million years ago) southern mammal faunas differ taxonomically from those in the northern Western Interior. This mammalian faunal transition seems to correspond with that of other vertebrates, at paleolatitudes between 45° and 49° N. Even stronger evidence supports the existence of separate northern and southern mammalian faunas in the earliest Cenozoic (65-64 million years ago). What about the intervening time, at the very end of the Cretaceous? The obstacle to finding out has been that knowledge of latest Cretaceous mammalian faunas has been restricted to the north. This is why my work in the San Juan Basin, done in collaboration with Dr. Tom Williamson and Dr. Steven Semken, is so interesting - what we've found there is a lot farther south than other faunas of the same age.

The teeth of Late Cretaceous multis are distinctly different from those of Paleocene multis, even when sampling is done in the same geographic area and in the same sedimentary facies. Distinguishing characteristics include the shape of the lower premolars, the number of molar cusps, and differences in gross tooth wear patterns. Parallel morphological transformations seem to take place in several multituberculate lineages in the 750,000-year period following the end-Cretaceous extinction. Although multis are generally thought to have been herbivores, herbivory is a broad category; the Paleocene diversification of multis seems to involve a shift in diet or in feeding strategies.

My preliminary study suggests that the morphological difference is correlated with a dietary shift toward hard-object feeding. However, the number of teeth examined thus far does not provide a statistically significant sample. Duke undergrad Francesca Pignataro is currently working under my supervision to examine a larger sample. We hope that our findings will provide a baseline for studies of contemporaneous faunas in other areas (for instance, the San Juan Basin of New Mexico) as well as for studies of multituberculate evolution over longer time scales. Click here for more about Francesca's research.