The Late Cretaceous and Paleogene span an interval marked by tremendous changes in global climate and terrestrial faunas. My research focuses on the anatomy, systematics, biostratigraphy, and biogeography of Late Cretaceous through Paleogene fossil vertebrates to interpret their evolutionary history. The hub for much of this work is the San Juan Basin of northwestern New Mexico where an incredibly rich and diverse vertebrate fauna.

Late Cretaceous (Campanian) Vertebrates

Near the beginning of the Late Cretaceous, the earth underwent a rapid increase of global temperature that was accompanied by a global rise in sea level. The San Juan Basin contains a long and nearly continuous record of Late Cretaceous time. For much of this interval, New Mexico straddled the western shore of the Western Interior seaway. As global sea level rose and fell, the shoreline alternately moved landward and seaward, at times submerging the landscape or leaving it high and dry.

The San Juan Basin contains a diverse vertebrate fauna of including mammals and dinosaurs. My collaborative work investigates some of these animals to better understand their evolutionary history and Late Cretaceous vertebrate diversity and biogeography.
 
Publications resulting from this research and suggested reading

K-PG boundary and Post-Extinction Ecological Recovery

Much of this work was in collaboration with my colleague and friend Anne Weil (Oklahoma State University, Center for Health Sciences, Department of Anatomy and Cell Biology; Anne.weil@okstate.edu). We collaborated on a projected funded by the National Science Foundation, “Evaluating the contribution of Late Cretaceous biogeography to earliest Cenozoic biodiversity in North America” (2002-2007)

The mass extinction event at the end of the Cretaceous is the youngest of the “Big Five” mass extinctions in Earth’s history. It marked the extinction of non-avian dinosaurs and other animals and approximately 50% of mammalian lineages, resulting to profound restructuring of terrestrial ecologies.

Many of the lineages of earliest Paleocene mammals that appear soon after the K-PG mass extinction are not known from the latest Cretaceous of North America. However, until recently, the only record of latest Cretaceous mammalian faunas was geographically restricted to the northern region of the Western Interior. The San Juan Basin of northwestern New Mexico contains one of the most diverse latest Cretaceous faunas of North America outside of the northern Rocky Mountain Region. In addition, it contains microvertebrate faunas that include mammals. By examining this record, our study tested the hypotheses that (1) there was mammalian faunal provinciality in the latest Cretaceous (late Maastrichtian) of North America; and (2) that recovery faunas of earliest Cenozoic North America are derived at least in part from diverse provinces present in Late Cretaceous North America.

As part of this study, we carefully examined the Naashoibito Member, Kirtland Formation. This stratum is relatively thin and is well-exposed over a small geographic area of the southwestern San Juan Basin. The Naashoibito Member is particularly well exposed in badlands within the Bisti/De-na-zin Wilderness area, managed by the Bureau of Land Management. The age of the Naashoibito Member is contentious. Some workers have suggested that it is early Maastrichtian in age, while others have argued that it is early Paleocene and provides evidence of Paleocene non-avian dinosaurs.
 
Here are some of the results of our study:

We recovered a significant mammal fauna from the Naashoibito Member. The mammalian fauna includes the first diagnostic therian mammals reported from the Naashoibito Member. We documented the presence of the metatherian mammal Glasbius, a taxon otherwise known only from latest Cretaceous faunas of Wyoming and Montana. We also recovered a diverse multituberculate assemglage that includes the taxon Essonodon, a taxon also known only from the latest Cretaceous of Wyoming and Montana. The presence of Glasbius and Essonodon supports a latest Cretaceous age for the Naashoibito Member and also demonstrates that some latest Cretaceous metatherian mammalian taxa were geographically widespread over western North America. However, whereas Glasbius is a rare component in northern faunas, it is relatively abundant in Naashoibito microfaunas.

As part of our study, we examined older Cretaceous (Campanian) strata of the Fruitland and lower Kirtland Formations (link to research on Late Cretaceous faunas) and younger early Paleocene fossil localities of the Nacimiento Formation (link to research on early Paleocene faunas). This resulted in the better understanding of the stratigraphic distrubition of animal and plant taxa in Late Cretaceous and Paleocene strata of the San Juan Basin.

We incorporated many of the results of our study in a permanent exhibit at the New Mexico Museum of Natural History and Science, “The Cretaceous Seacoast Exhibit.” We also included Native American undergraduate students from Diné College and the University of New Mexico. Native American students are the most underrepresented groups in the geosciences and our project provided significant training in geological and paleontological research and field and lab techniques.

Publications resulting from this research and suggested reading

Early Paleocene

The explosive diversification of mammals at the beginning of the Cenozoic was one of the most important events in mammalian history and is considered a classic example of an evolutionary radiation. After the mass extinction of dinosaurs and other organisms at the Cretaceous-Paleogene (K-Pg) boundary, mammals underwent a dramatic radiation that resulted in a rapid increase in species diversity, morphological disparity, and ecological diversity (Alroy 1999, 2000a, b; Archibald, 1996; Rose, 2006; Stucky, 1990). However, the record of this diversification is poorly documented, especially at its beginnings in the early Paleocene when rates of taxonomic and morphological change are expected to be at their greatest.The Nacimiento Fm. is the most diverse, longest, and most complete record of early Paleocene eutherian mammal succession in the world, spanning nearly four million years. Importantly, this record can be tied to the time scale using paleomagnetostratigraphy and radiometric dating.

I have long been interested in the taxonomy, phylogeny, and evolution of early Paleocene therian mammals and early Paleocene mammalian biostratigraphy and biochronology. I am also working collaboratively with other workers including Stephen Brusatte (Ph.D. candidate, Columbia University ), Ross Secord ( University of Nebraska ), and Dan Peppe ( Baylor University ) on improving our understanding of this critical interval. We plan to examine the early Paleocene radiation of mammals by studying this record in concert with a detailed chronologic study. We intend to correlate this record to several climate proxies including megaflora and stable isotopes in order to examine the potential interplay of climate, mammal faunal composition, and the larger picture of mammalian macroevolution. We will also examine different macroevolutionary measures of some mammal lineages such as diversity, morphological disparity, and ecological disparity to better document the overall pattern of the mammalian radiation.

Publications resulting from this research and suggested reading