Thomas E. Williamson

    Vertebrate Paleontologist


The San Juan Basin contains a record that brackets the Late Cretaceous and early Paleogene, a time that witnessed the abrupt extinction of the non-avian dinosaurs and the ensuing explosive radiation of mammals that rose to become the dominant animal in terrestrial ecosystems. Moreover, these events occurred during a time of fluctuating climate in a hothouse world. My research focuses on how vertebrate communities changed over time and in response climate change. I also study certain groups of animals in order to learn about their evolutionary history.

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 Late Cretaceous vertebrate fauna. I have led an active field campaign to collect Late Cretaceous vertebrates from the Fruitland and Kirtland Formations. I have collected large dinosaur specimens and also employ underwater screening to collect microvertebrates. In addition, my reseach has focused on the taxonomy and phylogeny of certain groups of Late Cretaceous dinosaurs, particularly pachycephalosaurs and tyrannosauroids.



K-PG boundary and Post-Extinction Ecological Recovery

The mass extinction event at the end of the Cretaceous is the youngest of the “Big Five” mass extinctions of the Phanerozoic in Earth’s history. It marked the extinction of non-avian dinosaurs and other animals and approximately 50% of mammalian lineages, resulting in profound restructuring of terrestrial ecosystems. This record has been well-studied in the northern Rocky  Mountain  region. However, this record is poorly  known outside of that area. The San Juan Basin contains one of the few records outside of the San Juan Basin that  closely brackets the Cretaceous - Paleogene (K-Pg) boundary.


The Cretaceous portion of this record is preserved by 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; land managed by the Bureau of Land Management. However, 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 latest Cretaceous or even early Paleocene in age.

Age of the Naashoibito Member:

In 2002 Anne Weil (Oklahoma State University) and I were awarded a grant from the National Science Foundation for the collaborative project “Evaluating the contribution of Late Cretaceous biogeography to earliest Cenozoic biodiversity in North America” (2002-2007). As part of this study, we employed underwater screening techniques to samples retrieved from the Naashoibito Member. We were trying to recover microvertebrate fossils including isolated teeth of small mammals. We intended to use these to better establish the age of the Naashoibito Member.

We recovered numerous teeth representing several mammal taxa. Some of these teeth represented the first diagnostic therian mammals reported from the Naashoibito Member. Several of these teeth were identified as 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 multituberculate mammal taxon Essonodon, a genus also known only from the latest Cretaceous of Wyoming and Montana. The presence of both Glasbius and Essonodon gives strong support for 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 shortly before the end-Cretaceous mass extinction. 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 Campanian, Late Cretaceous and early Paleocene strata of the San Juan Basin.

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 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 have been at their greatest.The Nacimiento Formation contains 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. The study of the succession of mammalian faunas of the Nacimeinto Formation formed the central focus of my Ph.D. dissertation (Williamson, 1996).


The early Paleocene marks an important turning point in Earth history during which ecosystems recovered from a mass extinction and mammals arose to dominance in terrestrial ecosystems. The Paleogene was also a time in which climate was much warmer today and unstable. My colleagues and I are interested in the early Paleocene climate record and how it may have impacted mammal faunas. I have been working with several researchers for the past several years in order to better understand early Paleocene climate. My collaborators include Stephen Brusatte (University of Edinburgh), Ross Secord (University of Nebraska-Lincoln ), and Dan Peppe, Stacy Atchley, and Lee Nordt (Baylor University). We have been applying a number of different techniques to reconstruct early Paleocene climates and environments including fossil plants and stable isotope analysis. In September of 2013, we were awarded a grant from the National Science Foundation for the collaborative project "Testing for a link between climate and mammalian faunal dynamics in the early Paleocene record of the San Juan Basin, New Mexico."

Stephen Brusatte and I are also working together on questions regarding the early radiation of mammals. How severely were mammals impacted by the K-Pg extinctin and which groups survived, and why? How rapidly did mammals recover and re-radiate in the Paleogene? Was this radiation associated with increased rates of speciation, body size evolution, and rates of anatomical change? How was mammal evolution related to early Paleocene climate change? We will be studying the evolutionary history of a number of groups including metatherians, the group that includes living marsupials, and "condylarths," a group of mammals that dominated early Paleocene mammal communities, but whose interrelationships are poorly known. "Condylarthra" is an unnatural group that includes close relatives to living hoofed mammals such as horses and pigs. We are working on reconstructing the genealogy of these groups to see where they fit on the mammalian family tree and we will use this information to examine the patterns of evolution during this critical time.

K-Pg Boundary Research Group

Presentations and publications resulting from this research and suggested reading

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