My research program sits at the intersection of paleontology, geology, and evolutionary biology and aims to elucidate the causes and consequences of major transitions in early mammalian evolution, such as the Cretaceous Terrestrial Revolution and the Cretaceous–Paleogene Mass Extinction. Using a combination of paleontological and geological fieldwork, systematic study of fossil collections, and analyses of the functional morphology, ecology, histology, and isotope geochemistry of modern and extinct mammals, my work scales from species-level studies of ecology, behavior, and life history, to ecosystem-level studies of biodiversity, tectonic and fluvial history, and climate.
My research program is guided by the following broad questions, which all center on understanding (1) the processes that promote or inhibit biodiversity and (2) how those processes change through time and space:
• How do terrestrial ecosystems (re)assemble during and after major environmental perturbations, such as periods of mountain building, mass extinction, and climate change?
• When in geologic time, where phylogenetically, and why (intrinsically or extrinsically) did complex behaviors and life histories evolve in early mammals?
• What are the long-term evolutionary effects of ecological innovations on mammalian species, clades, and communities?
Assembly of Terrestrial Ecosystems
My research program aims to understand how mammalian communities responded to environmental changes that occurred during the Cretaceous and Paleocene of North America. In particular, I am interested in whether the formation of the Rocky Mountains stimulated early mammalian diversification, and how abiotic and biotic changes associated with the Cretaceous–Paleogene mass extinction were modulated by mountain uplift and associated landscape change. This integrative work merges paleontological and geological fieldwork with specimen-based museum research and analytical methods spanning systematics, functional morphology, geochemistry, among others.
The upper Lance Formation (latest Cretaceous) and lower Fort Union Formation (earliest Paleocene) at Polecat Bench, Wyoming, U.S.A.
The Bighorn Basin in northwestern Wyoming hosts an impressive sequence of terrestrial and marine deposits spanning the Late Jurassic through early Eocene. The terrestrial units, in particular, capture important moments in the evolution of the Western Interior of North America, such as the onset of the Sevier and Laramide orogenies, and host mammalian faunas spanning the Cretaceous through Eocene. As such, the Bighorn Basin represents a geographically cohesive and stratigraphically extensive system in which to explore how mammalian faunas responded to landscape and climatic changes in the late Mesozoic through early Cenozoic.
Exposures from the D1 sequence of the Denver Formation that capture the first ca. 1 Ma of the Paleocene. Pikes Peak in the background. Just outside of Colorado Springs, CO, U.S.A.
The Denver Basin hosts exposures spanning the Cretaceous–Paleogene boundary and has yielded some of the most complete earliest Paleocene mammalian fossil specimens known anywhere in the world, including dozens of nearly complete skulls. Some mammal-bearing units directly abut the Rocky Mountain front range, whereas others occur farther out in the periphery of the Great Plains. By studying mammalian faunal dynamics through time in these mountain-proximal vs. mountain-distal localities I hope to test whether the Rocky Mountains buffered terrestrial communities from the Cretaceous–Paleogene mass extinction event and/or served as a cradle of biodiversity in its immediate aftermath.
Goler Formation, exposures of Member 4a, in the El Paso Mountains of the Mojave Desert region of southern California, U.S.A.
The Goler Formation, exposed in the El Paso Mountains of the Mojave Desert region, hosts the oldest mammalian faunas of California. These middle–late Paleocene mammals overlapped temporally with those well-known faunas from the Bighorn Basin, but lived in a very different paleoenvironment tucked between the rising Sierra Nevada mountains and the Pacific Ocean. Myself and an interdisciplinary team of geologists and paleontologists at the U of M aim to better understand the mammalian faunas, climate, and patterns of mountain uplift through the Goler Formation sequence, and to compare those with contemporary exposures in the Bighorn Basin.
Hell Creek Formation and lowermost Fort Union Formation exposures in McCone County, Montana, U.S.A.
The Hell Creek region of northeastern Montana is one of the best systems for studying evolution immediately before and after the Cretaceous–Paleogene mass extinction. The majority of my graduate student field research was based in the Hell Creek region, in particular, investigating the sedimentology, stratigraphy, and taphonomy of mammal-bearing deposits from the Hell Creek and Fort Union formations. This work is ongoing, and represents an interesting, tectonically quiescent, point of comparison with the comparatively tectonically active Bighorn and Denver basins.
Origins and Consequences of Biological Innovations
The early mammalian fossil record represents an excellent system in which to test hypotheses about the origin of higher taxa, adaptive radiation, and extinction. The origins and evolutionary consequences of quintessential mammalian traits such as lactation, chewing, and the detachment of the middle ear remain incompletely understood, but they have the potential to illuminate how intrinsic biotic factors like reproduction and development drive macroevolution. My entry point to these macroevolutionary questions was through multituberculates, an extinct group of superficially rodent-like mammals that lived from the Middle Jurassic through the Eocene and were the topic of my dissertation. My multituberculate work continues, but the current and future scope of my research program approaches questions about biological promoters and inhibitors in terms of early Mammalia more broadly.
Mammalian Life History Strategies
Histological cross section from the femur of Mesodma, a multituberculate mammal from the Late Cretaceous and early Paleocene of North America.
Innovations in reproduction and development are often cited as stimuli for evolutionary radiations in mammals; however, these hypotheses are based almost entirely on studies of extant mammals and little is known about the life histories of extinct mammals. My research program seeks to remedy this situation by using correlations between bone histology and life history traits in extant mammals to infer the reproductive and developmental biology of Mesozoic and early Cenozoic mammals. This aspect of my research program has the potential to change our understanding of mammalian life history evolution, illuminating when in geologic time, and where phylogenetically, the reproductive and developmental strategies characteristic of modern Mammalia arose and their long-term evolutionary consequences.
Mammalian Chewing Systems
An example of how understanding the evolutionary origins of mammalian chewing systems is central to understanding mammalian evolution is the controversies surrounding whether haramiyidans and multituberculates are closely related. If the dental similarities between these two groups are due to common ancestry, as suggested by the Bilateral Chewing Hypothesis, this implies a Late Triassic origin of mammals (Long-Fuse Model); if their dental similarities are due to convergent evolution, as suggested by the Homoplasy
Hypothesis, this implies an Early–Middle Jurassic origin of mammals (Explosive Model).
To understand mammalian evolution is to understand their teeth. Teeth and, less frequently, jaws make up the vast majority of early mammalian fossil record, and all major mammalian clades are chiefly defined by their distinct masticatory (chewing) systems. However, there is immense controversy in regards to the extent of conservation in these chewing systems: Are chewing systems plastic, evolving independently in numerous mammalian groups? Or are chewing systems fixed in lineages and indicative of major phylogenetic and ecomorphological divergences through the course of mammalian evolution? My research aims to explore these questions, as well as the long-term evolutionary and ecological consequences of these distinct mammalian chewing systems.
In the process of the measuring the postcranial bones of modern mammals to better understand the evolution of mammalian locomotion, and to infer the locomotor habits of early mammals.
Locomotion is a major axis of mammalian ecological diversity; however, the postcranial skeletons of Mesozoic and early Cenozoic mammals are rare. I am working with a group of paleontologists and evolutionary biologists to test hypotheses about the evolution of locomotor diversity using a large dataset (>200 specimens) of linear morphometrics on the limbs of modern mammals. Using a combination of techniques such as phylogenetic comparative methods and evolutionary modelling, we have are investigating the role body size plays in shaping mammalian ecomorphological diversity and the prevalence and drivers of convergence in the evolution of different mammalian ecomorphotypes.