Stephen Brusatte, Research

The central theme that unites my research is the use of fossils and the geological record to better understand how evolution works over long time scales. Fossils provide a deep-time perspective on evolution that is inaccessible if we focus solely on living organisms. Only by studying fossils can we appreciate the great diversity of life throughout earth history, understand how groups and faunas change in concert with a dynamically changing planet, untangle how body plans and major morphological character complexes are assembled, and understand how fluctuating climates and other environmental perturbations affect the diversification, distribution, abundance, and variability of living organisms on a long-term basis. I study the evolutionary history of major fossil groups such as dinosaurs, birds, and mammals by using a three-pronged approach. First, I collect primary fossil data through fieldwork, anatomical descriptions of specimens, and systematic revisions of important taxa. Second, I synthesize these primary data into large morphological phylogenetic analyses, which produce a genealogical framework for better understanding evolution. Third, I use morphological and phylogenetic information to study large-scale macroevolutionary patterns, which help us better understand the origin, initial radiation, evolutionary history, and extinction of clades over time. A quick introduction to my research can be seen in the following video:

My work has meandered around the family tree of vertebrates, across the geological time scale, and through a variety of methods. Some of the key topics I study include:

Building the Family Tree of Life: All living things, modern and extinct, evolved from a single common ancestor and are connected together in one great family tree. This fact was first recognized by Darwin, and over the past 150+ years countless scientists have been constructing this family tree branch by branch. One of my research specialties is morphological phylogenetics: using features of the anatomy to build large family trees showing how extinct vertebrates are related to each other and to modern species. The brunt of my work has focused on the relationships of three main groups: early archosaurs (dinosaurs, crocodiles, and their fossil relatives), theropod dinosaurs (the carnivorous species like T. rex and the birds that descended from them), and early placental mammals (the species living around the time of the end-Cretaceous extinction, which ultimately gave rise to the great diversity of today's mammals, including humans).

Evolutionary Radiations: The Origin and Rise of Major Groups: Some groups such as mammals and birds are clearly successful: they are diverse, distributed widely around the globe, and have evolved into a great range of size, shape, and ecological habits. But how did this happen? Was it gradual or rapid? Did it involve higher than normal rates of evolution? Was it enabled by anatomical or biological features that allowed them to speciate more profoundly or better weather extinctions? Did it involve outcompeting other groups or did it opportunistically follow unexpected contingencies of earth history? Were fluctuations in climate or mass extinctions major driving forces? I address these questions by examining evolutionary radiations in the fossil record: those periods of time when groups originate and first blossom in diversity, abundance, and behavioral and morphological variability. Much of my research has focused on two exemplary radiations: the rise of dinosaurs and close relatives during the Triassic Period (~250-200 million years ago) and the diversification of mammals after the extinction of the dinosaurs (66 million years ago).

Major Transitions in Evolution: Sometimes evolution takes one type of animal and turns it into something radically different, with a restyle body capable of remarkable new behaviors that allow it to colonize new environments. These major macroevolutionary transitions seem to be rare in the history of life, and require a transformational series of fossils to study. There remain huge questions about how they happen: do they occur rapidly, the result of a few freak events, or more slowly as organisms constantly adapt to changing environments over millions of years? Much of my research has centered on one of the most famous of these transitions: the evolution of small, feathered, flying birds from carnivorous theropod dinosaurs. I also work on another, much less celebrated transition: the thalattosuchians, an extinct group of crocodile relatives that evolved from land-living ancestors, then entered the water as semi-aquatic lagoon-dwellers, before eventually morphing their limbs into flippers and transforming into fast-swimming open-ocean predators.

Mass Extinctions and Recoveries: The history of life has occasionally been interrupted by mass extinctions: relatively sudden, global events in which a huge number of species die out simultaneously. These catastrophic events can devastate ecosystems and set evolution on new trajectories. They are also hugely important to understand: they are analogous to some of the sudden changes in climate and environment occurring in today's world, so they may hold the key to better understanding, planning for, and mitigating these changes. I work on the most infamous mass extinction of all: the asteroid impact at the end of the Cretaceous Period (66 million years ago) that killed off the non-bird dinosaurs and paved the way for mammals.