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, and evolutionary history of clades over time. A quick introduction to my research can be seen in the following video:
Evolutionary Radiations: The Origin and Rise of Major Clades: How do groups rise to dominance? Living groups such as mammals and birds are clearly successful: they are taxonomically diverse, distributed widely around the globe, and have evolved into a great range of size, shape, and ecological habits. But, these groups did not begin this way. Like all groups they began with a single ancestor that lived at a certain time and place. What, then, allowed these groups to become successful? Was it a gradual or rapid process? Did it involve higher than normal rates of evolution? Was it enabled by the acquisition of one or several "key innovations" of the anatomy or biology that allowed the group in question 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 and other major 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 early career research has focused on two exemplary radiations: the evolutionary transition between carnivorous theropod dinosaurs and birds and the rise of dinosaurs and close relatives during the Triassic Period (~250-200 million years ago).
Theropod Dinosaurs and the Origin of Birds: Much of my research over the past decade (as an undergraduate, MSc student, and PhD student) has focused on the evolution of one of the most familiar groups of dinosaurs: the theropods. Theropods are a major clade that includes carnivores such as Tyrannosaurus and Velociraptor, which evolved a remarkable range of size, shape, and diet during their ~230 million year evolutionary history. Perhaps the single most important fact ever discovered by dinosaur paleontologists is that living birds descended from Mesozoic theropods, and the rapidly expanding fossil record of early birds and their closest dinosaurian relatives provides a textbook example of major evolutionary transformation. My research focuses primarily on the anatomy and genealogy of Mesozoic theropods, which help place the origin and evolutionary radiation of birds in context. I have described several new taxa of theropods, including several large-bodied species from Africa (Carcharodontosaurus iguidensis, Eocarcharia, Kryptops), two tyrannosaurs (Raptorex, Alioramus altai), and an aberrant stocky and double-sickle-clawed dromaeosaurid from Romania (Balaur). My colleagues and I have also monographed the anatomy of several important theropods, including basal taxa (Monolophosaurus, Neovenator, Shaochilong) and members of the derived clade Coelurosauria, which includes birds and their closest extinct theropod relatives (Alioramus, Balaur, Dryptosaurus). This taxonomic and anatomical work has been published in Science, PNAS, and other leading journals. This detailed anatomical work has been instrumental in building morphological phylogenetic datasets, and my colleagues and I have revised the phylogeny of Allosaurus and its closest basal theropod relatives, as well as the iconic tyrannosauroids (the latter in a paper published in Science in 2010). Current work focuses on the phylogeny of Coelurosauria as a whole, as well as the disparity and evolutionary rates of early birds and other Mesozoic coelurosaurs. This project, which forms the brunt of my PhD dissertation, is funded by a National Science Foundation Doctoral Dissertation Improvement Grant. Thus far, we have shown (using geometric morphometric approaches) that theropod cranial shape evolution was constrained more by phylogeny than it was driven by function (2012, Journal of Evolutionary Biology). Much of my current work on theropods is done in collaboration with my advisor Mark Norell, my undergraduate research was done in conjunction with my former advisor Paul Sereno, I enjoy long-standing collaborations with Roger Benson (Oxford) and Thomas Carr (Carthage College), and I have also worked on theropods with a diverse set of colleagues. Currently I am involved in a field project in the Late Cretaceous of Romania, focused on discovering aberrant theropods and other dinosaurs, with Matyas Vremir, Zoltan Csiki, Mark Norell, and Gareth Dyke.
The Evolutionary Radiation of Dinosaurs and Archosaur Macroevolution in the Triassic: Another major focus of my work is the rise of dinosaurs and their closest relatives during the Triassic Period, approximately 250-200 million years ago. Dinosaurs belong to a wider group called the Archosauria, which is a speciose clade that includes living birds and crocodiles, as well as many extinct groups restricted to the Mesozoic (such as phytosaurs, aetosaurs, pterosaurs, rauisuchians, ornithosuchids, and non-avian dinosaurs). Archosaurs seemingly evolved in the immediate aftermath of the Permo-Triassic mass extinction, the worst period of mass death in earth history, and then proceeded to radiate throughout the remainder of the Triassic. During this period of time, many other major groups of living tetrapods also originated (such as turtles, mammals, and lizards), and ecosystems were dramatically restructured after the chaos of the P-T extinction. Beginning as a MSc student and continuing today, I have worked on the anatomy, phylogeny, and evolution of Triassic archosaurs, with a goal of better understanding the tempo and complexity of their evolutionary radiation. This work has included fieldwork in the Triassic of Europe (Portugal, Poland, and Lithuania), anatomical redescriptions of important European specimens (Ctenosauriscus, Polonosuchus), anatomical and phylogenetic assessment of basal dinosaurs (Camposaurus), and a focus on synthesizing information from the Triassic skeletal and footprint fossil records (based especially on new footprint finds in Poland, published in 2011 in Proceedings of the Royal Society of London). For my MSc thesis at the University of Bristol I conducted a novel higher-level phylogenetic analysis of Archosauria (2010, Journal of Systematic Palaeontology), which is a powerful framework for understanding broader evolutionary changes and trends during the archosaur radiation. I used the wealth of anatomical and phylogenetic information on Triassic archosaurs to calculate two important macroevolutionary metrics for Triassic archosaurs: morphological disparity (the variety of body plans and lifestyles) and evolutionary rates (the speed at which anatomical characters evolve). This research is summarized in detail here, but in short, we found that dinosaurs and other groups were evolving at the same rates and that crocodile-line archosaurs (crurotarsans) NOT dinosaurs had a much larger range of body plans. This research was published in two papers (in Science and Biology Letters) and was done in conjunction with Mike Benton, Marcello Ruta, and Graeme Lloyd. Much of my current work on Triassic archosaur evolution is done in conjunction with Richard Butler (Munich), and Richard and I enjoy fruitful fieldwork collaborations with Grzegorz Niedzwiedzki and Tomasz Sulej in Poland and Octavio Mateus, Jessica Whiteside, Alex Kasprak, and Seb Steyer in Portugal. I have also worked with an exceptional set of colleagues. Along with some of these colleagues, I have published major review papers on the origin of dinosaurs (2010, Earth-Science Reviews) and archosaurs in general (2011, Earth and Environmental Science Transactions of the Royal Society of Edinburgh). My recent and current research on Triassic archosaurs is funded by the Climate Center at Lamont-Doherty Earth Observatory, the German Research Foundation, the Jurassic Foundation, and other sources.
Methods for Studying Macroevolution: I am generally interested in many aspects of macroevolution, especially the grand evolutionary patterns of vertebrate clades over long time scales. I use a diverse array of methods to discover and analyze vertebrate fossils, including CT scanning and scanning electron microscopy (to study anatomy in detail), cladistics (to reconstruct genealogy), geometric morphometrics and analysis of discrete character disparity (to quantify the variability in morphological features exhibited by a clade), and analysis of rates of morphological evolution (to quantify how anatomical evolution has changed over time). A major focus has been on developing both new and refined methods for better identifying, quantifying, and studying macroevolutionary patterns in the fossil record, as well as better understanding and coping with biases that result from an unevenly sampled fossil record. Along with Graeme Lloyd and Steve Wang, I have developed new methods for identifying instances of morphological rate heterogeneity on a phylogeny (branches or clades with significantly higher/lower rates of change than the rest of the phylogeny), which we published in 2012 in Evolution. I have also devised a method, along with Shaena Montanari, Hong-yu Yi, and Mark Norell, for including phylogenetic information in studies of morphological disparity, which may help to fill gaps in the fossil record that are predicted to be present (by phylogenetic ghost lineages) but remain unsampled (2011, Paleobiology). Recently I have been working with Richard Butler, Roger Benson, and Brian Andres to better understand how morphological disparity patterns may be biased by an uneven fossil record. We published a paper in Evolution (2012) which showed that some disparity metrics are seemingly robust to large-scale sampling biases, whereas others are more prone to serious error. Additional recent work with Richard Butler and Christian Foth has focused on whether different morphological disparity proxies (discrete characters, geometric morphometric landmarks, measurements) converge on a common signal, which we indeed did find to be the case in an exemplar study using pterosaurs (2012, Journal of Evolutionary Biology). Finally, I am interested in integrating information from molecular and morphological studies to better understand the evolution and genealogy of major clades. I have worked with Chris Organ and Koen Stein in predicting the genome sizes of extinct dinosaurs based on the sizes of their osteocyte lacunae (based on the noted correlation of genome size and bone cell size in living animals) (2009, Proceedings of the Royal Society of London), and with Shaena Montanari, Wendy DeWolf, and Mark Norell on neontological studies for better predicting genome size based on cell sizes that are often variable across the skeleton (2011, Biology Letters). Current work with Matt Friedman and Richard Butler focuses on better predicting the origination ages of major clades, which are necessary for calibration points in molecular phylogenetic analyses.
The Post-Cretaceous Rise of Mammals: One of my major research objectives as a young scientist is to study the post-Cretaceous evolution of mammals, especially their evolutionary radiation within the first several million years after the extinction of the dinosaurs 66 million years ago. For the last several years I have been working collaboratively with Tom Williamson of the New Mexico Museum of Natural History and Science, Dan Peppe of Baylor University, Ross Secord of the University of Nebraska, and Anne Weil of Oklahoma State University. Our goal is to study the post-Cretaceous radiation of mammals using many of the quantitative macroevolutionary techniques I have used to study Triassic dinosaurs and the theropod-bird transition, as well as better map the San Juan Basin of New Mexico, provide new radioisotopic dates and paleomagnetic correlations for key units, and look at changes in local climate and precipitation during the first few million years after the K-Pg extinction. The first result of this project is a new phylogeny of Cretaceous-Paleocene metatherians (2012, Journal of Systematic Palaeontology), which provides a well-resolved genealogy of the group, helps resolve some thorny taxonomic issues relating to Paleocene specimens, and shows that there was a profound extinction of metatherians at or near the Cretacoeus-Paleocene boundary but then a burst of lineage splitting immediately after the boundary. Tom and I also recently published a monographic description of the bizarre "archaic" digging taeniodont Wortmania, based on a spectacular new specimen of this rare Paleocene genus (PLoS ONE, 2013). Our collaborative team is doing yearly fieldwork in New Mexico, and our research is supported by the National Science Foundation (USA), National Landscape Conservation System of the USA, and the US Bureau of Land Management.
Metriorhynchid Crocodylomorphs and Marine Reptile Macroevolution: An increasingly large part of my research focuses on one of the most unusual groups of archosaurs to ever live, the fully marine metriorhynchids (a subgroup of crocodylomorphs) that inhabited the world's oceans for tens of millions of years during the middle part of the Mesozoic. The primary goal of this research is to understand metriorhynchid anatomy, phylogeny, and macroevolution in great detail. We have named several new taxa of metriorhynchids from the Middle Jurassic-Early Cretaceous of Europe, described in detail the oldest known large-bodied metriorhynchid (2013, Journal of Systematic Palaeontology), monographed the colossal predatory genus Dakosaurus (2012, PLoS ONE), and presented an iteratively-updated phylogenetic dataset for reconstructing metriorhynchid phylogeny. This anatomical and phylogenetic data has been used to better understand major patterns in metriorhynchid evolution, especially trends in diversity and disparity (using both discrete characters and geometric morphometrics), the evolution of metriorhynchid body size and cranial shape over time (using likelihood-based evolutionary modeling), and the relationship between form and function (studied using Finite Element Analysis). We have published this work in recent papers in Zoological Journal of the Linnean Society (2010, 2011) and Biology Letters (2011). A recent focus has been on the detailed ornamental, functional, and structural morphology of metriorhynchid teeth, and crocodylomorph teeth in general (2010, Journal of Vertebrate Paleontology; 2012, Anatomical Record; 2014, Acta Palaeontologica Polonica; and several ongoing projects in conjunction with Brian Beatty). In general, our work has shown that: some derived metriorhynchids were well equipped for strong bite forces, that niche partitioning between species was likely enabled by cranial and dental differences, and that metriorhynchids became more taxonomically and morphologically diverse up until the Jurassic-Cretaceous boundary, after which they quickly crashed before going extinct. All of this work is in collaboration with two of the world's foremost metriorhynchid specialists, Mark Young and Marco Andrade, who were both fellow students of mine at the University of Bristol. Also collaborating on some of these projects are Mark Bell, Brian Beatty, Jeff Liston, and Manabu Sakamoto.
Other Random Areas of Interest: Oftentimes my research meanders into unforeseen directions. In the past I have worked on Pennsylvanian brachiopods from near my home in Illinois, the anatomy and systematics of Pennsylvanian chondrichthyans from Illinois, and large taxonomic and phylogenetic databases. I can only imagine what random areas of the research morphospace I may wander into in the future...
For more general information on my research, check out these two informal interviews that were conducted by Andrea Cau and Francisco Gasco and posted on their blogs.