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Within dinosaurs, carnivory was restricted to a single clade, the Theropoda.  Overwhelming anatomical evidence (now supported by recent, but limited, biochemical studies) nests living birds within this clade as part of a more exclusive group of predominantly small to midsized theropods in the clade Coelurosauria.  I have conducted systematic research on theropods since my time as a Masters student, and have published 34 papers and edited book chapters on this group including taxonomic descriptions of new taxa and specimens, descriptions of biologically important character systems, revisions of subclades within the group, new biogeographic insights, heterochronic shifts in life history, and detailed phylogenetic analyses of advanced theropods including birds based on firsthand observations of all specimens. Many of these papers are published in top journals such as Nature , Science, PNAS and Proceedings of the Royal Society B, as well as in specialist volumes such as The Dinosauria (2nd edition), the premier handbook in dinosaur paleontology. I was a PI on an NSF Assembling the Tree of Life collaborative project to elucidate the relationships among living and extinct archosaurs, which has its roots in the collaborative phylogenetic work that I have conducted on theropods. The principal aim of this project was to produce a densely sampled (generic level) phylogeny of archosaurs focusing on theropods including birds, through analysis of both morphological and molecular data in living and fossil taxa. At present, my fellow PIs and I are ready to begin large-scale analyses of the data and subsets thereof.

Among our chief goals will be to establish the ancestral state conditions for morphological characters in birds. We will particularly focus on important morphological character systems related to locomotion, flight, reproduction and development as they relate to avian origins. Theropods also provides ample opportunity to investigate broad scale evolutionary patterns and processes such as shifts in trophic habits. I have recently initiated a project with my post-doc Lindsay Zanno to test for purported dietary shifts to herbivory by using phylogenetic tests to determine which anatomical traits correlate with extrinsic evidence for herbivory such as gut content and gastroliths, and whether the repeated accrual of such traits follows similar patterns in different lineages providing evidence for common adaptive pathways and/ or evolutionary constraints. Our results suggest that herbivory was widespread among coelurosaurians, with six major sub-clades displaying morphological evidence of the diet and that contrary to previous thought, hypercarnivory was relatively rare and possibly secondarily derived. We find also evidence for a common succession of increasing specialization to herbivory in some subclades, substantiating intrinsic functional and/or developmental constraints. We are currently testing whether evolution in herbivory is linked with increasing body size and diversification in some or all of these lineages.

Another focus of my research program has been evolution of the Ceratopsia, or horned dinosaurs, a clade of small to gigantic herbivores, that were especially diverse in the Cretaceous, and which evolved a series of cranial display structures that have rendered them a dinosaurian model system for discussing questions of evolutionary mode. As in other dinosaurian clades, gigantism is a prominent trend in ceratopsian evolution, and ceratopsian body sizes range three orders of magnitude between small, basal forms and large, derived species. The step-wise accrual of ever more elaborate cranial display characters in progressively larger and more derived taxa, has inspired hypotheses that heterochrony, and in particular peramorphosis played an important role in ceratopsian evolution.

Results from my NSF funded research (with Greg Erickson of Florida Sate University) on ceratopsian evolution demonstrate that the proposed qualitative and poorly constrained hypotheses about heterochrony in horned dinosaurs pattern are overly simplistic and that heterochronic shifts in character expression are not tied to body size evolution. By examining remarkably complete growth series for three species of the basal ceratopsian Psittacosaurus, my collaborators and I documented heterochronic shifts (pre-displacement) in taxonomically significant traits between a basal taxon and more derived taxa, even though phylogenetic reconstructions indicate no shift in growth patterns as determined through histological examination and measurements of many specimens (n>30). Remarkably, a third species of Psittacosaurus exhibits an autapomorphic shift to a slower growth rate and smaller maximum size, yet does not contravene the evidence for pre-displacement in the more derived clade to which it belongs. Thus, rigorous analysis of the Psittacosaurus species complex provides clear evidence that heterochrony does play a role in evolution but does not necessarily accompany phylogenetic shifts in body size.

This conclusion is reinforced by our histological analysis of two stratigraphically disjunct populations of the more derived ceratopsian Protoceratops. A dramatic shift in growth rates between nominally conspecific populations, is not accompanied by obvious shifts in character expression or significant morphometric differences, providing evidence that significant changes in growth parameters do not correlate with trait displacement at micorevolutionary levels .

Results of our detailed analyses have significant implications for understanding the broader patterns of ceratopsian diversity and evolution. While ceratopsians are relatively speciose as a clade, they exhibit relatively low levels of sympatry between extant species. The speciose genus Psittacosaurus has been thought to represent an exception to this pattern, with up to 3 sympatric taxa of comparable or identical body size in some localities. Our results indicate that most of this apparent diversity is actually based on a poor understanding of the changes in traits over development and that only a single taxon is present at any locality, in keeping with the patterns observed in other ceratopsians.

DEVELOPING  & FUTURE PROJECTS

            Immediate goals include expanding the study on iterative evolution of herbivory toward all of Archosauria across the entire Mesozoic, a project for which I am seeking NSF support. Many of the herbivorous adaptations seen in coelurosaurs occur throughout Mesozoic archosaurs, allowing for a more comprehensive understanding of the evolution of this key trophic habit and its relation to floral evolution and major geologic events.

            Another developing project for the near future is to examine the function of the ceratopsian frill, an expanded bony collar that projects from the back of the skull in all but the most primitive ceratopsians and contributes to these animals having the largest skulls of any terrestrial vertebrates. This structure has traditionally been interpreted either as a frame for expanded muscle insertion for powering the beak and dental batteries or as a display structure. My research on basal ceratopsians suggests it may have originated for one function and later been co-opted for display, but our understanding of potential function is restricted to descriptive analyses of anatomy. I am developing s joint  project  (with Paul Gignac and Laura Porro) to investigate the potential functionality of the ceratopsian frill through comparative analyses of anatomy in living taxa with analogous structures (chameleons, turtles) as well as through Finite Element Modeling of the frill throughout ceratopsian evolution and ontogeny for CT-scans of well-sampled growth series. This combination of techniques and incorporation of data from ontogeny to look at changes in function through life history represent a new paradigm for dinosaur paleontology.

                     As a paleontologist, I have a strong interest in bridging the biological and geological aspects of my research. I recently developed a cladistic method that can chronologically order paleobiotas by using the temporal information inherent to rooted cladograms of groups that transcend these biotas. Future work will focus on expanding the method to take advantage of parameters such as sample size and sampling intensity to help overcome problems caused by gaps in the fossil record. An incomplete fossil record may also affect historical biogeographic analyses. Recently proposed time-slicing methods seek to avoid overprinting of different biogeographic patterns by limiting analysis to taxa that were present in that particular time-slice.  Although effective, these methods ignore informative biogeographic data from taxa that postdate the time slice, but whose presence is implied by phylogeny. Together with a student, I am working on an alternative time-slicing approach that takes such information into account but down-weights it according ghost lineage length. We are testing this on several datasets and hope to evaluate the method in the near future.