Smithsonian National Museum of Natural History

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Carboniferous

Dinosaur Phylogeny

and

Evolution

 

The evolution of dinosaurs documents the history of a diverse, widespread vertebrate group over more than 160 million years, and represents an excellent opportunity to study both small- and large-scale evolutionary patterns. My research has focused both on reconstructing the phylogenetic history of dinosaurs, and on interpreting the evolutionary patterns evident once that history is known.

From 1999–2004 I participated in fieldwork in the Late Cretaceous beds of northwest Madagascar, in collaboration with colleagues at Stony Brook University and the Université d’Anatananarivo.

 

This brought to light new materials of basal theropod dinosaurs (Majungasaurus and Masiakasaurus), and inspired a long-term research project focused on the phylogeny of that group.

Along with Scott Sampson and Roger Benson, I have been studying the evolutionary history of basal theropods, eventually covering more than 75 different species. We have published studies on coelophysoids and ceratosaurs, and are completing a final monograph on megalosaurs, spinosaurs, and other basal tetanuran theropods. Our recent ceratosaur phylogeny is shown below.

Most recently, we discovered that a number of enigmatic theropods in fact represented a previously unrecognized family, which we named Neovenatoridae. This group was related to allosaurs and carcharodontosaurs, but in some ways paralleled the more bird-like coelurosaurs. Importantly, it demonstrated that the allosaur radiation survived to the end of the Cretaceous, rather than becoming extinct some 30 my earlier as had been believed previously.

With these (and other) phylogenies at hand, I have studied large-scale evolutionary patterns within dinosaurs as a whole. In one study, I described the patterns of body-size evolution in dinosaurs, demonstrating that size increase was a persistent trend in most dinosaur groups, but that certain theropods (coelurosaurs) and sauropods (macronarians) were unusual in exhibiting a size decrease.

Dinosaur body size has important ecological implications, especially given the fact that dinosaurs and mammals have coexisted for their entire histories, yet barely overlapped in size.

One interesting possibility is that the different reproductive and growth strategies of dinosaurs may have created or allowed for important ecological interactions between Mesozoic mammals and juvenile dinosaurs.

My past work has also examined the evolution of locomotion in dinosaurs. This work focused on basic functional studies of dinosaur limb bones, in terms of their proportions and morphology, as well as muscle reconstructions. In this example, bony processes on the thigh bone are related to the positions and orientations of muscles that connect to the hip, and can reveal changes in muscle orientations and corresponding functions.

Actual muscle reconstructions can be attempted using morphology and information from living relatives, as with this example showing the hip and hind limb muscles of Tyrannosaurus.

By studying function in living birds and mammals, and relating that to differences in posture, morphology, and locomotion, I suggested that non-avian dinosaurs possessed a more “mammal-like” limb motion, in which the thigh bone played a significant role. The characteristic posture and limb movement of modern birds appears to have evolved after Archaeopteryx. The striking similarities between dinosaurs and mammals in limb proportions can be seen below.

Within dinosaurs, different groups independently acquired long, gracile limbs with long toe bones, and more proximally placed hip muscles. This occurred after the initial divergence of the major dinosaur lineages, which was more strongly driven by changes in feeding and diet.

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