640 AMER.ZOOL., 40:640–663 (2000) Theropod Locomotion 1 JAMES O. FARLOW, 2, *STEPHEN M. GATESY,² T HOMAS R. HOLTZ,JR.,‡ JOHN R. HUTCHINSON,§ AND JOHN M. ROBINSON *Department of Geosciences, Indiana-Purdue University at Fort Wayne, Indiana 46805 ² Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912 ‡Department of Geology, University of Maryland, College Park, Maryland 20742 §Department of Integrative Biology, University of California, Berkeley, California 94720 Department of Physics, Indiana-Purdue University at Fort Wayne, Indiana 46805 SYNOPSIS. Theropod (carnivorous) dinosaurs spanned a range from chicken-sized to elephant-sized animals. The primary mode of locomotion in these dinosaurs was fairly conservative: Theropods were erect, digitigrade, striding bipeds. Even so, during theropod evolution there were changes in the hip, tail, and hindlimb that undoubtedly affected the way these dinosaurs walked and ran, a trend that reached its extreme in the evolution of birds. Some derived non-avian theropods developed hindlimb proportions that suggest a greater degree of cursoriality than in more primitive groups. Despite this, fossilized trackways provide no evidence for changes in stride lengths of early as opposed to later non-avian theropods. However, these dinosaurs did take relatively longer strides—at least compared with footprint length—than bipedal ornithischian dinosaurs or ground birds. Judging from track- way evidence, non-avian theropods usually walked, and seldom used faster gaits. The largest theropods were probably not as fleet as their smaller relatives. INTRODUCTION Zoologists can relatively easily observe the motions of many extant animals in the field or the laboratory. For extinct species, particularly those with a body form not ex- actly like those of any living species, it is a different story. An extinct animal’s loco- motion cannot be observed directly; rather, the paleontologist must reconstruct how the creature may have moved from its mor- phology and other indirect sources of in- formation. In this paper, we summarize what has been inferred about the locomotion of car- nivorous dinosaurs (i.e., theropods; Fig. 1), based on the study of both body and trace fossils. Understanding how theropods moved is obviously necessary if we are to understand them as living animals. Given the likelihood that birds represent an extant subgroup of theropods, however, it is equal- ly important that we understand how non- avian theropods walked and ran in order to 1 From the Symposium Evolutionary Origin of Feathers presented at the Annual Meeting of the So- ciety for Integrative and Comparative Biology, 6–10 January 1999, at Denver, Colorado. 2 E-mail: farlow@ipfw.edu know which features of avian locomotion originated with (and within) birds, and which they inherited from their non-avian ancestors. Our study will therefore consider thero- pod locomotion in a phylogenetic context. We will compare and contrast features of locomotion among different theropod groups, including ground-living birds. Al- though it is reasonable to suppose that the- ropods could have swum well enough when the situation warranted (Coombs [1980] de- scribed possible trackway evidence for this), and it is conceivable that some small- bodied theropods could have scampered about in trees—at least on occasion—with- out the need for obvious scansorial adap- tations, we will restrict ourselves to consid- ering what was undoubtedly the most im- portant kind of locomotion for most non- avian theropods, namely walking and running on the ground. THEROPOD PHYLOGENY Modern studies of theropod phylogeny began with the question of the origin of birds, following Ostrom’s observations (e.g., 1975, 1976) of numerous features uniquely shared by primitive birds and at University of Maryland on August 11, 2010 http://icb.oxfordjournals.org Downloaded from