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89 5 Feet of the Fierce (and Not So Fierce): Pedal Proportions in Large Theropods, Other Non-avian Dinosaurs, and Large Ground Birds James O. Farlow, Thomas R. Holtz, Jr., Trevor H. Worthy, and Ralph E. Chapman The extent to which the makers of tridactyl dinosaur footprints can be identified depends on the extent to which their foot skeletons can be told apart. We examined this question for non-avian theropod dinosaurs (NATs) and large ground birds, making additional comparisons with functionally tridactyl, bipedal–or potentially bipedal–ornithischians. For birds we measured distances across the trochleae of the tarsometatarsus, and for NATs, the lengths of metatarsals II–IV. For birds, NATs, and ornithischians we measured the lengths and widths of individual phalanges and the aggregate lengths of digits II–IV. Metatarsal, digital, and phalangeal proportions distinguish some genera among dinosaurs (including birds). At higher taxonomic levels, pedal features are useful but not infallible proxies for the systematic affinities of birds and non-avian dinosaurs. Our results suggest that the parameters commonly used to describe tridactyl dinosaur footprints can often be used to provide a minimum estimate of the number of trackmaker taxa within an ichnofauna and that similarity in footprint shape is useful but not always a trustworthy indicator of phylogenetic relationships of trackmakers. In many Mesozoic stratigraphic units, footprints of dinosaurs are much more common than dinosaur body fossils and provide our best record of the kinds of dinosaurs living in certain regions during particular times (cf. Leonardi 1989; Thulborn 1990; Lockley 1991; Schult and Farlow 1992; Gierliński 1995; Lockley and Hunt 1995; Dalla Vecchia et al. 2000; Lockley and Meyer 2000; Kvale et al. 2001; Farlow and Galton 2003; Moratalla et al. 2003; Pérez-Lorente 2003; García-Ramos et al. 2004; Farlow et al. 2006; Milner and Spears 2007; Rainforth 2007; Sullivan et al. 2009). The resolution with which such trace fossils can be used for paleoecological or biostratigraphic studies depends on the degree to which particular footprint shapes are uniquely associated with given dinosaur taxa (Baird 1957; Farlow and Chapman 1997; Farlow 2001; Smith and Farlow 2003; Farlow et al. 2006). Abstract Introduction I submit that the characters most diagnostic for the classification of footprints as such, as well as most useful for comparison with skeletal remains, are those which reflect the bony structure of the foot. Baird (1957:469) Farlow, Holtz, Worthy, and Chapman 90 Tridactyl (three-toed) footprints of bipedal dinosaurs (Fig. 5.1) are often the most abundant dinosaur footprints found in track assemblages. Because dinosaur taxa are named on the basis of skeletal material, identifying the bipedal dinosaurs responsible for three-toed prints involves making correlations between footprints and foot skeletons. The “best case” of our ability to identify the makers of dinosaur tracks would therefore be that in which the shape of a footprint reflected the proportions of its maker’s foot skeleton (as revealed, perhaps, by relative digit lengths, the configuration of footprint digital [toe] pads, and interdigital angles of the footprint; Smith and Farlow [2003]) with perfect fidelity. This in turn leads us to inquire as to what features, and at what taxonomic level, we could use to tell foot skeletons apart. Although this question can be explored by a study of dinosaur foot skeletons (Farlow and Lockley 1993; Farlow and Chapman 1997; Farlow 2001; Smith and Farlow 2003), this approach is limited by the number of reasonably complete, well-preserved dinosaur feet available for study. The problem can be tackled in a more indirect fashion, however, by examining within-group and across-group variability in foot shape in ground birds–animals thought at the very least to be close relatives of dinosaurs (Feduccia 1996) and considered by most paleontologists to be extant dinosaurs (Gauthier and Gall 2001; Chiappe and Witmer 2002; Currie et al. 2003; Padian 2004). Living or recently extinct bird species are represented by many more complete foot skeletons in museum collections than are most non-avian theropod or ornithopod dinosaur species. It is therefore easier to obtain a large enough sample of foot skeletons to consider within-taxon and across-taxon variability in pedal proportions in birds than in dinosaurs. Here we examine within-taxon and across-taxon variability in foot skeletons of non-avian theropods (NATs; Fig. 5.2) and large ground birds (both extant and extinct), and consider similarities in foot shape among NATs, ground birds, and bipedal or potentially bipedal ornithischians (Appendix 5.1). Because we are interested in pedal...

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