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526 23 A Trackmaker for Saurexallopus: Ichnological Evidence for Oviraptorosaurian Tracks from the Upper Cretaceous of Western North America Gerard Gierlinski and Martin Lockley Identifying the trackmaker of Saurexallopus has been an intriguing dilemma since footprints of this kind were first described in 1997. The large bird-like tracks of Saurexallopus are known from the Upper Cretaceous of North America. At this time, the only body fossils of animals with tetradactyl bird-like feet large enough to produce these distinctive ichnites are oviraptosaurs. Introduction Saurexallopus lovei was originally described from the Harebell Formation (Maastrichtian) of Wyoming by Harris et al. (1996) on the basis of a small assemblage of tracks preserved as natural casts (Fig. 23.1A, B). These tracks were first named incorrectly as Exallopus, a name preoccupied by a polychaete genus. Thus, Harris (1997) later renamed the tracks as Saurexallopus . More recently, a well-preserved representative of this ichnogenus named as Saurexallopus zerbsti was reported from the Maastrichtian Lance Formation of Wyoming by Lockley, Nadon, and Currie (2004) (Fig. 23.1C), and less well-preserved tracks cf. Saurexallopus ichnosp. indet. were also identified in the Maastrichtian Laramie Formation of Colorado (Fig. 23.1D). The affinity of these tracks was discussed by previous authors without any conclusion. Harris et al. (1996) discussed and rejected such possible Saurexallopus trackmakers as psittacosaurids , protoceratopsians, pachycephalosaurs, hypsilophodontids , therizinosauroids, and several other nonavian theropods. Clearly the Saurexallopus trackmaker was a medium -sized gracile species, with very slender toes. Considering only the Saurexallopus footprint morphology, with a foot length (FL) which varies between 22 and 35 cm long, a hip height (h) of 110 to 175 cm is inferred from the formula h = 5 × FL. These dimensions in turn suggest a medium-sized bipedal trackmaker comparable in size to an emu or ostrich. Although an avian trackmaker seems plausible, no such large birds are known from body fossils in the Late Cretaceous of North America. In this regard, the traces of digits II–IV are reminiscent of tracks attributable to gracile coelurosaurian dinosaurs such as ornithomimisaurs. However, tracks attributed to ornithomimisaurs (Lockley et al., in press) lack any trace of a hallux, as is consistent with foot skeleton evidence. Thus, we must consider other possible trackmakers. Institutional Abbreviations CMN, Canadian Museum of Nature, Ottawa; CU-MWC, University of Colorado and Museum of Western Colorado joint collection, Colorado; DMNH, Denver Museum of Natural History, Denver, Colorado ; HGM, Hunan Geological Museum, Changsha, China; Muz.PIG, Geological Museum of the Polish Geological Institute , Warsaw. Description Saurexallopus tracks (Fig. 23.1) are functionally tetradactyl tracks with slender, widely divaricated toes and prominent hallux extended medially to posteromedially. Digits II and IV are subequal in length. Digit I is the shortest, whereas digit III is the longest one, typical for theropod tracks. We can observe some degree of morphological variations among Saurexallopus footprints. As noted by Lockley, Nadon , and Currie (2004), two existing ichnospecies of Saurexallopus display a significantly different hallux configuration. Saurexallopus lovei Harris, 1997, shows a straight hallux directed medially, while S. zerbsti Lockley, Nadon, and Currie, 2004, possesses a more reversed and curved hallux, extended posteromedially. Average divarication between lateral digits (I and IV) of Saurexallopus lovei equals 111.4°. The footprint length of S. lovei varies between 28.4 and 31.5 cm, while width varies between 23 and 31.5 cm. Saurexallopus zerbsti is 35 cm long and 30 cm wide, and divarication between lateral toes is ~175.0°. Both ichnospecies from Wyoming clearly show characteristic triangular, elongate metatarsophalangeal pads, located directly behind the third toe. A distinctive feature of Saurexallopus morphology is the anterior position of the hypex between digits II and III relative the digits III and IV. This is the reverse of the pattern seen in many theropod dinosaurs and birds (Lockley, 2007:fig. 14). A Trackmaker for Saurexallopus 527 The Colorado specimen of Saurexallopus (Fig. 23.1D) has broader proximal pad than these two ichnospecies from Wyoming and this proximal metatarsophalangeal pad is not so clearly separated, but this difference is evidently caused by extramorphologic factors attributable to its preservation. This footprint is 22 cm long and 27 cm wide. The angle between the lateral toes equals 137°. The hallux is directed medially as in S. lovei. Discussion The discrete, centrally located, triangular metatarsophalangeal pad of Saurexallopus suggests an arctometatarsalian arrangement of metatarsals. In such a configuration, the metatarsals II, III, and IV are closely packed together and the proximal phalanx of digit III (phalanx III-1) is often relatively long...

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