Three-toed Browsing Horse Anchitherium (equidae) from the Miocene of Panama

DURING THE Cenozoic, the New World tropics supported a rich biodiversity of mammals. However, because of the dense vegetative ground cover, today relatively little is known about extinct mammals from this region (MacFadden, 2006a). In an exception to this generalization, fossil vertebrates have been collected since the second half of the twentieth century from Neogene exposures along the Panama Canal. Whitmore and Stewart (1965) briefly reported on the extinct land mammals collected from the Miocene Cucaracha Formation that crops out in the Gaillard Cut along the southern reaches of the Canal. MacFadden (2006b) formally described this assemblage, referred to as the Gaillard Cut Local Fauna (L.E, e.g., Tedford et al., 2004), which consists of at least 10 species of carnivores, artiodactyls (also see recent addition of peccary in Kirby et al., 2008), perissodactyls, and as described by Slaughter (1981), rodents. Prior to the current report, the horses (Family Equidae) from the Gaillard Cut L.F consisted of only four fragmentary specimens including: two isolated teeth, i.e., one each of Archaeohippus sp. Gidley, 1906 and Anchitherium clarencei Simpson, 1932; a heavily worn partial dentition with p2-p4 of A. clarenci; and a partial calcaneum of Archaeohippus. Although meager, these fossils appear to represent two distinct taxa of three-toed horses otherwise know from the middle Miocene of North America, i.e., the dwarf-horse Archaeohippus sp. and the larger Anchitherium clarencei. Over the past decade the government of Panama has undertaken two monumental public works projects, including the construction of the new Centenario Bridge across the Gaillard Cut (completed in 2004) and the most extensive expansion of the Panama Canal since the original excavations by the United States in the early twentieth century, the latter of which is scheduled to continue well into the next decade. Both of these projects have yielded an extensive series of new exposures of the Cucaracha Formation. Consequently, new collections of fossil vertebrates are being made and studied as a result of collaborations between the Smithsonian Tropical Research Institute and the University of Florida. In 2008 a well preserved and relatively complete specimen of Anchitherium clarenci was collected that confirms the previous taxonomic identification based on poorly preserved and fragmentary materials (MacFadden, 2006b), while also significantly increasing our understanding of the dental morphology of this species as it is known from Panama. The purpose of this paper, therefore, is to describe this new specimen and discuss its significance with regard to morphology, distribution, and paleoecology of Anchitherium clarencei at the extreme southern extent of the known range of this genus, which is otherwise widely distributed throughout Holarctica (occurrence data were downloaded from The Paleobiology Database [www.paleodb.org] on 30 January, 2009, using the genus name "Anchitherium"). The following institutional abbreviations are used for specimens described in the text: MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts; UF, University of Florida Vertebrate Paleontology Collection, Gainesville, Florida; USNM, United States National Museum, Washington, D. C. The following abbreviations are used for morphological descriptions: APL, greatest anterior-posterior length; CRNHT, crown height of enamel measured at mesostyle; HI, hypsodonty index (CRNHT/APL); L, left; M, upper molar; R upper premolar; R, right; TRN, greatest transverse width.