A New Genus of Horse from Pleistocene North America
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A new genus of horse from Pleistocene North America The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Heintzman, P. D., G. D. Zazula, R. D. MacPhee, E. Scott, J. A. Cahill, B. K. McHorse, J. D. Kapp, et al. 2017. “A new genus of horse from Pleistocene North America.” eLife 6 (1): e29944. doi:10.7554/ eLife.29944. http://dx.doi.org/10.7554/eLife.29944. Published Version doi:10.7554/eLife.29944 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:34493380 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA RESEARCH ARTICLE A new genus of horse from Pleistocene North America Peter D Heintzman1,2*, Grant D Zazula3, Ross DE MacPhee4, Eric Scott5,6, James A Cahill1, Brianna K McHorse7, Joshua D Kapp1, Mathias Stiller1,8, Matthew J Wooller9,10, Ludovic Orlando11,12, John Southon13, Duane G Froese14, Beth Shapiro1,15* 1Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, United States; 2Tromsø University Museum, UiT - The Arctic University of Norway, Tromsø, Norway; 3Yukon Palaeontology Program, Government of Yukon, Whitehorse, Canada; 4Department of Mammalogy, Division of Vertebrate Zoology, American Museum of Natural History, New York, United States; 5Cogstone Resource Management, Incorporated, Riverside, United States; 6California State University San Bernardino, San Bernardino, United States; 7Department of Organismal and Evolutionary Biology, Harvard University, Cambridge, United States; 8Department of Translational Skin Cancer Research, German Consortium for Translational Cancer Research, Essen, Germany; 9College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, United States; 10Alaska Stable Isotope Facility, Water and Environmental Research Center, University of Alaska Fairbanks, Fairbanks, United States; 11Centre for GeoGenetics, Natural History Museum of Denmark, København K, Denmark; 12Universite´ Paul Sabatier, Universite´ de Toulouse, Toulouse, France; 13Keck-CCAMS Group, Earth System Science Department, University of California, Irvine, Irvine, United States; 14Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada; 15UCSC Genomics Institute, University of California, Santa Cruz, Santa Cruz, United States *For correspondence: [email protected] (PDH); [email protected] (BS) Competing interests: The Abstract The extinct ‘New World stilt-legged’, or NWSL, equids constitute a perplexing group authors declare that no of Pleistocene horses endemic to North America. Their slender distal limb bones resemble those of competing interests exist. Asiatic asses, such as the Persian onager. Previous palaeogenetic studies, however, have suggested Funding: See page 13 a closer relationship to caballine horses than to Asiatic asses. Here, we report complete mitochondrial and partial nuclear genomes from NWSL equids from across their geographic range. Received: 26 June 2017 Although multiple NWSL equid species have been named, our palaeogenomic and morphometric Accepted: 02 November 2017 Published: 28 November 2017 analyses support the idea that there was only a single species of middle to late Pleistocene NWSL equid, and demonstrate that it falls outside of crown group Equus. We therefore propose a new Reviewing editor: George H genus, Haringtonhippus, for the sole species H. francisci. Our combined genomic and phenomic Perry, Pennsylvania State approach to resolving the systematics of extinct megafauna will allow for an improved University, United States understanding of the full extent of the terminal Pleistocene extinction event. Copyright Heintzman et al. DOI: https://doi.org/10.7554/eLife.29944.001 This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use Introduction and redistribution provided that The family that includes modern horses, asses, and zebras, the Equidae, is a classic model of macro- the original author and source are ~ credited. evolution. The excellent fossil record of this family clearly documents its 55 million year evolution Heintzman et al. eLife 2017;6:e29944. DOI: https://doi.org/10.7554/eLife.29944 1 of 43 Research article Genomics and Evolutionary Biology eLife digest The horse family – which also includes zebras, donkeys and asses – is often featured on the pages of textbooks about evolution. All living horses belong to a group, or genus, called Equus. The fossil record shows how the ancestors of these animals evolved from dog-sized, three-toed browsers to larger, one-toed grazers. This process took around 55 million years, and many members of the horse family tree went extinct along the way. Nevertheless, the details of the horse family tree over the past 2.5 million years remain poorly understood. In North America, horses from this period – which is referred to as the Pleistocene – have been classed into two major groups: stout-legged horses and stilt-legged horses. Both groups became extinct near the end of the Pleistocene in North America, and it was not clear how they relate to one another. Based on their anatomy, many scientists suggested that stilt-legged horses were most closely related to modern-day asses living in Asia. Yet, other studies using ancient DNA placed the stilt-legged horses closer to the stout-legged horses. Heintzman et al. set out to resolve where the stilt-legged horses sit within the horse family tree by examining more ancient DNA than the previous studies. The analyses showed that the stilt- legged horses were much more distinct than previously thought. In fact, contrary to all previous findings, these animals actually belonged outside of the genus Equus. Heintzman et al. named the new genus for the stilt-legged horses Haringtonhippus, and showed that all stilt-legged horses belonged to a single species within this genus, Haringtonhippus francisci. Together these new findings provide a benchmark for reclassifying problematic fossil groups across the tree of life. A similar approach could be used to resolve the relationships in other problematic groups of Pleistocene animals, such as mammoths and bison. This would give scientists a more nuanced understanding of evolution and extinction during this period. DOI: https://doi.org/10.7554/eLife.29944.002 from dog-sized hyracotheres through many intermediate forms and extinct offshoots to present-day Equus, which comprises all living equid species (MacFadden, 1992). The downside of this excellent fossil record is that many dubious fossil equid taxa have been erected, a problem especially acute within Pleistocene Equus of North America (Macdonald et al., 1992). While numerous species are described from the fossil record, molecular data suggest that most belonged to, or were closely related to, a single, highly variable stout-legged caballine species that includes the domestic horse, E. caballus (Weinstock et al., 2005). The enigmatic and extinct ‘New World stilt-legged’ (NWSL) forms, however, exhibit a perplexing mix of morphological characters, including slender, stilt-like dis- tal limb bones with narrow hooves reminiscent of extant Eurasian hemionines, the Asiatic wild asses (E. hemionus, E. kiang)(Eisenmann, 1992; Eisenmann et al., 2008; Harington and Clulow, 1973; Lundelius and Stevens, 1970; Scott, 2004), and dentitions that have been interpreted as more con- sistent with either caballine horses (Lundelius and Stevens, 1970) or hemionines (MacFadden, 1992). On the basis of their slender distal limb bones, the NWSL equids have traditionally been consid- ered as allied to hemionines (e.g. Eisenmann et al., 2008; Guthrie, 2003; Scott, 2004; Skinner and Hibbard, 1972). Palaeogenetic analyses based on mitochondrial DNA (mtDNA) have, however, con- sistently placed NWSL equids closer to caballine horses (Barro´n-Ortiz et al., 2017; Der Sarkissian et al., 2015; Orlando et al., 2008, 2009; Vilstrup et al., 2013; Weinstock et al., 2005). The current mtDNA-based phylogenetic model therefore suggests that the stilt-legged morphology arose inde- pendently in the New and Old Worlds (Weinstock et al., 2005) and may represent convergent adaptations to arid climates and habitats (Eisenmann, 1985). However, these models have been based on two questionable sources. The first is based on 15 short control region sequences (<1000 base pairs, bp; Barro´n-Ortiz et al., 2017; Weinstock et al., 2005), a data type that can be unreli- able for resolving the placement of major equid groups (Der Sarkissian et al., 2015; Orlando et al., 2009). The second consist of two mitochondrial genome sequences (Vilstrup et al., 2013) that are either incomplete or otherwise problematic (see Results). Given continuing uncertainty regarding the phylogenetic placement of NWSL equids—which impedes our understanding of Pleistocene equid evolution in general—we therefore sought to resolve their position using multiple mitochondrial and Heintzman et al. eLife 2017;6:e29944. DOI: https://doi.org/10.7554/eLife.29944 2 of 43 Research article Genomics and Evolutionary Biology partial nuclear genomes from specimens representing as many parts of late Pleistocene North Amer- ica as possible. The earliest recognized NWSL equid fossils date to the late Pliocene/early Pleistocene (~2–3 mil- lion years ago, Ma) of New Mexico