The rich fossil record of the family Equidae (Mammalia: Perissodactyla) over the past 55 MY has made it an icon for the patterns and processes of macroevolution. Despite this, many aspects of equid phylogenetic relationships and taxonomy remain unresolved. Recent genetic analyses of extinct equids have revealed unexpected evolutionary patterns and a need for major revisions at the generic, subgeneric, and species levels. To investigate this issue we examine 35 ancient equid specimens from four geographic regions (South America, Europe, Southwest Asia, and South Africa), of which 22 delivered 87-688 bp of reproducible aDNA mitochondrial sequence. Phylogenetic analyses support a major revision of the recent evolutionary history of equids and reveal two new species, a South American hippidion and a descendant of a basal lineage potentially related to Middle Pleistocene equids. Sequences from specimens assigned to the giant extinct Cape zebra, Equus capensis, formed a separate clade within the modern plain zebra species, a phenotypicically plastic group that also included the extinct quagga. In addition, we revise the currently recognized extinction times for two hemione-related equid groups. However, it is apparent that the current dataset cannot solve all of the taxonomic and phylogenetic questions relevant to the evolution of Equus. In light of these findings, we propose a rapid DNA barcoding approach to evaluate the taxonomic status of the many Late Pleistocene fossil Equidae species that have been described from purely morphological analyses.DNA taxonomy ͉ equid evolution ͉ macroevolution ͉ phylogeny ͉ ancient DNA T he original sequence of horse fossils found in the 1870s by paleontologist Othaniel Charles Marsh, and popularized by Thomas Huxley (1), has been enriched by a large fossil record over the years and has now become one of the most widely known examples of macroevolutionary change (2). The original linear model of gradual modification of fox-sized animals (Hyracothere horses) to the modern forms has been replaced by a more complex tree, showing periods of explosive diversification and branch extinctions over 55 MY (3). The end of the Early Miocene (15-20 MYA) marks a particularly important transition, separating an initial phase of small leafy browsers from a second phase of more diverse animals, exhibiting tremendous body-size plasticity and modifications in tooth morphology (4). This explosive diversification has been accompanied by several stages of geographic extension from North America to the rest of the New and Old Worlds, so that by the end of the Miocene (5 MYA) more than a dozen distinct genera are represented in the fossil record (4) (Astrohippus,

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The taxonomy of the European stenonoid horses is revised using multivariate statistical techniques in this paper. The objectives are: to identify the taxonomic status of this group; to revise the specific and subspecific diagnoses, and the valid taxa; and to analyze their paleoecology and evolution. We conclude that: 1) there are six species among the Plio-Pleistocene European stenonoids: Equus major Depéret, in Delafond and Depéret 1893, ex Boule; Equus livenzovensis Bajgusheva, 1978; Equus stenonis Cocchi, 1867; Equus senezensis Prat, 1964; Equus altidens von Reichenau, 1915; and Equus sussenbornensis Wüts, 1901; 2) There are seven subspecies, three of E. stenonis, two of E. senezensis, and two of E. altidens; 3) There are two monophyletic groups according to the “neighbor-joining method,” one composed of the middle-to-small-size species, and the other of the biggest ones; 4) There is a trend toward decreasing body size through time in stenonoid horses. This trend, that follows the inverse direction of Cope's rule, can be related to the climatic-environmental changes that occurred from the middle Villafranchian to the early Galerian; 5) Large stenonoids were recorded in the middle Villafranchian and were associated with open and dry habitats (steppes), whereas the small ones lived in more closed and wet habitats (savanna-mosaic, woodlands) from late Villafranchian to earliest Galerian.

Evolutionary theory has long proposed a connection between trait evolution and diversification rates. In this work, we used phylogenetic methods to evaluate the relationship of lineage-specific speciation rates and the mode of evolution of body size and tooth morphology in the Neogene and Quaternary radiation of horses (7 living and 131 extinct species). We show that diversification pulses are a recurrent feature of equid evolution but that these pulses are not correlated with rapid bursts in phenotypic evolution. Instead, rapid cladogenesis seems repeatedly associated with extrinsic factors that relaxed diversity bounds, such as increasing productivity and geographic dispersals into the Old World. This evidence suggests that diversity dynamics in Equinae were controlled mainly by ecological limits under diversity dependence rather than rapid ecomorphological differentiation.

The importance of body mass prediction from several cranial, dental and appendicular variables in living Equini are studied. Relationships between the body mass changes and the ecogeographic picture of Equini evolution are also analysed. The metapodial and phalanx variables, particularly antero‐posterior diameters, are better correlated with body mass than cranial variables in living Equini. Large sized species are correlated with cold climates, open habitats and/or soft soils; small ones are correlated with warm climates, more closed habitats and/or hard soils. Pleistocene horses from Europe and Africa follow an evolutionary trend opposite to their North American counterparts, from larger sized species to smaller ones. In South America the pattern of body size is different to those of the other continents. Species of Hippidion reaching large body mass, whereas some species of Equus, E. andium, follow a diminishing trend.