Systematic, stratigraphic, and paleoenvironmental contexts of first-appearing Hipparion in the Vienna Basin, Austria

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Abstract
The tridactyl horse Hipparion (s.l.) has long attracted the attention of paleontologists and geologists alike for its potential use as a stratigraphic and paleoenvironmental index. A central dogma surrounding this horse's record is that it first occurred abruptly and instantaneously throughout the Old World ca. 12.5 Ma, heralding a general late Miocene environmental shift from forest to savanna ecosystems. We present data on the oldest known Central European hipparions, “Hipparion” primigenium (s.s.), that bear on these issues. Our analysis of the local Vienna Basin stratigraphie sequence and Paratethys geochronology suggests that hipparion's first provincial occurrence was ca. 11.0–11.5 Ma. We falsify the hypotheses of an Old World “Hipparion Datum” and the purported initial association of this horse with savanna habitats. An explicit cladistic analysis of “Hipparion” primigenium (s.s.), its North American sister taxon Cormohipparion occidentale, and several Old World primitive hipparions (“Group 1”) suggests that several distinct lineages may have been derived from “Hipparion” primigenium (s.s.). We attribute trans-Eurasian and North African speciation of “Group 1” horses to provincial environmental change which fragmented species ranges (environmental vicariance), and adaptations to newly emergent ecosystems.