Cormohipparion and Hipparion (Mammalia, Perissodactyla, Equidae) from the Late Neogene of Florida

Abstract

Five species of Cormohipparion are recognized from the late middle Miocene to late Pliocene (late Barstovian through Blancan) of Florida. A small sample of isolated teeth collected from a late Barstovian horizon of the lower Bone Valley Formation, south-central Florida, represents Cormohipparion sp., cf. C sphenodus. In the Clarendonian, a larger species of Cormohipparion, cf. C occidentale, is represented by three isolated teeth, also from an older horizon of the Bone Valley Formation. The very late Clarendonian Love Site produced numerous dentitions of two species of Cormohipparion, one small-sized and one medium-sized, here referred to C. ingenuum and C. plicatile, respectively. The completeness of the referred material allows for new phylogenetic interpretations concerning these previously poorly known species described by Joseph Leidy over a century ago on isolated teeth. The first appearance of C. ingenuum was in the early Clarendonian of Texas and Florida, and it persisted in Florida until the late early Hemphillian, a duration of about five million years. It is also recorded from the early Hemphillian Gracias Fauna of Honduras. C ingenuum is not the senior synonym of Nannippus lenticularis (Cope), as proposed by some authors, as the two share few characters but size. C plicatile is limited to the late Clarendonian-early Hemphillian of Florida, but is usually the more common of the two at sites where both occur. The range of C. emsliei is extended to the early Hemphillian (ca. 7 Ma), based on its presence at the Moss Acres Racetrack Site. It persisted through the Blancan (to about 2.0 Ma), and is the youngest known representative of the genus in North America. These three species form a monophyletic group, Cormohipparion (Notiocradohipparion) n subgen., united by their relatively very elongated muzzles. This clade was apparently limited to the Gulf Coastal Plain and Central America. Three major trends observed in Florida Cormohipparion over this ten million year period are increasing hypsodonty, increasing enamel complexity (in fossette plications, pli caballins, styles, and pli caballinids), and decreasing depth of the dorsal preorbital fossa.

Hipparion is rarely recorded from the late Miocene of Florida. Referable samples include those from the lower Bone Valley Formation (?Clarendonian), the Love Site, the Moss Acres Racetrack Site, and the Withlacoochee River 4A local fauna (late early Hemphillian). The latter two are among the youngest records of Hipparion from North America. They are all tentatively referred to the medium-sized species, Hipparion tehonense.

Cladistic analysis based on 58 dental and cranial characters suggests the following phylogenetic hypotheses: (1) the Tribe Hipparionini is monophyletic and contains a minimum of six North American genera and several species of uncertain generic affiliation; (2) Hipparion, Merychippus s.s., Nannippus, , and Old World hipparionines form one monophyletic clade, the Hipparion-genus group; (3) Neohipparion, Pseudhipparion, and “Merychippus” coloradense form a second clade, the Neohipparion-genus group, which is the sister taxon of the Hipparion-genus group; (4) "Merychippus" goorisi (MacFadden and Skinner) is the sister taxon to Nannippus + Cormohipparion, but its uniquely derived facial region prevents it from being ancestral to either genus; and (5) Cormohipparion from Florida (Notiocradohipparion) is the sister taxon to C. occidentale + Old World "Cormnohipparion" (i.e. Group 1 hipparionines).