HIPPARION. 



173 



GENUS HIPPARION Christol 1832. 

 Ann. Sri. Indust. du Midi dc France, Vol. I, 1S32, p. 215. 



Genotype: II. prostylum Gervais, 1849, from the Miocene of Cueuron, Vaucluse, France. 



Synonyms 1 : Hippotherium, Kaup, 1S33, Hipparitherium Christol, 1847, Neohipparion Gidley, 1903. Genotype, 

 Equus (Hippotherium) gracilis Kaup, from the Pliocene of Epplesheim. Leidy began to use the term Hipparion in 1856; 

 he used Hippotherium in 1859. Cope used the term Hippotherium following Leidy 's usage of 1859, 1883, 1885, etc. 

 Neohipparion whiineyi of the Little White River, South Dakota, Upper Miocene, Procamelus-Hipparion zone. 



Gidley (1903, p. 465) selected the superb type of Hipparion {Neohipparion) whitneyi as the genotype of Neohipparion, 

 which in his revised diagnosis (1907, p. 924) was defined as follows: "Generic characters. Protocone free except at base, 

 as in Hipparion. Protocone comparatively large and usually much expanded anteroposteriorly. Enamel foldings simple 

 as compared with Hipparion, but usually more complex than in Protohippus or Pliohippus. So far as known the facial 

 fossae are never pocketed, nor are their borders sharply defined. The median external basal column present in the lower 

 milk molars as in Hipparion, but shorter and much more expanded anteroposteriorly. Digits, in general, more slender 

 than in //. gracile, and the lateral digits much more reduced." The genus was proposed by him to include practically all 

 the North American species of Upper Miocene horses described by Leidy and Cope under the synonyms // ipparion and 

 Hippotherium. Exceptions noted by the same author (1907, p. 905) are the species from Florida //. venustum, II. ingenuum, 

 and //. plicatile, which resemble more closely the European Hipparion of the //. gracile type. "It seems not improbable," 

 continues Gidley (op. dt., p. 906) "that these species belonged to an American branch of the Hipparion group of the Old 

 World rather than to the Neohipparion group more typical of this continent." In the present revision (Matthew, 1913, 

 Osborn, 1918) it has not been found practicable to maintain a clear line of definition between species belonging to Hip- 

 parion and to Neohipparion although such clear distinction of separate phyla may be established later, in which case 

 Neohipparion will resume its generic rank. 



The very clearly defined and sharply circumscribed stage of equine evolution, known as Hipparion, was first recog- 

 nized in America in 1856 when Leidy described his Hipparion occidental c from the Upper Miocene of the White River, South 

 Dakota, now in the Procamelus-Hipparion zone. Since then twenty-one additional species, more or less distinct, have 

 been named, distributed geologically through the entire Pliocene and possibly extending into early Pleistocene time in 

 Florida. This equine type dies out in the Pliocene of Europe, but is recorded by Pomel in the early Pleistocene of Africa. 

 It is thus believed to be more long-lived than either Protohippus or Pliohippus-. 



A highly distinctive character is the entire separation of the protocone as an independent column from the protoconule, 

 a feature foreshadowed in certain species of Merychippus, in which the protocone is separated nearly half way down the 

 crown. Combined with this highly special character and with the usually extreme enamel plications of the grinding teeth, 

 is the conservative retention of the lateral digits, Hipparion being persistently anisotridacty] so far as this form is known 

 at present. 



The progressive evolution of Hipparion is especially witnessed in the elongation and plication of the crowns of the 

 grinding teeth. The genus is certainly polyphyletic, there being positive evidence of three parallel phyla. 



The characters which appear to be common to the twenty-two known species of Hipparion are the following: 



1. Grinding teeth long, hypsodont, fully cemented, deciduous premolars short, hypsodont, fully cemented, inner 

 basal cusp prominent (unlike Pliohippus in which the inner basal cusp is rudimentary or absent). 



2. Protocone round-oval to flattened, completely separate to base of crown; in primitive forms giving off a spur in the 

 direction of the protoconule; protocone enlarged, hypocone reduced, as in Pliohippus and Equus. 



3. Characters of proto- and metaeonules, of crochet, of hypostyle, founded on the Merychippus pattern. 



4. Enamel borders of fossettes progressively and complexly plicated, with numerous foldings in addition to the 

 duplication and triplication (//. plicatile) of the pli caballin, pli crochet, pli hypostyle, etc. 



5. In the lower grinders the metastylid column widely separated from the metaconid column to the base of the crown, 

 not fused half way down as in Pliohippus. 



1 Palmer, T. S. "Index Generum Mammalium," U. S. Dept. Agric, Jan. 23, 1904, pp. 9S4. 



