140 



osborn: oligocene, Miocene, pliocene equid^e. 



GENUS PLIOHIPPUS Marsh 1874. 

 Amer. Jour. Sci. and Arts, (3) Vol. VII, Mar. 1S74, pp. 252-253. 



Genotype. — Pliohippus pernix Marsh, from the Niobrara River, Nebraska, "Nebraska" formation, Proeamelus- 

 Hipparion zone. 



The seventeen species now referred to the Pliohippus stage of evolution, include the Pliohippus (Merychippus) mirabilis 

 Leidy 1858 from the Upper Miocene of the Niobrara River, also P. supremus, P. pernix, P. robustus from the same life zone, 

 namely, the Procamelus-Hipparion zone. To this life zone belong also the P. fossulatus and P. inter pol at us Cope from the 

 Clarendon formation of Texas. Of slightly more recent age is the P. nobilis Osborn from the Republican River, Peraceras 

 zone. There follow geologically the specimens referred to P. interpolatus and P. supremus from Snake Creek, western 

 Nebraska, and the slightly more recent P. tantalus and P. fairbanksi from the Upper Ricardo bordering the Mohave 

 Desert of Southern California. P. coalingensis of the Lower Etchegoin is evidently older than P. simplicidens (J. C. M.). 

 The Middle Pliocene yields the P. spectrins of the Rattlesnake formation of Oregon, and in full Middle Pliocene time there 

 occur the P. simplicidens, P. cumminsii, and P. minutus of the Blanco formation, Glyptotherium zone of Texas. Of the" 

 same or slightly more recent age is the P. proversus of the Upper Etchegoin of San Joaquin Valley, Southern California. 

 The latter species is extremely close to Equus in the crown pattern of its grinding teeth, in fact, the progressive evolution 

 into the Equus molar crown pattern is a most distinctive character of Pliohippus. No less important is the monodacty- 

 lism observed by Marsh in P. pernix and confirmed by Troxell. Thus the Pliohippus stages range gradually into the Equus 

 stage. 



The characters which prevail in these sixteen species and which exhibit marked evolution between Upper Miocene 

 and Middle or Upper Pliocene times are chiefly the following: 



1. Grinding teeth long, hypsodont, fully cemented. 



2. Deciduous premolars short, hypsodont, fully cemented, inner basal cusp rudimentary or absent. 



3. Protocone invariably united with protoloph, progressively larger than hypocone, of primitive round-oval, or 

 progressive elongate anteroposterior oval form, as in Equus'. 



4. Characters of proto- and metaconules, of crochet, and of hypostyle derived from the M erychi ppns-Protohi ppus 

 pattern. 



5. Pre- and postfossettes typically broad, with relatively few plications of the enamel borders (unlike progressive 

 Merychippus and Hipparion), i. e. single pli caballin, pli crochet, pli metaloph, pli hypostyle. 



6. Superior grinders incurved (P. supremus). 



7. Like Merychippus campestris and unlike known species of Protohippus a large malar fossa as well as a more or 

 less distinct lachrymal fossa (P. mirabilis, P. supremus, P. nobilis). 



8. In the lower grinders metaconid-metastylid column widely separate, fusing only at base. 



9. Anteroexternal fold of lower teeth vestigial or absent. 



10. Feet comparatively massive and heavy, ungual phalanges broad and flat. 



11. A horizontal facet for cuboid on Mts. Ill, also a facet for mesocuneiform. 



12. Fore and hind feet monodactyl or solipedal. 



(W. D. M.) The species of Pliohippus in which the skull is known appear to be derived rather from a Merychippus 

 calamarius stage than from any known species of Protohippus, according to Matthew's observation of 1913 on generic stages 

 of Protohippus, Pliohippus, and Hipparion. They appear to represent three more or less parallel groups or phyla. They 

 are distinct in the following features: 



Characters Protohippus Pliohippus Hipparion 



Upper grinders Straight or curved Curved Straight 



Protocone Small Progressively enlarging Progressively enlarging 



Protocone United to protoloph United to protoloph Separated from protoloph 



Protocone Typically round-oval Expanding oval Round or expanding flattened oval 



Enamel fossette borders Rather simple Rather simple, fossettes large Very complex, fossettes reduced 



Metaconid-metastylid column Partly separated Partly separated Widely separated 



Preorbital fossae Lachrymal fossa only Lachrymal and malar fossa? Lachrymal, or lachrymo-malar fossx 



p 0 ot structure Tridactyl Monodactyl or tridactyl Persistently tridactyl 



