PBDB Taxon

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Protohippus (Calippus)

Mammalia - Perissodactyla - Equidae

Taxonomy

Protohippus (Calippus) was named by Matthew and Stirton (1930) [named as a subgenus of Protohippus by Matthew and Stirton 1930Stirton 1935 [raised to genus level]. It is the type subgenus of Calippini.

It was reranked as Calippus by Johnston (1937), Stirton (1940), Quinn (1955), Forsten (1975), Hulbert (1988), Carroll (1988), Prothero and Schoch (1989), Kelly (1995), Prado and Alberdi (1996), MacFadden (1998) and May (2019).

It was assigned to Protohippus by Matthew and Stirton (1930); to Calippini by Quinn (1955); to Equinae by Forsten (1975); to Equidae by Johnston (1937), Stirton (1940) and Carroll (1988); to Protohippini by Kelly (1995); to Protohippina by Hulbert (1988) and Prado and Alberdi (1996); and to Equini by Prothero and Schoch (1989), MacFadden (1998) and May (2019).

Synonymy list

Year	Name and author
1930	Protohippus (Calippus) Matthew and Stirton
1937	Calippus Johnston p. 905
1940	Calippus Stirton p. 188
1955	Calippus Quinn p. 27
1975	Calippus Forsten p. 41
1988	Calippus Carroll
1988	Calippus Hulbert, Jr. p. 231
1989	Calippus Prothero and Schoch p. 532
1995	Calippus Kelly p. 25
1996	Calippus Prado and Alberdi p. 676
1998	Calippus MacFadden p. 550
2019	Calippus May

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Rank	Name	Author
kingdom	Animalia	()
—	Bilateria
—	Eubilateria	Ax 1987
—	Deuterostomia
phylum	Chordata	Haeckel 1874
subphylum	Vertebrata
superclass	Gnathostomata
—	Osteichthyes	()
subclass	Sarcopterygii	()
subclass	Dipnotetrapodomorpha	(Nelson 2006)
subclass	Tetrapodomorpha	()
—	Tetrapoda
—	Reptiliomorpha
—	Anthracosauria
subclass	Amphibiosauria	Kuhn 1967
—	Cotylosauria	()
—	Amniota
subclass	Synapsida
—	Therapsida	()
infraorder	Cynodontia	()
—	Mammaliamorpha
—	Mammaliaformes
class	Mammalia

Rank	Name	Author
—	Theriamorpha	(Rowe 1993)
—	Theriiformes	()
—	Trechnotheria
—	Cladotheria
—	Zatheria
subclass	Tribosphenida	()
subclass	Theria
—	Eutheria	()
—	Placentalia
—	Boreoeutheria
—	Laurasiatheria
—	Scrotifera
—	Euungulata
—	Panperissodactyla
superorder	Perissodactylamorpha
order	Perissodactyla	()
superfamily	Equoidea
family	Equidae
subfamily	Equinae
tribe	Equini
genus	Protohippus	()
subgenus	Calippus

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

Subg. †Protohippus (Calippus) Matthew and Stirton 1930

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Diagnosis

Reference	Diagnosis
J. H. Quinn 1955	Animals of small size; cheek teeth strongly hypsodont; uppers slightly curved; ectoloph flattened, styles delicate, valleys shallow and ribbed; fos- settes tranversely compressed;· pli proto- loph and pli hypoloph reduced to absent; protocone oval, elongate and directed posterolingually, always connected with protoselene; hypoconal groove open to half the length of the crowns in early . species, closed or nearly closed in late species; lower teeth with large metaconid, metastylid small and progressively re- duced, metaconid and metastylid sep- arated, intervening valley not deep but persistent; parastylid present but narrow and projecting a little forward; no pli caballinid; no protostylid on either milk or permanent teeth.
R. C. Hulbert 1988	Very small to medium-sized equids, with toothrow lengths less than 140 mm. DPOF long, relatively shallow, not pocketed; or absent. Malar fossa absent. Muzzle region greatly expanded (width 45% or greater than UTRL), with relatively large 112/i12. First and second incisors positioned in a straight row of four teeth, with the I3/i3 angled sharply posteriorly. Postcanine diastema relatively short. Cheekteeth moderate to extremely hypsodont. Upper cheekteeth with elongate or oval protocones broadly connected to protoselene (very rarely isolated in early wear), generally simple fossettes with plications absent or limited to early wear-stages, and single, unbranched pli caballins that fade with wear and are typically much stronger on premolars than molars. Lower cheekteeth typically with relatively non-persistent lingual flexids, shallow premolar ectoflexids, and lacking plications (except for pli entoflexids in early wear-stages) and pli caballinids. Deciduous lower premolars lack ectostylids.