|Basic info||Taxonomic history||Classification||Relationships|
|Morphology||Ecology and taphonomy||External Literature Search||Age range and collections|
Mammalia - Perissodactyla - Equidae
It was recombined as Merychippus placidus by Hay (1902) and Trouessart (1905); it was recombined as Protohippus (Calippus) placidus by Matthew and Stirton (1930); it was recombined as Calippus (Calippus) placidus by Hulbert (1988) and Prado and Alberdi (1996); it was considered a nomen dubium by Macdonald (1992); it was recombined as Calippus placidus by Stirton (1935), Johnston (1937), Stirton (1940), Quinn (1955), Dalquest and Hughes (1966), Webb (1969), Voorhies (1990), MacFadden (1998) and May (2019).
|Year||Name and author|
|1869||Protohippus placidus Leidy|
|1893||Protohippus placidus Cope p. 26|
|1902||Merychippus placidus Hay|
|1905||Merychippus placidus Trouessart|
|1907||Protohippus placidus Gidley p. 887|
|1918||Protohippus placidus Osborn p. 133 figs. Plate 34.4. Text Figs. 106, 107, 116a|
|1930||Protohippus (Calippus) placidus Matthew and Stirton p. 354|
|1935||Calippus placidus Stirton|
|1937||Calippus placidus Johnston|
|1940||Calippus placidus Stirton p. 188|
|1955||Calippus optimus Quinn p. 35 figs. Pl. 6; Pl. 7, Fig. 1|
|1955||Calippus placidus Quinn p. 39|
|1966||Calippus placidus Dalquest and Hughes|
|1969||Calippus placidus Webb|
|1975||Pseudhipparion optimum Forsten|
|1988||Calippus (Calippus) placidus Hulbert, Jr. p. 234 fig. 9|
|1990||Calippus placidus Voorhies|
|1996||Calippus (Calippus) placidus Prado and Alberdi p. 676|
|1998||Calippus placidus MacFadden p. 550|
|2019||Calippus placidus May|
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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.
|H. F. Osborn 1918|| (Leidy, 1869) (1) Separated from P. perditus as an animal of small stze. (2) Enamel plications in the type and some of the paratypes relatively simple; (3) crowns less curved than in P. perditus; (4) central lakes [fossettes] wide and gaping.
Neotype characters.- (Gidley, 1906, 1907) Based on fragment of skull, Amer. Mus. 10830. (1) Protocones directed well backward, as in P. perditus; (2) enamel borders of fossettes relatively simple; (3) upper molariform teeth of smaller size than in P. perditua, i. e. diameters of crown; (4) of greater comparative length than P. perditus; (5) of less curvature than P. perditua; (6) fossettes in true molars of less transverse width. (7) A slight depression in the malar bone immediately in front of the orbit, otherwise the facial region full and smooth as in P. perditus, (8) the lachrymal fossa shallow
and not sharply defined.
|J. H. Quinn 1955 (Calippus optimus)||Size intermediate to C. anatinus and C. placidus; styles of upper teeth delicate; ribs moderately prominent; fossettes angular with some bifid and tri- fid plications; protocone shorter and hy- poconal groove more open than in C. anatinus and C. placidus; lower molars with reduced and deflected metastylids; me- dian valley of molars deep; parastylid present; pli cahallinid absent; milk teeth high crowned, heavily cemented; parastylid present; hypostylid, protostylid and pli caballinid absent.|
|J. H. Quinn 1955||Largest of the species of Caliprpus; protocone longer and plications heavier than in the other species; upper teeth less tapered than in C. optimus; upper milk teeth wider transversely.|
|R. C. Hulbert 1988||Small equid with toothrow lengths of about 95 to 105 mm; unworn molar crown heights about 51 mm. DPOF long, oval and relatively deep (for a Calippus). Higher-crowned teeth than C. proplacidus. Larger than C. regulus and C. elachistus, with more complex fossettes, deeper linguaflexids, and a larger, moderate-sized DPl.|