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Merychippus seversus

Mammalia - Perissodactyla - Equidae

Taxonomy
Stylonus seversus was named by Cope (1878). Its type specimen is AMNH 8180, a tooth (isolated upper molar), and it is a 3D body fossil.

It was recombined as Hippotherium seversus by Wortman (1883), Cope (1886) and Cope (1889); it was recombined as Hipparion seversus by Matthew (1889), Trouessart (1898) and Hay (1902); it was recombined as Neohipparion seversus by Trouessart (1905); it was recombined as Merychippus (Merychippus) seversus by Stirton (1940); it was recombined as Pliohippus seversus by Quinn (1987); it was considered a nomen dubium by Macdonald (1992), Kelly (1995) and Kelly (1998); it was recombined as Merychippus seversus by Gidley (1907), Matthew (1909), Osborn (1918), Matthew (1924), Hay (1930), Downs (1956), Downs (1961), Shotwell (1968) and MacFadden (1998).

Synonymy list
YearName and author
1878Stylonus seversus Cope p. 76
1883Hippotherium seversus Wortman
1886Hippotherium seversus Cope
1889Hippotherium seversus Cope p. 457
1889Hipparion seversus Matthew
1898Hipparion seversus Trouessart
1902Hipparion seversus Hay
1905Neohipparion seversus Trouessart
1907Merychippus seversus Gidley
1909Merychippus seversus Matthew
1918Merychippus seversus Osborn p. 101 figs. Plate 13.8, 17.1. Text Fig. 74
1924Merychippus seversus Matthew
1930Merychippus seversus Hay
1940Merychippus (Merychippus) seversus Stirton p. 181
1956Merychippus seversus Downs
1961Merychippus seversus Downs
1968Merychippus seversus Shotwell
1987Pliohippus seversus Quinn
1988Stylonus seversus Kelly and Lander
1998Merychippus seversus MacFadden p. 547

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RankNameAuthor
kingdomAnimalia()
Triploblastica
Nephrozoa
Deuterostomia
phylumChordataHaeckel 1847
subphylumVertebrata
superclassGnathostomata
Osteichthyes()
Sarcopterygii
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
Tetrapoda()
Reptiliomorpha
Anthracosauria
Batrachosauria()
Cotylosauria()
Amniota
Synapsida()
Therapsida()
RankNameAuthor
infraorderCynodontia()
Epicynodontia
infraorderEucynodontia
Probainognathia
Mammaliamorpha
Mammaliaformes
classMammalia
subclassTribosphenida()
infraclassEutheria()
Placentalia
orderPerissodactyla()
suborderLophodontomorpha
infraorderEuperissodactyla
Hippomorpha
superfamilyEquoidea
familyEquidae
subfamilyEquinaeSteinmann and Döderlein 1890
genusMerychippus
speciesseversus()

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.

Diagnosis
ReferenceDiagnosis
E. D. Cope 1878Two superior molar teeth were accompanied by a num- ber of inferior molars as having been all found together, but whether they belong to one individual is uncertain. The dentinal lakes of the superior molar are confluent by the median transverse valley, and increased wear would probably join the posterior pair by their posterior angles. The bor- ders of the cementum lakes are simple, except one or two plicatioins on their opposed adjacent borders, and one at the posterior inner part of the posterior. The internal columns are small, and their sections form two equal ovals with their long axes antero-posterior. The anterior dentinal
lake sends off a narrow loop towards the posterior part of the anterior column. The shaft of the tooth is incurved, and the external face is un- equally divided by the usual ridge. The wide gutters on each side of the latter are uniformly concave, and contain a rather shallow deposit of cementum.
H. F. Osborn 1918 (1) Crown of superior molar moderately elongate [hypsodont], curved; (2) section of anterointernal column [protocone] an elongate oval with an anterior spur directed toward the internal median group [protoconule]; (3) posterointernal column [hypocone] also elongate oval in section, distinct from postero- internal crescent [metaconule]; (4) crescents [protoconule and metaconufe] separated by wide lakes [fossettes] entirely filled with cement; (5) borders of metaconule crescent ptychoid, protoconule crochet junction with pli caballin; (6) ecto- loph without median paracone and metacone.