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Merychippus isonesus tertius

Mammalia - Perissodactyla - Equidae

Taxonomy
Merychippus isonesus tertius was named by Osborn (1918) [named as a subspecies of M. isonesus by Osborn 1918; raised to species level]. Its type specimen is AMNH 14180, a partial skeleton (Palate and complete series of grinding teeth, pLm3, lower jaws and parts of skeleton.), and it is a 3D body fossil.

It was recombined as Merychippus (Protohippus) tertius by Stirton (1940); it was recombined as Acritohippus tertius by Kelly (1995) and Kelly (1998); it was recombined as Merychippus tertius by Hulbert and MacFadden (1991) and MacFadden (1998).

Synonymy list
YearName and author
1918Merychippus isonesus tertius Osborn p. 105 figs. Plates 13.5,6, 14.3, 41.2, 46.2, 52.2. Text Fig. 80
1940Merychippus (Protohippus) tertius Stirton p. 181
1991Merychippus tertius Hulbert, Jr. and MacFadden p. 33
1995Acritohippus tertius Kelly p. 9
1998Acritohippus tertius Kelly
1998Merychippus tertius 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()
infraorderCynodontia()
RankNameAuthor
Epicynodontia
infraorderEucynodontia
Probainognathia
Mammaliamorpha
Mammaliaformes
classMammalia
subclassTribosphenida()
infraclassEutheria()
Placentalia
orderPerissodactyla()
suborderLophodontomorpha
infraorderEuperissodactyla
Hippomorpha
superfamilyEquoidea
familyEquidae
subfamilyEquinaeSteinmann and Döderlein 1890
genusMerychippus
speciesisonesus()
subspeciestertius

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
H. F. Osborn 1918 (Matthew, 1913, Osborn, 1918) (1) This type is slightly larger than the preceding. (2) Crochetuniting
with protoconule in half worn teeth, giving off a prominent internal fold, or pli caballin; (3) protocone a long oval, uniting by a strong interior spur with middle compression of protoconule crescent in half worn teeth; (4) prominent plications of the metaloph entering pre- and postfossettes; (5) rudiments of the second plication. (6) Limb bones as in M. isonesus secundus, but shafts more robust than in the preceding type; (7) shafts of lateral digits as in M. isonesus secundus; (8) ulna fused for a short distance with radius. This type is separable from M. isonesus secundus in the antero-posterior elongation of the protocone. It is a premutation which is closely united to the preceding but shows a slight advance in the pli caballin, in the early union of protoconule and crochet, and the somewhat greater anteroposterior elongation of the protocone. The referred specimen, Amer. Mus. 14183, a topotype, exhibits the manner in which all the enamel plications and folds disappear in much worn crowns.