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Acritohippus isonesus

Mammalia - Perissodactyla - Equidae

Taxonomy
Hippotherium isonesum was named by Cope (1889) ["Hypotherium"]. Its type specimen is AMNH 8175, a partial skeleton (A nearly complete skull, cervical vertebrre, pelvis and hind limbs), and it is a 3D body fossil.

It was recombined as Hipparion isonesum by Trouessart (1898), Matthew (1899) and Hay (1902); it was recombined as Neohipparion isonesum by Trouessart (1905) and Lull (1907); it was recombined as Merychippus (Merychippus) isonesus by Stirton (1940); it was synonymized subjectively with Merychippus seversus by Downs (1956), Downs (1961), Kelly and Lander (1988) and Fremd (1994); it was recombined as Stylonus isonesus by Kelly and Lander (1988); it was recombined as Acritohippus isonesus by Kelly (1995) and Kelly (1998); it was recombined as Merychippus isonesus by Gidley (1907), Matthew (1909), Osborn (1918), Matthew (1924), Scharf (1935), Wallace (1946), MacFadden and Nelson (1980), Tedford et al. (1987), Hulbert and MacFadden (1991) and MacFadden (1998).

Synonyms
  • Merychippus praecocidens was named by Russell (1933). Its type specimen is NMC 8624, a tooth (right M3), and it is a 3D body fossil. Its type locality is Wood Mountain, which is in a Barstovian terrestrial horizon in the Wood Mountain Formation of Canada.

    It was recombined as Merychippus (Merychippus) praecocidens by Stirton (1940); it was synonymized subjectively with Merychippus isonesus by Storer (1975); it was synonymized subjectively with Stylonus isonesus by Kelly and Lander (1988); it was synonymized subjectively with Acritohippus isonesus by Kelly (1995).
Synonymy list
YearName and author
1889Hippotherium isonesum Cope p. 451
1898Hipparion isonesum Trouessart
1899Hipparion isonesum Matthew
1902Hipparion isonesum Hay
1905Neohipparion isonesum Trouessart
1907Merychippus isonesus Gidley p. 909
1907Neohipparion isonesum Lull p. 179
1909Merychippus isonesus Matthew
1918Merychippus isonesus Osborn p. 101 figs. Plates 6.8, 10.4, 13.1,2, 17.3,4, 42.1,2. Text Figs. 75, 77
1924Merychippus isonesus Matthew
1933Merychippus praecocidens Russell p. 11 figs. 1-6
1935Merychippus isonesus Scharf
1940Merychippus (Merychippus) isonesus Stirton p. 181
1940Merychippus (Merychippus) praecocidens Stirton p. 181
1946Merychippus isonesus Wallace
1980Merychippus isonesus MacFadden and Nelson
1987Merychippus isonesus Tedford et al.
1988Stylonus isonesus Kelly and Lander
1991Merychippus isonesus Hulbert, Jr. and MacFadden
1995Acritohippus isonesus Kelly
1998Acritohippus isonesus Kelly p. 6
1998Merychippus isonesus 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
genusAcritohippus
speciesisonesus()

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(1) Molars comparatively short-crowned, of true Merychippus
type; (2) protocone and hypocone subequal in size, elongate-oval, with internal spurs approaching protoconule and
metaconule respectively; (3) hypocone early joining metaconule; (4) protocone separated half way down the crown; (5) median crest of metaconule and crochet region ptychoid; (6) a pli caballin. (7) Facial region relatively short; (8) a single lachrymo-malar fossa, not well separated.
T. S. Kelly 1998Differs from Acrito- hippus tertius, A. isonesus, and A. quinni by having the following characteristics: (1) larger size (mean UTRL = 134 mm); (2) relative depth of malar fossa greater (ratio of depth to UTRL = 0.06); (3) upper cheek teeth with slightly less complex fossette pli- cations; (4) P2-M 1 protocones connect with proto- lophs in earlier wear stage (before M3 begins to wear); (5) P2-M 2 plis caballin less persistent, usu- ally worn away when M3 in onset of wear (specif- ically, when M3 metaloph and hypocone beginning to wear). Further differs from A. tertius by having the following characteristics: (1) cheek teeth higher crowned; (2) upper cheek teeth with less curvature; and (3) upper molar protocones slightly more an- teroposteriorly elongated. Further differs from A. isonesus and A. quinni by having P2-M 1 hypoconal grooves worn away at earlier wear stage (when M3 in onset of wear). Further differs from A. quinni by having the following characteristics: (1) relative muzzle length longer (UDL = 40-45% of UTRL); and (2) DPOF not pocketed posteriorly. Also see Table 2 for comparison of species.