Vera Eisenmann

E. mexicanus, Introduction and References

Sunday 19 April 2020 by Véra Eisenmann

Hibbard (1955) described as Equus Hesperohippus mexicanus a cranium from Tajo de Tequixquiac (No. 48 (HV-3) Museo Nacional de Historia Natural now catalogued as IGM 4009. He considered its age as Late Pleistocene.

Because of the size of its cranium E. mexicanus has currently been considered as synonym with any and all large North American species in particular E. scotti and E. occidentalis whose crania are well documented (see detailed discussion in Alberdi et al. 2014). However, their cranial morphologies differ from that of E. mexicanus and differ from each other (see the ratio diagrams).
Winans (1989) includes E. mexicanus in her E. laurentius group. Whatever its status of fossil or not (Scott et al. 2010), the cranium of E. laurentius is that of a caballine very lose to E. missi (ratio diagram). E. scotti is also a caballine while E. mexicanus is certainly not. Hibbard rightly stressed this point which seems somewhat to have escaped proper notice.

The upper cheek teeth are rather worn. Surprisingly they have caballine characters : long protocones, grooved styles, occurence of plis caballins.

There are no associated material but Hibbard referred to E. mexicanus a right ramus, n° 401 (1). The ectoflexid (vestibular valley) is shallow on all teeth; the stem of the double boucle is notably high on most; the double knot varies in shape, rather caballine on the M1, not at all on the other teeth.

No limb bones ar known.

From the Late Pleistocene of Cedral, Central Mexico, Alberdi et al. (2014) have described three species of equids. The largest, referred to as E. mexicanus, is represented by upper and lower cheek teeth and limb bones. The enamel pattern of both illustrated uppers and lowers is very different from E. mexicanus from Tequixquiac.

Fossils possibly belonging to E. mexicanus.
1. FC 678, cranial fragment from Cedazo (Mooser et Dalquest 1975, p. 806).
2. LACM 12391, a mandible from Barranca del Muerto (Azzaroli 1998).
As in the cheek teeth figured by Hibbard the stem of the double boucle is notably high. Infundibula are present on I1 and I2, absent on I3. By its size and proportions this mandible fits well with the type cranium.

References

Alberdi M.T, Arroyo-Cabrales J., Marin-Leyva A.H., Polaco O.J., 2014. Study of Cedral Horses and their place in the Mexican Quaternary. Revista Mexicana de Ciencias Geologicas, v. 31, num. 2, p. 221-237.

Azzaroli A., 1998. The Genus Equus in North America. The Pleistocene species. Palaeontographia Italica, 85 : 1-60, 21 pl.

Hibbard C. 1955. Pleistocene vertebrates from the Upper Becerra (Becerra Superior) Formation, Valley of Tequixquiac, Mexico, with notes on other Pleistocene forms. Contributions of the Museum of Paleontology of the University of Michigan, 12, 47-96.

Mooser, O., Dalquest, W.W., 1975. Pleistocene mammals from Aguascalientes, Central Mexico: Journal of Mammalogy, 56(4), 781-820.

Scott, E., Stafford T.W.Jr., Graham R.W., MARTIN L.D., 2010. Morphology and Metrics, Isotopes and Dates: Determining the Validity of Equus laurentius Hay, 1913. Journal of Vertebrate Paleontology, 30(6):1840-1847.

Winans M.C., 1985. - Revision of North American fossil species of the genus Equus (Mammalia : Perissodactyla : Equidae). Dissertation, Univ. of Texas : 264pp., 26 fig., tables, Austin.

WINANS, M. C. 1989. A quantitative study of North American fossil species of the genus Equus, 263-297. In D. R. Prothero and R. M. Schoch (eds.), The Evolution of Perissodactyls. Oxford University Press, Oxford, USA.

E. missi Myssy, Introduction (Français, English)

E. occidentalis E. occidentalis, Introduction (English)

E. scotti E. scotti, Introduction


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