Equus suessenbornensis Wüst, 1901

Eisenmann, Véra & Kuznetsova, Tatiana, 2004, Early Pleistocene equids (Mammalia, Perissodactyla) of Nalaikha, Mongolia, and the emergence of modern Equus Linnaeus, 1758, Geodiversitas 26 (3), pp. 535-561 : 535-561

Equus suessenbornensis Wüst, 1901 s.l.

During early-middle and middle Pleistocene of the Old World and northwestern America, many fossils point to the existence of a widely distribut- ed but poorly known group, referred here as E. suessenbornensis s.l. The limb bones are best represented at Akhalkalaki, Georgia ( Vekua 1962, 1986; Gabunia & Vekua 1989). The teeth sample of Süssenborn is richer but unfortunately not homogeneous ( Forsten 1986). The same is true for the samples of northeastern Siberia (Chukochya, Adycha) and Yukon (Old Crow). The skull of E. coliemensis (northeastern Siberia) is isolated and certainly too small for E. suessenbornensis Wüst, 1901 s.s. Thus, although very probable, the taxonomical closeness of all these fossils is not absolutely certain.

As noted above, the skull of E. coliemensis ( Fig. 19 ; Annexe: Table 2) shows some similarities with hemiones, but it is much larger. The pattern of the upper cheek teeth of E. coliemensis is very peculiar, with the extremely complicate enamel and wide plis caballins also observed in E. verae and E. suessenbornensis . Inside the Süssenborn, Akhalkalaki, northeastern Siberian and Yukon samples, large lower cheek teeth exhibit at times extremely deep ectoflexids and rather shallow, hemione-like lingual valleys. Moreover, some lower teeth of Yukon, Akhalkalaki and northeasten Siberia display isolated ectostylids – an exceptional feature for monodactyl equids. If we rightly referred the large and robust metapodials of Chukochya to E. verae , and if the teeth resemblances do indicate a close relationship, E. suessenbornensis s.l. had a rather hemione-like skull but unlike-hemione cheek teeth and limb bones and ranged from Beringida to Western Europe.

MOLECULAR BIOLOGY

Biomolecular studies bring additional insight. On the basis of an age of 0.7 Ma for E. mauritanicus ( Geraads et al. 1986) , Oakenfull et al. (2000) calculate an age of about 2 Ma for the emergence of the first Equus . This is considerably less than the frequently cited age of 3.5 Ma (or more), based on the assumption that Plesippus of North America was the ancestor of modern Equus . As noted above, Plesippus and Allohippus were side lineages, not at the origin of modern Equus . Again according to Oakenfull et al. (2000), caballine equids were the first to branch out of the common line, and they did it shortly after the emergence of Equus . That may be so, but there is no sound paleontological proof of that, since the first clearly caballine fossils are younger. The next to branch out would have been the hemiones, possibly about 1 Ma ago. By skull characters, E. coliemensis could be close to the emergence of this group in Eurasia. Still according to Oakenfull et al. (2000), asses, Grevy’s zebras, and mountain zebras diverged two or three hundred thousand years later, and very close to each other, just before the emergence of plains zebras (the reference point in time).

EQUUS NALAIKHAENSIS, E. COLIEMENSIS AND E. APOLLONIENSIS

These last points are very interesting in relation to the osteology of E. nalaikhaensis . We have noted

above in this species a mixture of similarities: the skull has both hemione and zebra characters, some lower cheek teeth resemble extant hemiones, first phalanges at times look like Grevy’s zebras, or like hemiones, or like asses. One explanation for this mosaic would be that E. nalaikhaensis was close in time to the branching out of true hemiones and to the common ancestor of E. africanus and zebras.

The following attempt at more precision is very tentative. If we accept as plesiomorphic the skull proportions of hemiones, asses and all zebras share a wider supra-occipital tuberosity and a lower face. Only zebras share a shorter vomer, and a narrower skull.

E. nalaikhaensis had the narrow skull of zebras but the face was high and the supra-occipital crest narrow. Accordingly, it would best be considered as branching out before the hypothetical common ancestor of asses and zebras but developing skull parallellisms with zebras.

The skull of E. graziosii lacks the occiput so that important points like Franck’s Index and width of the supra-occipital tuberosity are uncertain. Some points are clear: the muzzle is very narrow and constricted and the frontal is wide (like in some extant zebras, in particular E. grevyi ) but the vomer is long like in hemiones and asses. The author of the species ( Azzaroli 1966, 1979) believes E. graziosii to be an ass. If so, it is another case of parallel evolution with zebras.

Unlike the two species above, E. coliemensis shows no zebra-like tendency: the muzzle is rather broad, even at the constriction, and thus resembles asses. The supra-occipital crest, however, is very narrow – more like hemiones. Accordingly, it should be best considered as branching out before the common ass-zebra ancestor, but in a rather ass-like way.

Because of its imperfect preservation, all that can be said about E. apolloniensis skull is that it does belong to an Equus ( Koufos et al. 1997) and that it differs both from E. coliemensis and E. nalaikhaensis .

Figures

FIG. 19. — Ratio diagrams of Equus skulls. Measurements 16, 23, 3, etc. are defined in Annexe (Table 8).