Hippotherium cf. brachypus (Hensel, 1862)

Geraads, Denis, Spassov, Nikolaï, Hristova, Latinka, Markov, Georgi N. & Tzankov, Tzanko, 2011, Upper Miocene mammals from Strumyani, South-Western Bulgaria, Geodiversitas 33 (3), pp. 451-484 : 462

Hippotherium cf. brachypus (Hensel, 1862)

There is only one skull of a young individual, FM- 2326, with deciduous teeth (DP1-DP4), M1 and the tips of M2 ( Fig. 6A ). The region above the cheek teeth, with the left preorbital fossa, is preserved. The orbit is about 7 mm behind M2. The preorbital fossa is oriented antero-ventrally, sub-triangular, and deep. The peripheral rims are well defined, with a slight posterior pocket. The infraorbital foramen is located at the antero-ventral border of the preorbital fossa, above the limit DP2-DP3. The facial crest ends at the limit P4-M1. The choanae reach the middle of M1. Enamel plication of the deciduous teeth is rich, with multiple plis caballins and rounded protocon. M1 is slightly worn, the posterior fossette is still open and the enamel features are not clearly visible. The protocon is elliptical, the pli caballin is simple.

As the specimen is not well-preserved and belongs to a young individual, it is difficult to draw reliable conclusions about its taxonomical appartenance. The species with well-developed, sub-triangular preorbital fossa situated far from the orbit are H. primigenium , H. giganteum , and H. brachypus . The preserved portion of the skull resembles all of them. We can exclude H. primigenium as the rest of the fauna is unlikely of Vallesian age. Hippotheriun brachypus is known from several Turolian localities: Hadjidimovo, Kalimantsi, AkkasdaĞI, Pikermi and Samos ( Hristova et al. 2003; Hristova & Kovachev 2005; Koufos 1987a; Koufos & Vlachou 2005; Vlachou & Koufos 2009). Gromova (1952) described a closely related species, H. giganteum , from the early Turolian of Grebeniki (GREB), Ukraine, but its differences from H. brachypus are not clear. Vlachou & Koufos (2009) think that they are related to the degree of retraction of the nasal slit, but the problem looks more complex. Gromova erected the species H. giganteum on the basis of one juvenile and one adult skulls. From her description, one of the distinctive features of the species is a nasal slit positioned slightly anterior to P2. Gabunia (1959) expanded the diagnosis of the species, based upon this feature, on the basis of eight new skulls from the type locality. According to his observations, the level of the bottom of the nasal slit varies from just before P2 to above its anterior half. Thus, it partly overlaps its range in the sample of H. brachypus from Pikermi (above P2) and Hadjidimovo (usually above P2; only one specimen has a nasal slit above the anterior end of P3). The slightly shorter, more primitive, nasal slit of some specimens from Grebeniki, could be related to the earlier age of this site. The material from the type locality of H. giganteum needs revision but from the recent data, the position of the nasal slit cannot be used to separate H. giganteum from H. brachypus .

Figures

FIG. 6. — Fossil mammals from the Miocene of Strumyani (SW Bulgaria): A, Hippotherium cf. brachypus (Hensel, 1862), lateral view of skull FM-2326; B, Cremohipparion mediterraneum (Roth & Wagner, 1855), lateral view of skull FM-2028;C, Hipparion dietrichi (Wehrli, 1941), lateral view of skull FM-2029. Scale bar: 20 cm.