Hipparion, de Christol, 1832

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 : 466-468

“ Hipparion ” sp. (small sized)

A distal metatarsal of small size (FM-2814) from Str-1 differs from all other hipparion remains, having the dimensions of Cremohipparion macedonicum and C. matthewi . The close size of the metapodials from these two species and the bad preservation of this metatarsal prevent its exact species affiliation.

Hipparion postcranials are abundant at Str-2, but most of them are deformed or badly preserved. Metacarpals can be divided into two groups ( Fig. 10). The first one includes robust metacarpals that match Hippotherium in size (FM-2035, FM-2296A). The second one consists of slender metacarpals (FM- 2295, FM-2705, FM-2776, FM-2704, FM-2709A, FM-2703) close in size and proportions to H. dietrichi but, as some metacarpals of C. mediterraneum (e.g., from Hadjidimovo and Perivolaki) are of the same length and are as slender, it is hard to provide a definite species identification.

Metatarsals can be sorted into three groups ( Fig. 11). The robust ones (FM-2340A, FM-2722A, and FM-2317) fall within the range of H. brachypus from Pikermi and Hadjidimovo and probably belong to this species; the second group includes long, slender metapodials (FM-2405, FM-2355, FM-2728A, FM-2315); the third one is represented also by slender but slightly shorter metatarsals (FM- 2727, FM-2729A, FM-2077, FM-2085A), but a more or less gradual transition is observed between the three groups. As the skulls of H. dietrichi and C. mediterraneum from Strumyani-2 have similar lengths (380-420 mm for the first and 388 mm for the second species), and as no complete hipparion skeletons has been found at the locality, it is impossible to reach a final conclusion about the specific identity of each individual metatarsal.

The hipparions constrain the age of the locality between the end of the early Turolian and the first half of the middle Turolian. Most significant are the advanced features of C. mediterraneum . The absence of subnasal fossa, the position of its preorbital fossa, slightly shorter nasal notch and the moderately developed enamel plication show affinities with the Pikermi sample and could indicate a close age. However, the presence of H. dietrichi indicates a pre-Pikermian age for Str-2, since this species seems to become extinct before the level of Pikermi.

All the material identified above is from Str-2. The species composition of the hipparion material from Str-1 is probably similar to that of Str-2 and includes several postcranials (metapodials, astragali), plus mandibular and maxillary fragments from young individuals, but is too scarce to reach precise taxonomic conclusions. There are two metacarpals. One is slender (FM-2785), resembling C. mediterraneum , and its measurements are close to the mean value of the Pikermian sample of this species ( Koufos 1987a, b); the second metacarpal (FM- 2784) is robust: its line on the Simpson diagram is more or less parallel to the lines of H. brachypus from Hadjidimovo and Pikermi ( Koufos 1987a, b; Hristova et al. 2003), but it has more slender proximal and distal ends. Metatarsals can be sorted in three groups. The most robust metatarsal (FM- 2786) resembles those of H. brachypus from Pikermi, but with larger antero-posterior diameter of the diaphysis and smaller dimensions of the proximal and distal epiphyses. The second “group” consists of slender metatarsals (FM-2786, FM-2787, FM- 2788), close to the morphotype of H. dietrichi . As a third “group” could be separated the single metatarsal of the small hipparion (FM-2814). The single astragalus (FM-2794) is comparatively small (maximal length 45 mm), close in its dimensions to the astragalus of H. dietrichi from Nikiti 2 (46.3 mm), but larger than the astragali of C. macedonicum (41.5 for “Ravin des Zouaves”-5 and 39.7 mm for Nikiti-2) ( Vlachou & Koufos 2002). The hipparion fauna of Str-1 looks slightly different to the better known fauna of Str-2. The presence of a small hipparion might indicate a difference in age and/or paleoecological conditions in comparison with Str-2.

Order ARTIODACTYLA Owen, 1848