Hipparion brachypus Hensel, 1862

Koufos, George D. & Vlachou, Theodora D., 2005, Equidae (Mammalia, Perissodactyla) from the late Miocene of Akkaşdağı, Turkey, Geodiversitas 27 (4), pp. 633-705 : 638-648

Hipparion brachypus Hensel, 1862

LOCALITY. — Akkaşdağı, near Keskin, Turkey.

AGE. — Middle Turolian, MN 12 (late Miocene).

MATERIAL. — Skull associated with the mandible, AKK-147; hemimandible with i1-m2 dex, AK6-23; mandible with p2-m1 dex and sin, AK3-57; mandibular fragment with p2-m3 sin, AK3-175; mandibular fragment with p2-p4 dex, AK11-138; mandibular fragment with p3-m3 sin, AK4-245; mandibular fragment with p4-m1 dex, AK4-127; mandibular fragment with p2-p3, dp4-m2 dex, AK7- 80; 2 mandibular fragments with p2-m3 dex, AK5a- 329, AK2-107; mandibular fragment with p3-m2 sin, AK12-9; 7 distal parts of humerus, AK3-10, 257, AK5-465, 536, AK12-26, 28, 29; radius, AK6-233; radius+carpals+McII+McIII+McIV, AK11-98a-l; 2 distal parts of radius, AK2- 391, AK 5-341; 2 McII+McIII+McIV, AK4-5, AK12-34; 4 McIII, AK2-97, AK4-149, AK5-75, 475; 2 proximal parts of McIII, AK4-151, AK5-395; distal part of McIII, AK3-6; 7 distal parts of tibia, AK2-20, AK4-154, 217, AK5-557, AK6-15, AK7-48, AK11-46; 10 astragali, AK2-26, 27, AK3-269, AK4-17, 29, AK5-13, 100, 472, AK10-14, AK11-54; 2 calcanei, AK5-174, AKK-172; MtII+MtIII+MtIV, AK11-37a-c; 8 third metatarsals, AK3-87, 276, AK4-95, AK5-158, 480, 564, AK7-135, AK11-135; 4 distal parts of MtIII, 279, 282, AK4-169, 219; 3 first phalanges, AK2-425, AK3-288, AK4-13; 4 second phalanges AK5-n.n., 119, 163, 416; 4 third phalanges, AK5-166, 171, AK6-43, 162.

DESCRIPTION

Skull and dentition

The studied skull belongs to a relatively young individual, preserving the dP4. The P2 and P3 are just rised without any attrition, while the M3 is almost inside the bone ( Fig. 3 ). The skull is very long and relatively narrow. The muzzle is very elongated and relatively broad. The narial opening is deep and narrow. Its posterior border is situated far from the orbit, above the middle of the P2 ( Fig. 3A ). The preorbital fossa is subtriangular, large and deep, anteroposteriorly oriented with a deep posterior pocketing, and placed far from the orbit. The anterior rim is not well expressed, and is placed approximately above the anterior part of the P2. The posterior one is situated above the anterior border of the M2. The ventral rim is straight, parallel to the toothrow and at the same line with the lower border of the orbit. The infraorbital foramen is situated slightly inferior to the anteroventral border of the fossa ( Fig. 3A ). The orbit is rounded and its anterior border is situated above the posterior part of the M3. The facial crest is quite strong; it is developed far from the alveoli and the ventral rim of the preorbital fossa (POF), while its anterior bor- der is situated above the anterior half of the dP4. The palate is elongated, wide and shallow. The index m2 × 100/m13 is 202. The correspondence index for H. brachypus of Pikermi is 221.5 on average, while in H. mediterraneum from Pikermi with shorter palate it is 183 on average ( Koufos 1987a). The choanae, although they are damaged, seem to be wide with their anterior border situated at the level of the contact between M1 and M2. The upper toothrow is almost complete including the dP1, P2-3-dP4-M1-2 and it seems to be quite long. The P2, 3 are unworn and their enamel morphology is not visible. The dP4 is quadrangular with moderate enamel plication in the fossettes. The protocone is elliptical and isolated. The hypocone is elliptical with relatively shallow distal hypoconal groove. The pli caballin is single and short. The M1 is worn enough to see its morphology. The enamel plication is rich (more than 17 plis) with deep plis. The protocone is large and elliptical, and the pli caballin is double. The distal hypoconal groove is deep while the lingual hypoconal groove is absent.

The mandible is elongated and high with long and relatively narrow snout ( Fig. 4A, B ). The index m2 × 100/m7 is 259 for AKK-147 versus 254 for H. brachypus of Pikermi, 213 for H. mediterraneum of Pikermi with shorter snout, and 167 for H. dietrichi of Prochoma (Axios Valley, Greece) with short and wide snout ( Koufos 1987a, b). The horizontal ramus is relatively short. The symphysis is relatively short and narrow. The toothrow is elongated and the teeth are large and wide ( Fig. 4C ). The parastylid is well developed and closed. The metaconid is rounded, the metastylid is elliptical to rounded and the entoconid elliptical. The ectoflexid is shallow in the premolars, reaching the middle of the tooth while in the molars it is long and narrow reaching the linguaflexid. The last is shallow and V-shaped in the premolars, while in the molars it is U-shaped. A pli caballinid is sometimes present (AK6-23). It is stressing that the specimen AK6-23 seems to have short muzzle but the dental size and morphology, as well as the toothrow length are similar to the studied forms.

Postcranials

The metapodials are short and robust ( Fig. 5 ). The mean index Length McIII × 100/Length radius is 78.4 indicating a relatively short metacarpal. The absence of complete tibia cannot allow the calculation of the corresponding index for MtIII. The robusticity index (m11 × 100/m1) is 18.7 for McIII and 16.2 for MtIII indicating robust metapodials. The keel index ( Sen et al. 1978) is 121.1 for McIII and 128.4 for MtIII indicating that the keel is not very prominent. For comparison, H. primigenium with short and robust metapodials has the following indices respectively: McIII: 15.3 and 122.8; MtIII: 17.1 and 115.7 (data from Bernor et al. 1997).

COMPARISONS

The large-sized skull from Akkaşdagwı shares the same morphological pattern with H. brachypus , H. giganteum ( Gromova 1952) and H. gettyi ( Bernor 1985) . All are known from the Turolian of the Eastern Mediterranean region and a comparison with these species is useful to determine the studied material.

Hipparion brachypus is known from Pikermi and it is characterized by large size, elongated skull with relatively wide muzzle and deep narial opening. The preorbital fossa is oval, anteroposteriorly oriented, well marked, frequently deeply posteriorly pocketed and situated far from the orbit. The upper cheek teeth are highly plicated ( Koufos 1987a).

The skull morphology of the large-sized hipparion from Akkaşdagwı (AKK-147) is similar to that of H. brachypus from Pikermi. The unworn teeth lack morphology, but the little worn M1 indicates rich enamel plication with deep plis, like in H. brachypus . Figure 6 contrasts the proportions of the large-sized skull (AKK-147) with H. brachypus specimens from Pikermi and Hadjidimovo ( Bulgaria) and indicates that the three skull forms are almost identical. Comparatively to H. brachypus from Pikermi, the muzzle of AKK-147 is longer and the facial height larger (measurements 1, 25 in Fig. 6 ). However, AKK-147 belongs to a young individual (M3 is almost inside the bone) and consequently both measurements are probably unreliable. Additionally, the facial height depends strongly upon the preservation of the skull, and a slight deformation leads to different values. The premolar length (measurement 7 in Fig. 6 ) is also longer, but this is possibly due to the low attrition of the teeth. Not much more can be said for the contrast between AKK-147 and H. brachypus from Hadjidimovo except that the later one appears to have narrower snout ( Hristova et al. 2003) (measurements 14, 15 in Fig. 6 ).

The skull morphology of H. gettyi and H. giganteum is also very close to that of H. brachypus and to the Akkaşdagwı skull AKK-147. Hipparion gettyi is known from the Lower Maragha. The type specimen (KNHM RLB 8401) is a medium-sized skull with deep preorbital fossa (POF), triangular-shaped, posteriorly pocketed, anteroposteriorly oriented and placed far from the orbit and facial crest. The slender snout and the remarkably short facial length (measurements 1, 31 in Fig. 6 ) distinguish this species from H. brachypus and AKK-147 ( Fig. 6 ). Additionally, the premolar length is significantly shorter reinforcing the idea about a medium-sized form, smaller than that of H. brachypus and AKK-147. The upper cheek teeth seem to be less plicated and the plis are not so deep. On the other hand, H. giganteum from Grebeniki proportionally resembles the studied form, but the facial region seems to be shorter. Gromova (1952) described H. giganteum as a large-sized hipparion with single, elliptical and deep POF, strongly posteriorly pocketed and placed far from the orbit and facial crest. There is no canine fossa, but in its place there is an elongated groove, connecting the preorbital fossa with the buccinator one. The last character is also observable in some slightly deformed specimens of H. brachypus from Pikermi and Hadjidimovo, rendering this character unreliable for species determination.

A large-sized skull (AMNH 22838) from Samos Q4 was described by Sondaar (1971) as H. cf. proboscideum . This skull has a single deep and not extended preorbital fossa ( Sondaar 1971), being in that different from the type skull of H. proboscideum that is characterized by double and extremely deep preorbital fossa. This is probably the reason why Sondaar (1971: 431) wrote: “AMNH 22838 […s with considerable doubt referred to H. proboscideum ”. The morphology of the preorbital fossa is similar to that of the Akkaşdagwı skull. Furthermore, the proportions of this skull are very close to those of H. brachypus from Pikermi and Hadjidimovo ( Fig. 6 ). The distance vomer-basion (measurement 4 in Fig. 6 ), however, seems to be slightly longer than that of H. brachypus . This measurement is estimated for the studied skull, and for this reason it is given with a question mark. The comparatively shorter premolar length of AMNH 22838 is possibly due to the relatively advanced stage of wear, while the facial height is doubtfully measured as the skull is dorsoventraly compresed, hence the question mark. Recently, a partly preserved skull (MTLB- 30) with strong morhological similarities to H. brachypus , was found in “Mytilinii-B” (MTLB), Samos locality. Several specimens from Samos with morphology similar to that of H. brachypus exist in the collections of some European museums (Weimar, Lausanne, München). These observations suggest the possible presence of H. brachypus in the Samos fauna, dated to about 7.5 Ma (Q4) to 7.1 Ma (MTLB), being in agreement with the age (7.1 Ma) of the Akkaşdagw ı locality ( Kostopoulos et al. 2003; Karadenizli et al. 2005).

The metapodials of the studied form appear to be similar to those of H. brachypus from Pikermi, but they are larger. This difference is well reflect- ed in the PCA of MCIII and MTIII ( Fig. 7 ). The robusticity index of MCIII and MTIII (m11 × 100/m1) is 18.7 and 16.2 versus 17.9 and 16 for H. brachypus from Pikermi respectively, indicating that the Akkaşdagwı sample proportionally resembles H. brachypus from Pikermi.

The metacarpal of H. brachypus from Hadjidimovo is similar to the Pikermi one, but it differs in having stronger distal articular facet as on the Akkaşdagwı metacarpal (m11, m12, m13, m 14 in Fig. 8 ). The metacarpal from Samos Q4 referred to “ H. cf. proboscideum ” ( Sondaar 1971) is generally larger than that from Pikermi, and close to the Akkaşdagwı sample in its length, the midshaft width and the depth of the distal articular keel. On the other hand, they sharply differ in their reduced mid-shaft depth, proximal and distal articular dimensions ( Fig. 8 ). The sample from Samos Q1 differs from Akkaşdagwı one in being slightly shorter and in having narrower proximal articular depth (m6) and narrower distal supra-articular and articular breadth (m10, m11). Since the large-sized skull from Samos Q4 (AMNH 22838) is closer to H. brachypu s, there is a great possibility for the metacarpals from Q4 to belong to the same species. Similarly, the largesized form from Kemiklitepe A-B, that had been referred as Hipparion sp. with affinities to H. proboscideum (Koufos F Kostopoulos 1994) , must also belong to H. brachypus since its metacarpals are similar to “ H. cf. proboscideum ” from Q4 ( Fig. 8 ). The metacarpals from Polgardi and Grebeniki, referred to H. brachypus by Eisenmann (1995), are basically similar each other and significantly smaller than all H. brachypus samples ( Fig. 8 ).

On the contrary, the metatarsal of H. brachypus giganteum ( Eisenmann 1995) from Grebeniki ( Ukraine) is closer to H. brachypus group ( Fig. 9 ). It tracks closely the third metatarsal from Hadjidimovo and both differ from Pikermi in having larger proximal and distal articular dimensions ( Fig. 9 ). The Akkaşdagwı metatarsal is striking by its larger size comparatively to that of Pikermi and diverges from Hadjidimovo and Grebeniki samples in having deeper proximal articular facet and larger distal articular dimensions (m10, m11, m 12 in Fig. 9 ). The metatarsal of “ H. cf. proboscideum ” from Samos Q4 provides similar results as the metacarpal, being closer to the Akkaşdagwı one, but more robust ( Fig. 9 ). The sample from Kemiklitepe A-B ( Turkey) includes two incomplete metatarsals, KTA-577 (proximal part) and KTA-579 (distal part) both described as Hipparion sp. (Koufos F Kostopoulos 1994). The distal part seems to be closer to the Akkaşdagwı metatarsal, but slightly larger (probably due to the scanty material), while the proximal part is better compared to H. mediterraneum , the smaller hipparion form of KTA-B locality ( Fig. 9 ). Therefore, contrary to the initial suggestion, the two metatarsals from KTA-B possibly belong to H. brachypus (KTA-579) and to H. mediterraneum (KTA-577).

The astragalus dimensions of the Akkaşdagwı form are also larger than those of Pikermi confirming once more the size difference between the two samples. The significantly larger tuber calcaneum (measurements C4, C 5 in Fig. 10 ), compared to H. brachypus from Pikermi, and the larger dimensions of the distal part of the calcaneum (measurements C6, C 7 in Fig. 10 ), which is possibly correlated to the size and body weight of the animal (Vlachou F Koufos 2002), reinforce the size superiority of the studied form.

The correlation of the first and second phalanx of the Akkaşdagwı sample to those of H. brachypus from Pikermi and Hipparion sp. from KTA-B is depicted in Figure 11 . The first phalanx of the studied form compared to H. brachypus from Pikermi is longer, its diaphysis narrower, its supra differences

Log

1 3 4 5 6 10

11 12 13 14 7 8

articular width larger, and its distal articular depth smaller ( Fig. 11 ). Compared to Hipparion s p. f r o m K T A -B t h e first p h a l a n x f r o m Akkaşdagw ı is shorter, and it has larger supra articular width ( Fig. 11 ). The two samples, however, seem to be close to each other and could belong to the same species. The second phalanx of the studied species is slightly longer and more slender than that of H. brachypus from Pikermi ( Fig. 11 ), while the second phalanx of Hipparion sp. from Kemiklitepe has closer proportions to the Akkaşdagwı sample, but it has significantly shorter anterior length (measurement PHII 2 in Fig. 11 ).

The front and hind leg of the studied hipparion are compared in Figure 12 . The Akkaşdagwı form is generally slightly larger than H. brachypus from Pikermi with significantly broader distal epiphysis in the metapodials. This fact correlated to the slightly larger skull ( Fig. 3 ) indicates that the Akkaşdagwı form is larger than the typical H. brachypus from Pikermi although its proportions as well as the skull and dental morphology are similar. Consequently, the Akkaşdagw ı form can be determined as H. brachypus . The similarity of some large-sized forms from Samos Q4 ( Greece) and Kemiklitepe ( Turkey) with the Akkaşdagwı H. brachypus indicates that this species must also be present in these localities.

Figures

FIG. 3. — Hipparion brachypus, Akkaşdağı, Turkey, skull AKK-147; A, lateral view; B, dorsal view; C, occlusal view. Scale bar: 2 cm.

FIG. 4. — Hipparion brachypus, Akkaşdağı, Turkey, mandible AKK-147; A, lateral view; B, dorsal view; C, left toothrow, occlusal view. Scale bars: 2 cm.

FIG. 5. — Hipparion brachypus, Akkaşdağı, Turkey; A, third metacarpals, AK5-75, AK5-5a, AK11-98k; B, third metatarsals, AK11- 371, AK11-37b. Scale bars: 2 cm.

FIG. 6. — Logarithmic ratio diagram comparing the skull of Hipparion brachypus from Akkaşdağı with H. brachypus, H. cf. proboscideum, H. gettyi and H. giganteum from various localities. Standard H. mediterraneum, Pikermi, n = 1-9 (Koufos 1987a).

FIG. 7. — PCA plots giving the relations of the metacarpal (A) and metatarsal (B) of Hipparion brachypus from Akkaşdağı with other similar forms from various localities. o, H. brachypus (Akkaşdağı); n, H. brachypus (Hadjidimovo); s, H. brachypus (Pikermi); c, robust form (Maragha).

FIG. 8. — Logarithmic ratio diagram comparing the third metacarpal of Hipparion brachypus from Akkaşdağı with H. brachypus, H. cf. proboscideum and H. sp. from various localities. Standard H. mediterraneum, Pikermi, n = 12-39 (Koufos 1987a).

FIG. 9. — Logarithmic ratio diagram comparing the third metatarsal of Hipparion brachypus from Akkaşdağı with H. brachypus, H. cf. proboscideum from various localities and H. mediterraneum and H. sp. from Kemiklitepe A-B. Standard H. mediterraneum, Pikermi, n = 11-65 (Koufos 1987a).

FIG. 10. — Logarithmic ratio diagram comparing the astragalus and calcaneum of Hipparion brachypus from Akkaşdağı with H. brachypus from Pikermi. Standard H. mediterraneum, Pikermi, n = 6-27 (Koufos 1987a).

FIG. 11. — Logarithmic ratio diagram comparing the first and second phalanges of Hipparion brachypus from Akkaşdağı with H. brachypus from Pikermi and H. sp. from Kemiklitepe A-B. Standard H. mediterraneum, Pikermi, n = 2-8 (Koufos 1987a).

FIG. 12. — Logarithmic ratio diagram comparing the front and hind legs of Hipparion brachypus from Akkaşdağı with H. brachypus from Pikermi. Standard H. mediterraneum, Pikermi (Koufos 1987a).