Hipparion de Christol, 1832

Koufos, George D., 2016, Hipparion macedonicum revisited: New data on evolution of hipparionine horses from the Late Miocene of Greece, Acta Palaeontologica Polonica 61 (3), pp. 519-536 : 520-525

Genus Hipparion de Christol, 1832 Hipparion macedonicum Koufos, 1984

Figs. 1–3.

1980 Hipparion sp. ; Koufos 1980: 292.

1984 H. macedonicum ; Koufos 1984: 386, figs. 2, 3.

1986 H. macedonicum Koufos, 1984 ; Koufos 1986: 71, pls. 1–3.

2013 H. macedonicum Koufos, 1984 ; Vlachou 2013: 318.

Type material: Holotype: mandible with i1–m3 dex and i1–m2 sin, LGPUT RPl-21 ( Fig. 3A ) . Paratype: skull and associated mandible, LGPUT RPl-125 (Figs. 1, 3C) .

Type locality: Ravin de la Pluie (RPl), Axios Valley , Macedonia, Greece . Type horizon: Late Vallesian, MN 10 ; Late Miocene; GPTS = ~9.3 Ma Koufos 2013 and references cited therein ).

Material.—The given list includes only the new undescribed previously specimens. Skull and associated mandible, LGPUT RPl-125; right frontal part of the skull with the muzzle and the tooth row, LGPUT RPl-142; left maxillary fragment with P3–M2, LGPUT RPl-287; partial mandible with i1–m2 dex and sin, LGPUT RPl-286; partial mandible with p2–m2 dex and p2–m3 sin, LGPUT RPl-290; left mandibular fragment with p2–m3, LGPUT RPl-281; right mandibular fragment with p2–m3, LGPUT RPl-282; left mandibular fragment with p2–p4, LGPUT RPl-291; proximal part of McIII, LGPUT RPl-285; proximal part of tibia, LGPUT RPl-284; distal epiphysis of tibia, LGPUT RPl-288; distal part of MtIII, LGPUT RPl-292.

Emended diagnosis.—Small size; relatively elongated and narrow muzzle; shallow narial opening, the nasal notch is retracted above the middle of the C-P2 diastema; elliptical, shallow, antero-posteriorly oriented, not pocketed posteriorly with moderately developed margin, and open anteriorly POF; moderate POB; infraorbital foramen encroaches upon the antero-ventral border of the POF; moderate enamel plication in the upper cheek teeth with narrow and deep plis; simple-double pli caballin; elliptical-oval and isolated protocone; often connection of the fossettes; very rare presence of a weak lingual hypoconal groove; often presence of functional dP1; elongated and slender metapodials.

Hipparion matthewi differs from H. macedonicum having smaller size, shorter muzzle, deeper narial opening (nasal notch is retracted above the mesial half of P2), ovoid with well-defined borders, deeper, dorso-ventrally oriented POF and infraorbital foramen situated in front but outside of POF. Hipparion nikosi differs from H. macedonicum having smaller size (similar to H. matthewi ) and remarkably deeper narial opening (nasal notch is retracted above the P3–P4) distinguishing it well from all known small hipparions of the Eastern Mediterranean region. Hipparion moldavicum with larger size, longer muzzle, wider incisive row, deeper narial opening (nasal notch is retracted above or in front of P2), larger, deeper and posteriorly pocketed POF, infraorbital foramen situated below the antero-ventral rim of POF and larger metapodials differs from H. macedonicum . Hipparion sithonis Koufos and Vlachou, 2016 is different in the slightly larger size, the deeper narial opening (nasal notch is retracted above the mesostyle of P2), the presence of canine fossa, the antero-ventrally oriented and with well-defined borders POF, and the relatively shorter and robust metapodials. Hipparion uzunagizli differs from H. macedonicum in the longer and wider muzzle, the deeper narial opening (nasal notch is retracted above the mesial margin of P2), the longer POB, the larger, subtriangular, antero-ventrally oriented and posteriorly pocketed POF. Hipparion kecigibi with larger size, short and wider muzzle, subtriangular, deeply pocketed posteriorly and larger POF is separated from H. macedonicum . Hipparion elegans differs in the larger size, the subtriangular with well-defined borders POF, the longer and wider snout, the longer tooth rows, and the large and slenderer metapodials.

Measurements.—The measurements of the new material are given in Supplementary Online Material (SOM: tables 1, 2) available at http://app.pan.pl/SOM/app61-Koufos_SOM.pdf.

Fig. 1. Equid Hipparion macedonicum Koufos, 1984 (LGPUT RPl-125) → from Ravin de la Pluie, Axios Valley ( Macedonia, Greece), late Vallesian, MN 10 (Late Miocene). A–D. Skull in right (A) and left (B) lateral, lateral-dorsal (C), and ventral (D) views. E. Left tooth row in occlusal view.

Description.—LGPUT RPl-125 is a partially preserved skull lacking the occipital part (Fig. 1A–D); its left side is better preserved (the ventral and distal part of the orbit and the zygomatic arch are preserved) than the right one which is more damaged. The skull is laterally compressed and the dimensions are slightly influenced being smaller. The muzzle is relatively elongated and narrow; the narial opening is shallow; the nasal notch is retracted above approximately ½ of the distance between the canine and P2. The incisive row is rounded and all incisors are well developed. The buccinator fossa is very deep and pocketed posteriorly. The POF is better preserved in LGPUT RPl-142 ( Fig. 2A 1) and in the right side of LGPUT RPl-125 (Fig. 1A). It is elliptical, shallow, antero-posteriorly oriented, not pocketed posteriorly with moderately developed margin, and open anteriorly. The infraorbital foramen encroaches upon the antero-ventral border of the POF. The partially preserved right orbit of LGPUT RPl-125 indicates that the POB is probably moderate and the mesial orbital margin is situated above the middle of M3. The palate is laterally compressed and gives the impression that it is very narrow. The choanae are narrow and their anterior margin is at the middle of M1.

The upper cheek tooth row is well preserved, short and moderately worn in all studied specimens; in LGPUT RPl-142 it is unworn-little worn (IId stage; the M3 is erupted) (Figs. 1E, 2A 3, B). The tooth rows of LGPUT RPl-125 and LGPUT RPl-142 preserve a functional dP1. The P2 has a short and rounded anterostyle, well separated from the protoloph and connected fossettes in all specimens, except LGPUT RPl-37 ( Fig. 2B ). The enamel in the fossette’s borders is plicated and the plis are elongated and moderately deep; this morphology is clear in the moderately worn teeth of LGPUT RPl-37. The mean plication number ranges between 7–17 (mean 14.1) in the premolars and 10–17 (mean 13.6) in the molars. The pli caballin is single in all teeth, except LGPUT RPl-37 where it is double in the premolars. The protocone is isolated in all teeth, rounded-oval in the premolars and oval-lenticular in the molars; a weak spur occurs in the little worn teeth of LGPUT RPl-142 ( Fig. 2A 3 , B). The hypocone is elliptical-angular but it is rounded in M3 of all specimens. The distal hypoconal groove is deep in the less worn teeth and shallower in the more worn ones. In all M3 there is a deep lingual hypoconal groove separating well the hypocone while a very weak lingual hypoconal groove occurs in some teeth.

Although the mandible and the lower teeth of H. macedonicum from RPl have been described earlier on the basis of the holotype LGPUT RPl-21 and LGPUT RPl-17, the five new mandibles ( LGPUT RPl-125, 281, 282, 286, 290) add more data ( Fig. 3 ). The mandible relative to its size has high mandibular corpus; its height is remarkably reduced and became small in front of p2. The snout is relatively elongated and narrow. The incisive row is rounded and narrow between the distal margins of the i3s; in LGPUT RPl-125 seems to be wide but this is due to a strong dorso-ventral compression and deformation which flattened the incisive area. The symphysis is relatively short, like in the holotype LGPUT RPl-21 .

The lower teeth are well preserved in most of the available mandibles ( Fig. 3 ). The paraconid of the p2 is elongated and projects mesially. The parastylid is open mesially in the less worn teeth but it is closed and joined with the protostylid in the much worn ones ( LGPUT RPl-17). The protostylid is moderately high occurred in the half-worn teeth; it is well distinguished in LGPUT RPl-17, 125, 290, while in the holotype LGPUT RPl-21 just appeared and it is clearly separated from the parastylid. The metaconid is elliptical-rounded in the premolars, and rounded in the molars. The linguaflexid is open, V-shaped and deep touching the ectoflexid in the worn and much worn teeth. The ectoflexid is narrow and very deep, especially in the molars. The metastylid is elliptical in the premolars and more rounded in the molars. There are plications or crenulations in the preflexid and postflexid enamel which gradually disappear by the attrition .

The postcranial remains of H. macedonicum from RPl are a few; except those desribed earlier ( Koufos 1984, 1986), the new material includes some bone fragments, the size of which correlates them to the RPl small-sized hiparion.

Remarks.— Hipparion macedonicum was originally described on a small suite of mandibles, maxillary fragments and post cranial remains from RPl ( Koufos 1984, 1986) but soon it was recognized in several localities of Axios Valley (PNT, RZ1, RZO, PXM, VATH) and later in Nikiti (NKT, NIK), as well as in Perivolaki (PER) ( Koufos 1987b, c, 1988, 2000; Vlachou and Koufos 2002, 2006; Koufos and Vlachou 2016). The cranial characters of the taxon are mainly known from an almost complete skull from the Turolian locality PXM and some partial skulls from the other Turolian localities of northern Greece ( Koufos 1987c, 1988; Vlachou and Koufos 2006; Koufos and Vlachou 2016). LGPUT RPl-125, as it originates from the type locality, allows to study the cranial morphology of H. macedonicum in its typical site, as well as to see its evolutionary relationships with the younger forms of the species, and other small-sized hipparions.

The mean values of the RPl cranial and mandibular remains are compared with the corresponding ones of H. macedonicum from other localities, as well as with other Hipparion taxa known from Eurasia. The PCA for the skull separates well the various Hipparion taxa according to their size ( Fig. 4A ). The small-sized hipparions match in the lower and the medium-sized ones in the upper half of the PC2, while the very small and large forms are displayed in the two extremes of the PC1. The RPl skull matches H. macedonicum suggesting that metrically is very close to this taxon. The type of H. matthewi Abel, 1926 and the skulls described as H. cf matthewi are well separated from the RPl skull being quite smaller. Three other small-sized species have skulls comparable in size to the RPl one ( Fig. 4A ). Hipparion elegans Gromova, 1952 differs from the RPl in several morphological features (see below). The other two are H. sithonis Koufos and Vlachou, 2016 and aff. H. forstenae Zhegallo, 1971 and both are separated from H. macedonicum and RPl skull having canine fossa. The PCA for the mandible, although the distinction of the various taxa is not sufficient, the RPl mandible is closer to H. macedonicum and H. sithonis than to H. matthewi which has smaller mandible ( Fig. 4B ).

The MtIII sample of the various species is rich and can allow a good comparison. The PCA separates well the MtIII of the various taxa according to their mid-shaft length and width ( Fig. 4C ). The RPl MtIII is into the area of H. macedonicum and can be referred to it; however, it is clear that there is a great overlapping between the various taxa e.g. the MtIII of H. macedonicum and H. sithonis cannot be distinguished, as they overlap each other ( Fig. 4C ). The distinction of hipparions having similar size and the attribution of the postcranials to the one or the other taxon (except having complete skeletons) is, in most cases, arbitrary. The correlation is mainly based on size or some morphological features, if there are, e.g., H. macedonicum and H. sithonis are well-separated by their skull morphology (presence or absence of canine fossa) but the distinction of the postcranial remains is difficult or even impossible. The presence of two hipparions in RPl, a large- and a small-sized one makes the distinction of the postcranials easier, but we cannot exclude the possibility of a second large or small hipparion in the sample.

Figures

Fig. 3. Equid Hipparion macedonicum Koufos, 1984 from Ravin de la Pluie, Axios Valley (Macedonia, Greece), late Vallesian, MN 10 (Late Miocene). A. Holotype LGPUT RPl-21, mandible in right lateral (A1) and occlusal (A2) views. B. LGPUT RPl-286, mandible in right lateral (B1) and occlusal (B2) views. C. LGPUT RPl-125, mandible in left lateral (C1) and occlusal (C2) views.

Fig. 2. Equid Hipparion macedonicum Koufos, 1984 from Ravin de la Pluie, Axios Valley (Macedonia, Greece), late Vallesian, MN 10 (Late Miocene). A. LGPUT RPl-142, right frontal part of skull in lateral (A1) and occlusal (A2) views, right tooth row in occlusal view (A3). B. LGPUT RPl-37, left tooth row in occlusal view.

Fig. 4. Principal component analysis of the skull (A), mandible (B), and third metatarsal (C) of the RPl small-sized hipparion in comparison with other small-sized forms of Eurasia, indicating its strong size-similarities with Hipparion macedonicum. As the available specimens are fragmentary, lacking measurements the mean cranial dimensions for each locality are used. The used variables (various measurements of the skull, mandible and MtIII) and their influence to the distinction of the specimens is given in the left part of the diagrams. The convex hulls indicate the allocation of the measurements in the space for its sample. Hipparion dietrichi, Hipparion mediterraneum, and Hipparion proboscideum or Hipparion cf. proboscideum are used as comparative samples. Hipparion macedonicum: 1, NKT; 2, NIK; 3, RZO; 4, PXM; 5, VATH; 6, PER. Hipparion matthewi: 7, Samos (unknown locality), holotype; 8, KTA-B. Hipparion cf. matthwei: 9, MTLA. Hipparion sithonis: 10, NIK. aff. Hipparion forstenae: 11, MTLA; 12, Samos Q1. Hipparion →