Ancylotherium pentelicum (Gaudry F Lartet, 1856)

Saraç, Gerçek & Sen, Sevket, 2005, Chalicotheriidae (Mammalia, Perissodactyla) from the late Miocene of Akkaşdağı, Turkey, Geodiversitas 27 (4), pp. 591-600 : 592-596

Ancylotherium pentelicum (Gaudry F Lartet, 1856)

MATERIAL. — A juvenile skull having the dorsal part crushed and damaged and the ventral part and incomplete dentition including left P2, P3, M1, M2 and erupting M3, right M1, M2 and erupting M3 (AK2- 293); a left duplex (AK5-4) and a left calcaneum (AK2-438). This material was unearthed from two different bone pockets about 3 m apart.

DESCRIPTION

Skull ( Fig. 1A )

The skull belongs to a young individual, ontogenically similar to that of Hadjidimovo- 1 in Bulgaria ( Geraads et al. 2001), but younger than the one from Titov Veles in Macedonia ( Garevski 1974). In the Akkaşdagwı specimens, almost nothing is preserved from the top part of the skull, while the ventral part is nearly complete. The skull anatomy of this species is still poorly known; apart from the material from Hadjidimovo-1 and Titov Veles, only some upper jaw fragments were described from Pikermi ( Gaudry 1862 -1867; Thenius 1953). However, an almost complete skull from Gülpınar (Department of Çanakkale, northwestern Turkey) is preserved at the Ankara Natural History Museum, and it belongs to an adult individual. This skull has not been the subject of any publication. We compared the Akkaşdagwı skull with this one in order to appreciate the proportions of missing and damaged parts.

On the skull AK2-293, the dorsal part is completely crushed preventing detailed observation of the nasal, frontal and parietal bones. Also, the posterior part of the skull is missing. Some parts of the temporal bones are preserved, as well as the zygomatic arches, maxillae, palate, glenoid fossae, post-glenoid processes, and some cheek teeth (see Fig. 1A for details). The M3s are just erupting.

In ventral view, the palate narrows slightly forward (narrowest width of the palate between M2s is 67 mm, and between M1s 52 mm). The choanae are large and elliptical in outline. Their anterior border is behind the M1s. The median keel of the basisphenoid forms a bony septum which narrows anteriorly and divides the choanae into two parts. The zygomatic arch is rounded, and it narrows anteriorly. It reaches the maxilla immediately behind the M1. The maximum width of the skull at the level of zygomatic arches is 212 mm. The mandibular fossa is rather flat and, just behind it, the retro-articular process is massive and rounded. In lateral view, the infra-orbital foramen is situated 20 mm above the posterior edge of P3.

Dentition ( Fig. 1B ; Appendix: Table 1)

On the Akkaşdagwı specimen the premolars are still erupting and are unworn, and thus they preserve all details of the occlusal pattern. We emphasize that such fresh premolars of Ancylotherium have not previously been described. The premolars are not molarized, so their occlusal pattern is different from that of the molars. The P3 is much smaller than the P4. Both have strong cingula on their anterior, lingual and posterior faces. The lingual cingulum is stronger in P4. Both premolars have a slightly oblique protolophule which is shorter than the metalophule. The protocone is without any crest. It is relatively conical in P3, but rather pinched mesio-distally and lined up with the protolophule in P4. Both premolars have the metalophule thinner than the protolophule and oriented rather transversely. The buccal surface of the premolars is flat, thus distinguishing it from that of Chalicotherium Kaup, 1833 which has P3 and P4 with folded or depressed buccal surface ( Zapfe 1979). The width/length ratio is 125.3 for P3 and 127.3 for P4.

The M1 is much larger than the P4 but notably smaller than the other molars. Its outline is almost square with a W/L ratio of 98.9. However, the anterior part of the M1 is narrower than the posterior part. The ectoloph is W-shaped. Wear has affected only the anterior faces of the paracone and metacone crests. The protocone is an isolated cusp situated close to the anterolingual corner of the tooth; it is without any crest or furrow. The protolophule is short. The metacone is slightly higher than the paracone, and their lingual tips protrude lingually beyond the median line. The paracone rib is weak and there is no metacone rib. The parastyle protrudes antero-buccally, the hypocone lingually. The median valley is directed toward the mesostyle. The anterior and posterior cingula are relatively strong; there is no cingulum along the lingual and buccal borders. The crochet is absent as on the specimen HD-633 from Hajidimovo-1, while the other specimen (HD-631) from this locality has a crochet ( Geraads et al. 2001). The four roots of the M1 are situated under the main cusps.

The M2 is about 30% larger than the M1. Its occlusal outline is rather rectangular with a W/L ratio 87.5. Wear has slightly affected limited areas on the W-shaped ectoloph. The first lobe is larger and is twice as long as the second lobe. The median valley which separates these lobes is a little narrower than on the M1. In contrast to M1, the paracone is notably higher than the metacone. The conical protocone is a large isolated cusp which occupies a wider area than in the M1. The anterior cingulum is complete between the parastyle and the base of the protocone. The posterior cingulum is weaker and forms a tiny but continuous ridge from the mesostyle to the hypocone. On the lingual face, there is a short thickening which closes the median valley. There is a short crochet on the metaloph as HD-631 from Hadjidimovo-1. The presence of this structure is apparently related to the degree of wear; it disappears in worn teeth. The other characters are similar to that of the M1.

The M3 is still erupting. Only the antero-labial side of the tooth is visible. The paracone rib is weak, and there is no metacone rib. The parastyle and mesostyle are sharp.

Duplex ( Fig. 2 )

This bone is typical for Schizotherinae, and it is formed by the coalescence of the first and second phalanges of the second digit, in both hand and foot, as observed in the genera Ancylotherium Gaudry, 1863 , Tylocephalonyx Coombs, 1979 , Moropus Marsh, 1877 and Phylotillon Pilgrim, 1910 (Coombs F Rothschild 1999). The duplex from Akkaşdagwı belongs to the second digit of the left forelimb. Its articular facet with McII is heartshaped, concave and faced proximo-dorsally. It occupies almost half the surface of the dorsal side of the first phalanx. The proximal edge of the bone is bilobed because of the presence of an almost central notch. The proximo-lateral volar process is stronger but less protruding anteriorly than the medial one. On the dorsal face, the medial side is higher than the lateral one. The volar tuberosity is clear. The distal trochlea is deep (deeper on the medial side).

The duplex is larger in Ancylotherium pentelicum than in any species of Metaschizotherium von Koenigswald, 1932 . The duplex of A. pentelicum shows similarities with those of Moropus elatus Marsh, 1877 and Tylocephalonyx skinneri Coombs, 1979 in having a heart-shaped proximal articular facet. However, in T. skinneri the angle between the first and second phalanges is stronger, and the proximo-volar process is more robust ( Coombs 1979). The Akkaşdagwı duplex is smaller than those of the adult specimens from Pikermi and Pınaryaka (Roussiakis F Theodorou 2001; Saraç et al. 2002). It has much weaker muscular connections than in other specimens (Appendix: Table 2). These differences are interpreted as being due to the younger ontogenetic age of the Akkaşdagw ı individual and/or to sexual dimorphism.

Calcaneum ( Fig. 3 ; Appendix: Table 3)

AK2-438 is a complete calcaneum. Until now, only one complete calcaneum (collections of Vienna Museum, Zapfe 1979) was referred to Ancylotherium pentelicum , while the three others specimens from Pikermi (one in Paris and two in Athens; Gaudry 1862 -1867; Roussiakis F Theodorou 2001) are all incomplete. Other localities where this species is recognized did not yield any calcanea. The Akkaşdagw ı specimen preserves all articular facets well delimited and other morphological structures such as processes and grooves, whereas on the Pikermi specimens such structures are partly damaged, eroded or simply not yet well formed (juvenile).

In dorsal view, AK2-438 has a sustentaculum tali forming a strong lateral process; it bears on its dorso-anterior face an articular facet for the astragalus. This facet is separated from the main facet for the astragalus by a 5 mm wide notch. The main facet for the astragalus has two parts: the upper one (W = 53 mm) is ellipsoid in outline and covers all the dorsal face of the coracoid process; the lower part (L = 32, W = 29 mm) is round and covers two thirds of the anterior articular process. Both parts form a continuous large facet for the astragalus. On the anterior face, the facet for the cuboid is moon-shaped and extends from the medial edge up to the median notch.

On the medial face of the bone, a deep oblique scar divides the medial tuberosity for the upper portion of the tibial cranial muscle into two unequal parts, the lower one being much larger than the upper one.

An important character of AK2-438 is that the shaft of the tuber calcanei is rectilinear, with an almost regular medio-lateral width from the articular part up to the posterior edge of the bone. The left calcaneum from Pikermi (PG 95/20) on which this part is well preserved has the medial and lateral faces concave and, consequently, the shaft is notably narrower in the middle part than in other parts of the bone. Moreover, due to the lateral concavity, the angle between the sustentaculum tali and tuber calcanei is smooth in all three specimens from Pikermi ( Gaudry 1862 -1867; Roussiakis F Theodorou 2001), while it is sharper in the Akkaşdagwı specimen. This unique difference may be interpreted as an individual or regional variation.

Two calcanei from Omo and Laetoli in East Africa have been assigned to Ancylotherium hennigi Butler, 1965 ,a species that lived in East Africa from latest Miocene up to the latest Pliocene ( Butler 1965; Guérin 1985, 1987). The size and morphology of these specimens seem to be almost identical to those from Pikermi (Appendix: Table 3) according to descriptions and illustrations given by Guérin (1985, 1987: fig. 9.1).

Lastly, Guérin F Pickford (2005) described a new species, Ancylotherium cheboitense , from the latest Miocene Cheboit locality at Tugen Hills in Kenya. The material consists of a lower molar and some foot bones but nothing comparable to the specimens recorded at Akkaşdagwı.

Figures

FIG. 1. — Ancylotherium pentelicum (Gaudry & Lartet, 1856), juvenile skull (AK2-293), Akkaşdağı; A, ventral view; B, occlusal view of the left toothrow with P4-M2 and erupting M3. Scale bars: 50 mm.

FIG. 2. — Ancylotherium pentelicum (Gaudry & Lartet, 1856), left duplex (AK5-4), Akkaşdağı; A, dorsal view; B, medial view. Scale bar: 50 mm.

FIG. 3. — Ancylotherium pentelicum (Gaudry & Lartet, 1856), left calcaneum (AK2-438), Akkaşdağı; A, dorsal view; B, ventral view. Scale bar: 50 mm.