Acerorhinus sp.

Antoine, Pierre-Olivier & Saraç, Gerçek, 2005, Rhinocerotidae (Mammalia, Perissodactyla) from the late Miocene of Akkașdağı, Turkey, Geodiversitas 27 (4), pp. 601-632 : 619-621

Acerorhinus sp.

MATERIAL. — Distal part of a left humerus, AKB-47; distal part of a left humerus, AK5-1; distal part of a right humerus, AK6-88; right radius, AK5-629; left calcaneus, AK7-98; right calcaneus, AK6-130; right MtII, AK5-678.

DESCRIPTION

Humerus

Three distal parts have been unearthed. The bones seem to be rather slender and long (Appendix: Table 33). The diaphysis is triangular in cross section, below the lacking deltoid tuberosity. The lateral epicondyle is high and wide, with sharp ridges. In anterior view, its lateral border forms a sharp dihedron (130°), with a proximal tubercle oriented upward. The trochlea is narrow, with a broad (TD) lateral lip. The medial one is regularly conical, except for its last centimetre, where it further tapers. The median strangulation is not very marked. The trochlea is eggcup-like. The lateral lip is first conical, widening laterally, and then it tapers until its lateral tip. The frontier between these two parts is smooth. There is no synovial fossa on the trochlea. The fossa olecrani is wide and low. A distal gutter isolates the lateral epicondyle. In anterior view, the distal end is lower on the lateral side.

Radius

AK5-629 is complete. It is high and slender, thickening downward (Appendix: Table 34). In proximal view, the proximal articulation has a straight anterior border. This articulation is separate into two equally wide cochleae, the medial one being deeper (APD). The frontier between both cochleae is a smooth ridge, slightly higher posteriorly. The insertion for the m. biceps brachii is developed, but shallow, and medially displaced. The ulna-facets are vertical and widely separate. The medial one is low, crescent-like. The lateral one is higher, but still small. The diaphysis is twisted, with a convex medial border and a concave lateral border. The posterior side of the diaphysis shows a long trace corresponding to the contact with the ulna. It is essentially concentrated in the distal half of the bone, down from the spatium interosseum. This surface forms a rough triangle. The diaphysis widens distally, but it does not get deeper (APD). There is only one distal ulna-facet, almond-shaped. The gutter for the m. extensor carpi is shallow. There is no tuberculum dorsale laying on the anterior side. The distal end is lower on the medial side. The scaphoid-facet is just a little visible in anterior view. The distal articulation bears three facets. The scaphoid-facet is not deep (APD), but strongly furrowed. The median one, for the semilunate, is transversally flat and sagittally concave. The pyramidal-facet forms a long and straight oblique stripe.

Calcaneus

The bone is very thick and robust, with a very short tuber and processus calcanei. The dimensions are variable but the proportions are similar (Appendix: Table 35). The tuber calcanei is triangular and massive. The insertion for the m. fibularis longus is salient. The sustentaculum tali is broad (TD> APD) and thick. The astragalusfacet 1 is flattened in its proximal part. A distal circular expansion is always present, separate from the main facet 1 by a shallow groove. There is no fibula-facet, but a large tibia-facet on both specimens (AK6-130; AK7-98). The astragalusfacets 2 and 3 are widely separate. The facet 2 is oval and concave. The facet 3 forms a narrow stripe. The distal facet (for the cuboid) is saddleshaped, but the postero-lateral tip is broken on both specimens.

MtII

AK5-w.n. is rather slender, with a slightly curved diaphysis and distal widening. The proximal end is hemicircular in proximal view. The mesocuneiform-facet occupies most of the surface. It is also hemicircular and nearly flat. The entocuneiform-facet is high and narrow, in contact with the mesocuneiform-facet. On the lateral side, the anterior facet is broken; the posterior one is circular and split into two equivalent subfacets. The proximal subfacet corresponds to the ectocuneiform and the distal one to the MtIII. The insertion for the m. interosseus is very long. It nearly reaches the bottom of the diaphysis. An oblique groove runs on the antero-lateral side of the diaphysis. The distal trochlea is deep- er than wide (APD> TD; Appendix: Table 36), wider posteriorly. The depth is equal medially and laterally. The intermediate relief is nearly absent but it determines a large medial lip and a narrow lateral lip. The former is slightly concave.

DISCUSSION

These postcranial specimens display characteristic traits of several late Miocene aceratheriine genera from Eurasia, i.e. Alicornops Ginsburg F Guérin, 1979 , Acerorhinus Kretzoi, 1942 , and Chilotherium ( Antoine et al. 2003) . Their representatives are convergent with Neogene teleoceratines in that they have acquired independently more or less shortened limbs (“hippo-like”; e.g., Heissig 1999; Antoine et al. 2003).

The relatively slender proportions of these seven postcranials – which are likely to document a single taxon – prevent any assignment to Chilotherium , the most “hippo-like” aceratheriine genus. In return, these proportions and dimensions (Appendix: Tables 33-36) recall those o b s e r v e d i n A c e r o r h i n u s (R i n g s t r ö m 1 9 2 4; Cerdeño 1996; Fortelius et al. 2003), and at a lesser degree Alicornops ( Guérin 1980, 1988; C e r d e ñ o 1 9 9 7; C e r d e ñ o F S a n c h e z 2 0 0 0; Antoine et al. 2003). For instance, the humeri and the radius from Akkasdagw ı are strongly similar to the specimens referred to Acerorhinus zernowi ( Cerdeño 1996) but much larger than the largest ones identified as Al. simorrense and Al. complanatum (Appendix: Tables 33; 34; e.g., APDs). The calcanei are wider and more massive than any calcaneus assigned so far to Alicornops or to Acerorhinus , which is especially visible on the tuber calcanei (Appendix: Table 35). However, t h e c l o s e s t a f f i n i t i e s e x i s t w i t h A. z e r n o w i ( Cerdeño 1996) and “ Alicornops ” alfambrense Cerdeño F Alcalá, 1989, the generic assignment of which has already been challenged by Antoine et al. (2003), who tentatively referred it to Acerorhinus . In that respect, the characteristics of the MtII AK5-w.n. (morphology, proportions, and dimensions) are quite puzzling, in that they match perfectly those of Alicornops simorrense (Appendix: Table 36). The MtII is at the same time more robust than those of A. zernowi (middle Miocene; Cerdeño 1996) and much more slender than the McIV assigned to Acerorhinus sp. by Fortelius et al. (2003: 289, fig. 12.6; c. 8.1 Ma). The trend toward shorter metapodials is generalised – though probably independently acquired – within Chilotherium and Acerorhinus ( Fortelius et al. 2003) . Thus, and in the case of a linear trend, such morphology for the MtII would rather point to an intermediate age for Akkasdagw ı.

However and pending new discoveries – notably associated cranial, mandibular and/or dental material in Akkasdagw ı –, we propose to assign these postcranials to Acerorhinus sp. It should be added that Akkasdagw ı counts among the latest occurrences of Acerorhinus (MN 7-MN 12; Qiu F Qiu 1995; Cerdeño 1996; Deng 2000; Fortelius et al. 2003).