Omanitherium dhofarense SEIFFERT et al., 2012

Al-Kindi, Mohammed, Pickford, Martin, Al-Sinani, Yusouf, Al-Ismaili, Ibrahim, Hartman, Axel & Heward, Alan, 2017, Large Mammals From The Rupelian Of Oman - Recent Finds, Fossil Imprint 73 (3 - 4), pp. 300-321 : 305-312

Omanitherium dhofarense SEIFFERT et al., 2012

M a t e r i a l. See Table 2.

D e s c r i p t i o n. The fossils attributed to Omanitherium comprise 21 specimens out of 89 catalogue entries in the 2017 field catalogue (Tab. 2, Appendix 2), indicating that among the large mammals, this genus is dominant. Many of the specimens are broken pieces of mandibles and maxillae, and limb bone fragments, which yield little morphological information. There are also many enamel fragments. The following descriptions deal only with the more complete dental elements, comprising some rather complete lower incisors, and a reasonable sample of upper cheek teeth, which were poorly represented in previously available samples ( Seiffert et al. 2012, Pickford 2015b).

ONHM TN 2017-19 is a damaged left i/1 with apical wear ( Text-fig. 10 ). The section of the tooth is a compressed rectangle. The labial surface is almost complete and shows a shallow groove near the apex, positioned about one third of the distance from the mesial edge towards the distal margin. The labial surface is shallowly convex, almost flat; the mesial surface is at right angles to the labial one and measures about 8 mm from labial to lingual. The lingual surface is damaged, but remnants of enamel are preserved along the distal half, which shows that the lingual surface was originally covered in enamel, which is appreciably thinner than that on the labial side. The distal edge is damaged. The cervix is distinct labially, and rises towards the apex on the mesial side and then extends across the lingual side. The apex is somewhat damaged, so the exact shape of the wear facet cannot be determined.

ONHM TN 2017-51 is a complete, lightly worn right i/1 crown with part of the root preserved ( Text-fig. 11 ). The tooth has a rectangular section, much broader mesiodistally than the labio-lingual diameter. The labial surface has a subtle groove that extends from the apex towards the root, fading out as it goes. This groove is positioned about 1/3 of the distance between the mesial and distal margins of the crown. The mesial side of the tooth is almost flat, with a complete cover of thin enamel. The distal margin of the tooth is curved apically, but for most of its height it is parallel to the mesial margin. The mesial side of the tooth is flat and at right angles to the labial side. The wear facet is straight and at right angles to the long axis of the crown.

ONHM TN 2017-43 is a left i/2 with a wear facet at its apex. The section is ovoid, such that the mesio-distal diameter is slightly greater than the labio-lingual diameter. The labial surface is uniformly curved, but the lingual side is more angular. The cervix is far from the apex on the labial side, but rises a long way towards the apex mesially, before descending rootwards again. The wear facet is flat and at right angles to the long axis of the tooth.

ONHM TN 2017-50 is the distal lophid of an unworn lower molar, probably m/2 on the basis of its dimensions ( Text-fig. 12 ). The crest of the lophid has a mamellated appearance, with no central sulcus. The precristids are welldeveloped, in particular the one on the buccal cusp. The distal cingulum is broken off, but the posterior accessory tubercle is preserved, and lies in line with the pre-cristid of the buccal cusp, as in other specimens of the species (Textfig. 13).

ONHM TN 2017-16 is an unworn left P3/ crown. It has a prominent protocone, which is separated by a sulcus from the buccal cusp, which is comprised of closely fused paracone and metacone. The parastyle is bordered on either side by a cingulum, which is prominent near the parastyle, but fades out buccally and lingually. The distal cingulum is prominent in the centre-line of the tooth, but diminishes buccally and lingually. The pre-protocrista reaches mesiobuccally towards the pre-paracrista, thereby forming a lophlike wall with a shallow notch between the two cristae.

ONHM TN 2017-17 is an unworn right P4/ found close to the left P3/. It is slightly larger than the P3/, the loph is better formed and the central sulcus is broader than that in the P3/. The protocone is large, and the paracone and metacone are separated at their apices, but solidly fused at their bases. The crests of the tooth are comparable to those in the P3/, but show stronger beading along their edges. The same can be said about the beading of the cingula.

ONHM TN 2017-44 is a lightly worn right P4/, similar in construction to ONHM TN 2017-17, but slightly larger ( Text-fig. 14 ). The apices of the loph and the cingula are lightly worn, to the stage where the beading has been removed.

ONHM TN 2017-18 is a lightly worn right M1/, lacking the buccal part of the crown ( Text-fig. 14 ). It was found close to the left P3/. The protocone and paracone form a clear loph, as do the hypocone and metacone behind, separated from each other by a broad central valley. There is a narrow mesial cingulum, and a broader distal one. The mesial edge of each loph is straight, but the distal side is concave, due to the way that the post-cristae are arranged.

ONHM TN 2017-45 comprises two fragments of an unworn left M2/ ( Text-fig. 14 ). The tooth is bilophodont, with beaded apices of the lophs. The mesial cingulum is broken off, but the distal one is preserved, and shows a beaded apex. The distal cingulum blends into the post-metacrista.

ONHM TN 2017-79 is a damaged right M3/ with medium wear ( Text-fig. 14 ). The tooth is bilophodont, but the occlusal outline is more trapezoidal than it is in the M1/ and M2/, which are rectangular. This is due to the fact that not only is the hypocone more distally positioned relative to the metacone than it is on the anterior molars, but in addition, the post-hypocrista is more voluminous than the post-metacrista. As a result of this arrangement of the main cusps, the distal cingulum is oblique to the long axis of the tooth. In other respects, the M3/ is similar to the other molars.

D i s c u s s i o n. The first mentions of the presence of a lophodont proboscidean at Thaytiniti were by Thomas et al. (1989, 1999), summarised as? Barytherium sp. indet. (“cf. Barytherioidea”) by Sanders et al. (2010b), and now known as Omanitherium dhofarense SEIFFERT et al., 2012 .

When Omanitherium was described for the first time, Seiffert et al. (2012) reconstructed the mandible with only two incisors, separated from each other by a broad gap. In contrast, Pickford (2015b) thought that there would have been a pair of central incisors infilling the gap. The new fossils from the type locality and nearby sites indicate that Omanitherium did indeed possess four lower incisors, but instead of the central lower incisors being relatively low crowned as originally thought, they are taller. The difference of opinion is due to the fact that Pickford (2015b) used an upper central incisor as a proxy for the lower incisor.

The meristic position (upper central incisor) and orientation (steeply inserted in the premaxilla) of ONHM TH 6 inferred from the wear facet and general form of the tooth is confirmed by the discovery of two skulls of the closely related taxon, Arcanotherium savagei from Dor el Talha, Libya (classified as Numidotherium savagei by Jaeger et al. 2012).

The difference in crown height between the upper and lower central incisors of Omanitherium dhofarense is illustrated in Text-fig 11 . The upper central incisor is not only lower crowned than the lower incisor, but it has an undulating distal margin and thick enamel on the lingual surface, whereas the lower central incisor has no sign of undulations on its distal margin, and the lingual enamel is thin. Furthermore, the crown of the upper incisor is labiolingually compressed, and the wear facet is confined to the lingual surface of the tooth, where it forms a broad surface ending at a step about half way between the apex and the cervix, whereas the wear facet on the lower incisor is apical. This conformation indicates that the upper central incisor was steeply inserted in the jaw, whereas the lower incisor was procumbent, almost horizontal.

In Barytherium ANDREWS, 1901a , from Dor el Talha (also spelled Dur at Talha), Libya, in contrast to Omanitherium dhofarense , the upper central incisor is tusk-like, with a uniformly curved root and crown, compressed mesio-distally rather than labio-lingually, with a longitudinal groove on the distal aspect of the root and shallow grooves on the lingual side of the root ( Text-fig. 15 ). There is a lingual wear facet that extends onto the root, and there is abrasional wear all over the exposed part of the tooth, probably caused by rubbing against vegetation during acquisition of food (for example, debarking trees, or reaching upwards to high branches to draw them downwards towards the mouth). The upper central incisor of Barytherium evidently occluded with both the lower central and lateral incisors. An I1/ of Barytherium from Dor el Talha housed in the NHMUK, London ( Text-fig. 15 ), is considered to represent a female individual, because its crown and root are relatively gracile. A second specimen housed in the MNHN, Paris, has a much enlarged crown base and a huge root (broken off just beneath cervix), and is interpreted to represent a male individual.

As concerns the second lower incisor, the holotype specimen of Omanitherium dhofarense is unworn, so it is not possible to determine its occlusal relationships to the upper incisors. The new fossils available from the Ashawq Formation indicate that the second incisors also experienced apical wear. The significance of the new data is that it strengthens the affinities of the Omani proboscidean with the Libyan genus Arcanotherium , as deduced by Pickford (2015b), but it distances these two genera from Barytherium , and even more so, from Deinotherium KAUP, 1829 .

The original sample of Omanitherium did not have any elements of upper dentition. Pickford (2015b) described some upper premolars and part of an upper molar, which revealed that the species was similar in many respects to Arcanotherium savagei ( COURT, 1995) from Dor el Talha, Libya. Amongst the premolars attributed to Omanitherium by Pickford (2015b), there was a P4/ that turned out to belong to a different proboscidean taxon, probably a palaeomastodontoid (see below). The removal of the P4/ previously attributed to Omanitherium , and its replacement by a new specimen from the type locality, strengthens the affinities with Numidotherium ( Mahboubi et al. 1986, Noubhani et al. 2008, Adnet et al. 2010), Phosphatherium ( Gheerbrant et al. 1998, 2005) and Daouitherium ( Gheerbrant et al. 2002) .

The 2017 sample of Omanitherium teeth from Dhofar includes upper premolars as well as molars, all of which lessen residual doubts that might still exist concerning the systematic relationships of this species. Interestingly, the lower central incisors of Omanitherium have thinner enamel on the lingual surface than on the labial side, whereas in the second incisors, the enamel is thick all around, but lingually there is a tall anticline in the cervix, which means that on the lingual side of the crown, the enamel of the i/2 covers only half the height of the tooth compared with the enamel on the labial side.

The large barytherioid, Barytherium , in contrast, shows no enamel at all on the lingual surfaces of the i/1 and i/2 ( Pickford 2015b) ( Text-fig. 16 ). Not only that, but lingual doming of the dentine, best appreciated in the section, is vast, comprising the bulk of the volume of the tooth, providing a major contrast with the almost flat or even slightly concave lingual surface of the i/ 1 in Omanitherium and Arcanotherium . The wear facet at the apex of the tooth in Barytherium is more inclined distally than it is in Omanitherium , suggesting that the upper central incisor was oriented differently in the two genera.

Near its apex, the lower first incisor of Barytherium shows an interstitial contact facet on its distal margin caused by abrasion against the i/2 ( Text-fig. 16a 2 ). In Arcanotherium and Omanitherium , the interstitial wear facet of the lower incisors is not near the apex, but is well towards the cervix ( Text-fig. 16b, c ), indicating that the two lower incisors were separated at their apices, unlike Barytherium , in which the two incisors were in contact at their apices, and for quite a distance towards the cervix. The i/ 2 in Barytherium is poorly represented in the fossil record, but judging from its alveolus, it is highly compressed, being mesio-distally short and bucco-lingually broad, and its crown would have been almost as tall as that of the i/1. Note also the mammelated distal margin of the i/ 1 in Arcanotherium , and the nonmammellated margin in Barytherium . In Omanitherium , the distal margin of the i/1 is not mammelated ( Text-figs 10 , 11 ), in strong contrast to the distinct mammelae present in Arcanotherium ( Text-fig. 16b, c ).

In contrast to Barytherium , the i/1 and i/2 of Omanitherium and Arcanotherium show no contact near their apices, even though their roots are close together. Arcanotherium shows an interstitial facet on its distal margin close to the base of the crown, caused by contact with i/2 ( Text-fig. 16b, c ). Orientations of the lower incisors in Barytherium and Omanitherium were different, implying divergent occlusal relationships with the upper incisor battery. Arcanotherium and Omanitherium are much more similar to each other and to the older genera, Numidotherium , Phosphatherium and Daouitherium , than they are to Barytherium . This new evidence concerning lower incisor morphology in barytheres (sensu lato) does not resolve the issue of relationships between them and deinotheres ( Sanders et al. 2010b), but it means that a close relationship is unlikely.

The upper teeth of Omanitherium now available reveal that it is substantially smaller than Arcanotherium savagei ( Text-figs 17 , 18 ). The M3/ of the latter species ( Delmer 2009: fig. 3D), for example, measures 51 × 45 mm (length × breadth), which compares with 43.7 × 38.3 mm in Omanitherium dhofarense .

In summary, additions to the hypodigm of Omanitherium dhofarense collected in 2017 contribute to our understanding of its anterior dentition and upper cheek teeth, and thereby clarify its relationships to numidotheres (close) and barytheres (remote). However, there remain uncertainties, especially concerning the number of upper incisors, and the presence or absence of canines and anterior premolars. Furthermore, little is known about the skull and post-cranial skeleton of the species. In contrast, the recovery of relatively complete cranial remains of Arcanotherium savagei from Dor el Talha in Libya reveals that there were three pairs of upper incisors, a low-crowned spatulate central incisor with beaded distal margin, a taller, more tusk-like second incisor and a small upper third incisor and upper canine ( Jaeger et al. 2012). It remains to be discerned whether Omanitherium conforms in this respect to Arcanotherium . These Omani and Libyan discoveries reveal that knowledge about incisors is essential for sorting out phylogenetic relationships of Palaeogene proboscideans, and furthermore, for resolving the long-lasting debate about the meristic position of the tusks of elephants and mastodonts. In this respect, Omanitherium is closer to elephants than it is to barytheres, despite the fact that the cheek teeth of numidotheres and barytheres are morphologically comparable. For these reasons, we here adopt a modified classification of the Palaeogene lophodont proboscideans, recognising Barytherioidea ANDREWS, 1906, as a distinct superfamily from Numidotherioidea SHOSHANI et TASSY, 1992 (new rank).

Figures

Text-fig. 10. Lower incisors of Omanitherium dhofarense. a) ONHM TN 2017-19, left i/1, (a1 – stereo lingual view, a2 – distal view, a3 – mesial view, a4 – stereo labial view, a5 – apical view); b) ONHM TN 2017-43, left i/2 from Mohammed’s i/2 site (b1 – stereo distal view, b2 – lingual view to show the anticlinal form of the cervix, b3 – labial view, b4 – mesial view, b5 – apical view) (scale bar 10 mm).

Text-fig. 11. Upper and lower incisors of Omanitherium dhofarense. a) ONHM TN 2017-51, right i/1 from Mohammed’s molar site (a1 – mesial view, a2 – stereo labial view, a3 – stereo lingual view, a4 – distal view, a5 – apical view, a6 – mesial view of tooth occluded with the I1/); b) ONHM TH 6, from Thaytiniti (b1 – lingual view, b2 – mesial view of tooth occluded with the i/1) (scale bar 10 mm).

Text-fig. 12. Lower molars of (a) Omanitherium dhofarense and (b) Moeritherium chehbeurameuri. a) stereo occlusal views of ONHM TN 2017-50, posterior lophid of an unworn right lower molar, probably m/2, from Mohammed’s molar site, Dhofar, Oman, b) stereo occlusal views of MNHN 1890-14, left lower molar from “Khenchella”, Algeria (note the longitudinal sulcus in the centre-line of the tooth of Moeritherium and its absence in Omanitherium). Arrows show the course of the sulcus in the molar from “Khenchella” (scale bar 10 mm).

Text-fig. 14. Upper cheek teeth of Omanitherium dhofarense. a) ONHM TN 2017-16, left P3/, stereo occlusal view; b) ONHM TN 2017-17, right P4/, stereo occlusal view; c) ONHM TN 2017-44, right P4/, stereo occlusal view; d) ONHM TN 2017-18, right M1/ stereo occlusal view; e and f) ONHM TN 2017-45, fragments of left M2/, stereo occlusal view; g) ONHM TN 2017-79, right M3/, stereo occlusal view (scale bar 10 mm).

Text-fig. 15. NHMUK 17/2189, upper left central incisor (probably female) of Barytherium from Dor el Talha, Libya. a) mesial, b) lingual, c) labial, d) distal views (scale bar 10 cm).

Text-fig. 16. a) MNHN LBE 007, symphysis with left and right i/1 of Barytherium sp. from Dor el Talha, Libya. (a1 – superior stereo views of the symphysis, a2 – stereo distal view of right i/1 (arrow 1 shows the interstitial contact facet close to the apex caused by abrasion against the right i/2), a3 – section of right i/1 viewed from the radicular end to show the extent of the enamel cover (below the arrows 2) and the dome-shaped dentine mass on the lingual aspect (above the arrows), a4 – stereo occlusal views of right i/1 (note the orientation of the apical wear facet), a5 – stereo mesial view) (scale bar 5 cm); b–c) NHMUK M 82167b, distal stereo views of lower central incisors of Arcanotherium savagei from Dor el Talha, Libya, to show the interstitial wear facets near cervix caused by abrasion against the i/2s (b – left i/1, c – right i/1) (scale bar 10 cm).

Text-fig. 2. Fossiliferous localities in the Ashawq Formation north and north-east of Thaytiniti, Dhofar Governorate, Oman (for latitude, longitude and altitude, see App. 1). The large white area either side of the north-south road is the village of Aydim (map modified from Google Earth).

Text-fig. 17. Upper molars of a) Barytherium sp. and b) Arcanotherium savagei from Dor el Talha, Libya. a) stereo occlusal view of MNHN LBE 002, left maxilla containing M2/ and M3/, b) stereo occlusal view of MNHN LBE 020, left M3/ (scale bar 5 cm).

Text-fig. 18. Barytherium sp. from Dor el Talha, Libya. a) MNHN LBE 13, right upper premolar, stereo occlusal views (scale bar 1 cm) and b) MNHN LBE 004, right mandible containing m/2 and m/3 (b1 – stereo occlusal views, b2 – lingual view) (scale bar 5 cm).