Stegodon cf. orientalis Owen, 1870

Suraprasit, Kantapon, Jaeger, Jean-Jacques, Chaimanee, Yaowalak, Chavasseau, Olivier, Yamee, Chotima, Tian, Pannipa & Panha, Somsak, 2016, The Middle Pleistocene vertebrate fauna from Khok Sung (Nakhon Ratchasima, Thailand): biochronological and paleobiogeographical implications, ZooKeys 613, pp. 1-157 : 11-16

Taxon classification Animalia Proboscidea Stegodontidae

Stegodon cf. orientalis Owen, 1870

Referred material.

A right DP4 (posterior part), DMR-KS-05-03-28-14; a left DP4 (anterior part), DMR-KS-05-03-19-7; a left M2, DMR-KS-05-03-29-1 (posterior part); a right M3, DMR-KS-05-03-22-19 (posterior part); a fragmentary tusk, DMR-KS-05-03-15-2; a left dp3 (anterior part), DMR-KS-05-04-01-8; two mandibles with m3-DMR-KS-05-03-08-1 (right) and DMR-KS-05-03-08-2 (left); a right humerus fragment (proximal part), DMR-KS-05-03-10-5; a left humerus, DMR-KS-05-03-10-6; two ulna fragments (proximal parts)-DMR-KS-05-03-09-7 and DMR-KS-05-03-10-2; a femoral head fragment, DMR-KS-05-03-10-3; a right femur, DMR-KS-05-03-10-4; a right tibia fragment (distal part), DMR-KS-05-03-10-3; a right fibula, DMR-KS-05-03-00-124; two pelvis fragments-DMR-KS-05-03-10-11 (right) and DMR-KS-05-03-10-12 (left); five vertebrae-DMR-KS-05-03-17-11, DMR-KS-05-03-10-7, DMR-KS-05-03-09-18, DMR-KS-05-03-10-1, and DMR-KS-05-03-28-20; a sacrum fragment, DMR-KS-05-03-10-8; two ribs-DMR-KS-05-03-10-13 and DMR-KS-05-03-10-14; three rib fragments-DMR-KS-05-03-09-6 (body), DMR-KS-05-03-09-45 (body), and DMR-KS-05-03-09-4 (head and neck).

Material description.

Upper dentition: both fragments of DP4 (DMR-KS-05-03-28-14: Fig. 7A, B) and DMR-KS-05-03-19-7: Fig. 7C) are slightly worn and unworn respectively (for measurements, see Tab. 4). The first specimen lacks two or three anterior ridges, whereas the second specimen preserves only the anterior cingulum and the first ridge. DMR-KS-05-03-28-14 has a rectangular outline in occlusal view, a convex crown base in lateral view, and a posterior cingulum. These characters indicate that this specimen belongs to a posterior lobe of DP4. The buccal and lingual surfaces of ridges display subvertically developed grooves. A median cleft is well-developed and runs from anteriorly to posteriorly in the middle part of the tooth, starting from the halfway height of the crown. The second anterior ridge of DMR-KS-05-03-28-14 shows displacement between the pretrite and posttrite halves, a character sometimes present in deciduous molars of derived Stegodon . Each ridge bears ten to twelve mammillae.

DMR-KS-05-03-29-1 (M2) preserves three posterior ridges with a small cingulum (Fig. 7E, F and Tab. 4). Two anterior ridges bear slightly worn mammillae with stronger abrasion on the buccal side. The posterior-most ridge is unworn and reduced in width. The outline of the buccal side is concave in occlusal view and the base of the crown is nearly straight in lateral view. The median cleft is weakly developed. The number of the mammillae on each ridge ranges from eight to eleven.

DMR-KS-05-03-22-19 (M3) preserves only three posterior ridges with a cingulum (Fig. 7G, H and Tab. 4). The ridges are slightly worn with more abraded buccal surfaces. The general outline of this tooth is similar to that of M2, but is comparatively wider and displays a more developed posterior cingulum. The median cleft is poorly developed. Each ridge consists of eight to ten mammillae.

A fragmentary tusk (DMR-KS-05-03-15-2) contains dentine (outer and inner layers), cementum, and a pulp cavity (Fig. 7 I–K). It is slightly curved upward and sub-rounded in cross-section for both the proximal and the distal section. A median longitudinal groove is present on the dorsal surface. The Schreger pattern commonly developed in elephantoid tusks is visible on the inner dentine layer. The maximum length of DMR-KS-05-03-15-2 is 159.2 mm and the mediolateral and dorsoventral diameters of the proximal cross-section are 73.88 and 70.56 mm, respectively. The outline of the tusk (DMR-KS-05-03-15-2) resembles Stegodon trigonocephalus in its more medial-laterally than the dorso-ventrally compressed cross-section. The macroscopic distinctive features in cross-section are similar to Stegodon sompoensis ( van den Bergh 1999) but show the incremental lines more obviously.

Lower dentition: DMR-KS-05-04-01-8 (dp3) is heavily worn and comprises three preserved ridges and an anterior cingulum (Fig. 7D and Tab. 4). The buccal part of the third ridge is broken but it is presumably wider than the second ridge. The dp3 is subrectangular in outline or tapers towards the anterior part. The lateral sides between the first and second ridges are distinctly constricted.

Two hemi-mandibles of the same individual (DMR-KS-05-03-08-1 and DMR-KS-05-03-08-2) are moderately well-preserved (Tab. 4). The completely erupted m3 has eight ridges with small posterior cingulids (Fig. 7L, M). The symphysis and most of the ramus are broken away. The mandibular corpus is robust. We estimate the total number of ridges to be eleven based on the position on the corpus of the anterior root that supports two first lophs in Stegodon ( Saegusa et al. 2005). The anteriormost preserved ridge is thus the third ridge, broken at its anterior and lateral parts in both specimens. The third to sixth ridges are strongly worn, whereas more posterior ridges are successively less damaged by abrasion. Valleys between the ridges are moderately filled with abundant cement. There is no median cleft. The m3 is much more elongated and contains five mammillae on the posteriormost ridge. The mammillae increase in size successively from the anterior to posterior ridge.

Postcranial remains: postcranial elements include two humeri (Fig. 8A, B), two ulnae, two femora (Fig. 8C, D), a tibia, a fibula (Fig. 8E), two pelvis girdles (Fig. 8F, G), five vertebrae, a sacrum (Fig. 8J), and five ribs (Fig. 8K, L) (for measurements, see Appendix 1). All postcranial bones excluding some vertebrae belong to a single individual because they were found together in association with two mandibles with the m3 (DMR-KS-05-03-08-1 and DMR-KS-05-03-08-2) and show fully fused epiphyses. This individual is a senior adult due to the heavy wear on the anterior lophs on the m3. Only two vertebrae (DMR-KS-05-03-09-18: Fig. 8H and DMR-KS-05-03-10-7: Fig. 8I) were found in association with that individual. The specimen DMR-KS-05-03-26-38 is a juvenile because the vertebral body is not fused.

Taxonomic remarks and comparisons.

The proboscidean cheek teeth from Khok Sung are assigned to Stegodon because there are more than five ridges or loph(id)s on molars, V-shaped valleys between ridges on molars, and step-like worn surface reliefs on the enamel layer ( Saegusa 1996, Saegusa et al. 2005). The Khok Sung material shows well-developed cheek tooth features of derived Stegodon (e.g., a greater number of ridges and mammillae, high filled cements between the ridges, and a high angled cliff on the enamel surfaces (step-like structure "type 3", in Saegusa (1996)).

The morphologies and ridge sizes of upper molars from Khok Sung are congruent with Chinese Stegodon orientalis (Tabs 5-7). However, we suggest that some comparative upper third molars of Stegodon orientalis (e.g., IVPP V5216-9) represent a total ridge number of ten (excluding anterior and posterior halfridges), different from the ridge formula ( ×11× for this species) given by van den Bergh et al. (2008: table. 3). The ridge formula of the M3 of Stegodon orientalis therefore ranges from ten to eleven. The m3 of Stegodon orientalis commonly has a total number of twelve ridges (excluding anterior and posterior halfridges). According to the fact that only a few comparative specimens of the m3 of Stegodon orientalis are complete with the total ridge number of twelve, some of them (e.g., IVPP V1777 and IVPP V5216-16, based on our observations) display a total of 11 ridges (excluding anterior and posterior halfridges). In Stegodon orientalis , the number of ridges on the m3 thus ranges from eleven to twelve. Stegodon insignis has a total number of ridges ranging from eleven to thirteen ( van den Bergh et al. 2008). The ridge formula of Stegodon trigonocephalus trigonocephalus is almost thirteen (excluding anterior and posterior halfridges) ( van den Bergh 1999). Another subspecies, Stegodon trigonocephalus praecursor, has a lower number of ridges ( ×11×, van den Bergh et al. 2008: table. 3). The m3 of the Khok Sung stegodontid share a similar ridge formula ( ×11×) with Stegodon orientalis from South China and Stegodon insignis from Punjab (Siwaliks). But it differs from Stegodon insignis in having more delicately folded enamel, more pronounced curvature of the crown, and V-shaped valleys (between the two ridges) slightly less filled by cements. The ridge sizes of Khok Sung lower third molar are almost comparable to those of Stegodon orientalis and Stegodon insignis , but are distinctly larger than other derived Stegodon species from Indonesia (Tab. 8). We thus identify hereby all cheek teeth as belonging to Stegodon cf. orientalis .