Siamochoerus Ducrocq, Chaimanee, Suteethorn & Jaeger 1998
Creators: name: Ducrocq, Stéphane
name: Yamee, Chotima
name: Rugbumrung, Mana
name: Chaimanee, Yaowalak
name: Jaeger, Jean-Jacques
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Artiodactyla
Suidae
Siamochoerus

description: Genus Siamochoerus Ducrocq, Chaimanee, Suteethorn & Jaeger, 1998
TYPE SPECIES. — Siamochoerus banmarkensis Ducrocq, Chaimanee, Suteethorn & Jaeger, 1998.
INCLUDED SPECIES. — Siamochoerus viriosus Liu, 2001.
MATERIAL EXAMINED. — Holotype. Thailand. Fragmentary lower jaw with left p4-m3, TF 2905.
REFERRED MATERIAL. — Right lower jaw with i1-i3, c, p1-m3 associated with left lower jaw with i1-i3, c, p2-m3 (BM- 04-09-30-1); left lower jaw with p2 and m1-m3 (BM- 06-08-03 -5.1); right lower jaw with p2 and m1-m3 (BM- 06-08-03 -5.2); left m3 (BM- 04-07- 26-1 b); right M3 (BM- 04-07-26-1 a); right M3 (BM- 06-08-04-2); left M3 (BM- 06-08-04-3); right M1 (BM- 06-08-04-5).
EMENDED DIAGNOSIS. — Primitive suid close to Palaeochoerus typus Pomel, 1847 in size with small verrucosic lower canine (buccal side wider than the distal side), very short diastemata between c-p1 and p1-p2, p1-p3 without accessory cusps, p4 with a small metaconid and a moderately developed hypoconid. Lower molars with the mesial lobe wider than the distal lobe, and almost absent mesoconulid. Upper molars with a distinct centroconule and a small paraconule slightly mesial to the paracone and protocone, and divergent roots. Differs from Siamochoerus viriosus Liu, 2001 in being smaller, in having mesial lobe wider than the distal one on m1 and m2, and in having buccal cuspids in line with the lingual ones. Differs from Eocenchoerus savagei Liu, 2001 by its less distally developed M3 with a centroconule. Differs from Huaxiachoerus guangxiensis Liu, 2001 in being larger and in having a centroconule on upper molars.
TYPE LOCALITY AND HORIZON. — Bang Mark lignite pit, Krabi coal mine (southern Thailand), late Eocene.
DESCRIPTION
The better-preserved mandibles (BM-04-09-30-1) still exhibit a complete dentition, and the horizontal ramus that displays a rather constant depth from p2 to m3 (depth under p2: 23.5 mm; under p4: 21.8 mm; under m3: 23.5 mm). In dorsal view, it is swollen buccally beneath the molars with a shallow lingual fossa (Fig. 1B). The shallow and almost horizontal symphysis extends to the back of the p2. The two symphyses do not seem to be fused (Fig. 2A, C). The root of the ascending ramus rises behind the back of the m3, and the distal part of the coronoid apophysis that is still preserved on BM06-08-03-5.1 is vertical (Fig. 3). Two mental foramina are visible at mid-height of the ramus, one beneath the mesial root of the p2 and one beneath the p4 on BM-04-09-30-1 (Fig. 1A), and only one foramen is visible under p2 on BM-06-08-03-5.2 (Fig. 4A).
The three incisors are forwardly protruding, spatulate and mesiodistally short, the i1 being the longest with a horizontal apex. Its buccal face is convex and its lingual face is concave with a longitudinal central ridge extending from the cervix to about 3 mm under the apex. The i2 is slightly shorter than the i1 and displays a similar shape except for its oblique apex higher mesially than distally. It also exhibits a central longitudinal ridge on its lingual face that extends from the cervix to about 4 mm under the apex. The i3 is the smallest incisor with a shorter and oval-shaped crown in buccal view. It is a smaller version of the i2 rather than a triangular tooth in lateral view. The buccal face is convex and the concave lingual face also displays a central ridge that ends on a very slight enamel swelling above the cervix. There is no diastema between the incisors (Figs 1 A-C; 2A, B).
On BM-04-09-30-1 the small and slender lower canine (not much larger than the p1) is separated from the i3 by a short diastema (about 4 mm). It is slightly laterally splaying, its apex is broken about 6 mm above the root and the crown has a verrucosic section. The distal face is straight to slightly concave (Fig. 1 A-C). It is difficult to attribute BM-04-09-30-1 to a male or a female on the basis of the canine size because it is the only known mandible that preserve this tooth and sexual dimorphism is thus unknown. In addition, the size of male and female canines might have been smaller in primitive suoids than in younger taxa.
There is a very short diastema (3 mm) between the canine and the p1. The p1 is a small and very simple tooth with only one root preserved. It is triangular in lateral view. The buccal face is convex and the lingual face is concave. The apex is broken but two mesial and distal cristids can be distinguished on the crown (Fig. 1A).
The p2 is separated from the p1 by a diastema of about 3 mm. It is a triangular narrow tooth with two roots and the apex above the gap between both roots. The mesial half of the crown is damaged on BM-04-09-30-1 (Fig. 1A) but on the specimens that are better preserved (BM-06-08-03-5.1 and BM-06-08-03-5.2 that very likely belong to the same individual) two slight mesial and distal cristids are present that connect the apex and the mesial and distal end of the crown respectively (Figs 3; 4). There is no extended talonid distally, but a very small enamel spur meets the distal cristid. The buccal face is convex and the lingual face is concave.
The lower p3-p4 and molars have been described in detail in Ducrocq et al. (1998), and only variations in their morphology and additional characters that were not observed because of the preservation of the original material will be added here.
The p3 and p4 on BM-04-09-30-1 exhibit a triangular crown, a convex buccal face and a concave lingual face, and a small metaconid on the lingual face of the p4 slightly distal to the main cuspid. The talonid part is weakly developed and there is no distinct paraconid, but a very slight mesiodistal enamel swelling at the end of the preprotocristid (Fig. 1 A-C).
The lower molars of the new specimens are also similar in morphology and size to those of the holotype TF 2905 (Ducrocq et al. 1998). The m3 BM-04-07-26-1b however, displays a few minor differences compared with the holotype, such as its less well developed preprotocristid and postprotocristid, but the wear facet on the distal face of the protoconid might be responsible for the less distinct development of the postprotocristid (Fig. 5K).
The upper molars of S. banmarkensis that were first described (Ducrocq et al. 1998) are longitudinally cracked and their morphology is thus not known with accuracy. For example, it was noticed that there were no well-defined accessory cusps. The new material is thus described here in more detail. Three M3’s and a possible M1 are present in the new material. The M3 is narrower distally than mesially with the buccal cusps more mesially situated than the lingual ones. The paracone is the largest cusp and a small centroconule is present mesiobuccally to the metaconule (this cusp was not visible on TF 2907 due to the preservation of the molars). A very small paraconule occurs slightly mesial to the paracone and the protocone and is merged in the preparacrista, but tends to disappear with wear. A cingulum is present mesially and distally and is variably developed under the metaconule (strong on BM-06-08-04-2 and weak on BM-06-08-04-5 and BM-04-07-26-1a). The distal part on the M3 is represented by a small distostyle that is often connected to the distal end of the postmetacristule (Fig. 5 A-H). The M1 is morphologically similar to the M3 (Fig. 5 G-H) except for its more square occlusal outline. A very tiny entostyle is present at the lingual end of the transverse valley of the M1 (BM-06-08-04-5) and this structure is much less developed on the M3 (Table 1). Ducrocq et al. (1998) mentioned that the roots of the upper molars of TF 2907 are unfused, but it is unclear. In buccal view, the upper parts of the roots are separated and seem to be divergent (Fig. 5I). In lingual view, the roots of M2 are embedded in the bone but they seem to be separated and divergent at their junction with the crown (Fig. 5J).
description: Published as part of Ducrocq, Stéphane, Yamee, Chotima, Rugbumrung, Mana, Chaimanee, Yaowalak & Jaeger, Jean-Jacques, 2024, New remains of Siamochoerus banmarkensis Ducrocq, Chaimanee, Suteethorn & Jaeger, 1998 (Artiodactyla: Suidae) from the late Eocene of Thailand, pp. 257-268 in Comptes Rendus Palevol 23 (19) on pages 259-264, DOI: 10.5852/cr-palevol2024v23a19, http://zenodo.org/record/12704231