Siamodon nimngami Buffetaut & Suteethorn, 2011

Manitkoon, Sita, Deesri, Uthumporn, Warapeang, Prapasiri, Nonsrirach, Thanit & Chanthasit, Phornphen, 2023, Ornithischian dinosaurs in Southeast Asia: a review with palaeobiogeographic implications, Fossil Record 26 (1), pp. 1-25 : 1

Siamodon nimngami Buffetaut & Suteethorn, 2011

Material.

holotype PRC-4, a left maxilla (Fig. 3I ) and the referred materials, an isolated maxillary tooth (PRC-5) and a braincase (PRC-6).

Locality and age.

Ban Saphan Hin, Khok Kruat Subdistrict, Nakhon Ratchasima Province; late Early Cretaceous Khok Kruat Formation (Aptian) ( Buffetaut and Suteethorn 2011).

Previous study.

S. nimngami shows a combination of iguanodontian features: maxilla shaped like an isosceles triangle, with the dorsal process located at about mid-length of the bone; a strong longitudinal bulge on the medial surface of the maxilla; maxillary teeth bear a strong median primary ridge, one short weak subsidiary ridge or no subsidiary ridge; and mamillated denticles on the crown margins similar to Gongpoquansaurus mazongshanensis and Probactrosaurus mongoliensis from China ( Buffetaut and Suteethorn 2011). There may have been as many as 25 tooth positions in the maxilla ( Buffetaut and Suteethorn 2011). It differs from hadrosaurids in the jugal, which forms a tab-like process, whereas in hadrosaurids, the expanded anterior end of the jugal contacts and overlaps a large sutural area on the maxilla ( Buffetaut and Suteethorn 2011). S. nimngami and R. suranareae were mentioned as members of hadrosauroids ( Shibata et al. 2015), but the phylogenetic analysis recovered S. nimngami at the base of Hadrosauriformes ( Madzia et al. 2020).

Additional material and description.

WNM-Sp-001 (Fig. 8I, J ), an isolated dentary tooth from the same locality is referred to S. nimngami . It is well preserved and apparently from a right dentary. It generally resembles a previously reported S. khoratensis dentary tooth ( Shibata et al. 2015). The crown of the tooth is leaf-shaped with enamel covering only the lingual surface. The ratio of apicobasal length / mesiodistal width: 1.87. One prominent primary ridge situated slightly distal to the mid-line makes the crown asymmetric. The secondary ridge is positioned mesial and it is less prominent. In contrast to S. khoratensis dentary teeth, where the crowns appear to be apicobasally erect, whereas in S. nimngami , the dentary teeth appear to be curved apicobasally. Shibata et al. (2015) noted that there were no other accessory ridges on the crown of S. khoratensis , but WNM-KS-001 shows at least one very faint accessory ridge on the mesial side. Small denticles are present on the mesial and distal margins of the upper half of the crown.

Comment.

Some palaeontologists consider S. nimngami a nomen dubium as its material does not show any autapomorphic characters, and it might be referable to some of the other taxa from the same area ( Norman 2014). However, Shibata et al. (2015) showed the possibility of three iguanodontians in the Khok Kruat Formation. The holotype specimen of S. nimngami and S. khoratensis maxilla (NRRU-A2048) are similar in size. They likely to belong to same growth stage, but have distinct physical characteristics suggesting that they represent different animals, while comparisons to R. suranareae are not feasible as overlapping material has not been discovered.

There has been some disagreement about the type locality of S. nimngami given by Shibata and his team, and Buffetaut and Suteethorn; however, we would like to confirm that, after corroboration from the holotype collector, Mr. Witaya Nimngam, we now know that the type locality of S. nimngami is at Ban Saphan Hin. This is far from the type locality of S. khoratensis and definitely not from Ban Nong Rangka as previously suggested (Fig. 7 ).

Figures

Figure 3. Exceptional specimen of ornithischian dinosaurs from Thailand. Stegosaurid vertebra (A. SM 2011 - 1 - 001), Phu Noi neornithischian left dentary (B. PRC 149), Phu Noi neornithischian articulated skeleton (C. PRC 150), Dan Luang neornithischian left femur (D. SM 2016 - 1 - 081), Psitacosaurus indet. Right femur (E. SM 2016 - 1 - 080), Psitacosaurus sattayaraki right dentary (F. SM 2016 - 1 - 163), Khok Pha Suam iguanodontian dorsal vertebra (G. SM 2021 - 1 - 113), Khok Pha Suam iguanodontian left femur (H. SM 2021 - 1 - 118), Siamodon nimngami left maxilla (I. PRC- 4), Ratchasimasaurus suranareae left dentary (J. NRRU-A 2064), Sirindhorna khoratensis left maxilla (K. NRRU-A 2048), Sirindhorna khoratensis right dentary (L. NRRU 3001 - 167); A and G in anterior view; B-D, and H-J in lateral view; E, F and L in medial view. Scale bars: 10 cm (A, C-E, G-L); 5 cm (B, F).

Figure 7. Locality map of Nakhon Ratchasima Province, showing localities of Siamodon, Ratchasimasaurus and Sirindhorna.

Figure 8. Isolated Thai iguanodontian teeth; Sirindhorna khoratensis maxillary tooth (A, B. NRRU-A 1959) and dentary tooth (G, H. NRRU 3001 - 28); Siamodon nomngami maxillary tooth (C, D. PRC- 5) and dentary tooth (I, J. WNM-Sp- 001); Khok Pha Suam iguanodontian maxillary tooth (E, F. SM 2021 - 1 - 122) and dentary tooth (K, L. SM 2021 - 1 - 121). In labial (A, C, E), mesial (B, D, F), lingual (G, I, K), and distal (H, J, L) views. Abbreviations: ar; accessory ridges, pr; primary ridge, sr; secondary ridge. Scale bars: 0.5 cm (Modified from Buffetaut and Suteethorn (2011); Shibata et al. (2015); Manitkoon et al. (2022)).