Ratchasimasaurus suranareae Shibata et al., 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

Ratchasimasaurus suranareae Shibata et al., 2011

Material.

holotype NRRU-A2064, a left dentary (Fig. 3J ).

Locality and age.

Ban Pong Malaengwan, Khok Kruat Subdistrict, Nakhon Ratchasima Province; late Early Cretaceous Khok Kruat Formation (Aptian).

Previous study.

R. suranareae is a hadrosauroid ( Madzia et al. 2020; Shibata et al. 2015) and the material comprises a complete toothless left dentary with 18 alveoli ( Shibata et al. 2011). One autapomorphy of R. suranareae is its elongated and dorsoventrally shallow dentary ramus, with a ratio of length (from the rostral to the caudal margin)/height (at the middle of the dentary) of 6.9 ( Shibata et al. 2011). It shows both primitive and derived characters for iguanodontians, such as a caudally inclined coronoid process and alveolar trough with a primitive crown impression, and a derived buccal shelf between the tooth row and the coronoid process ( Shibata et al. 2011).

Comment.

The length of R. suranareae dentary is 19.81 cm, which is relatively small when compared to other skull material of iguanodontians from Thailand. It is not possible to determine if it is an immature or mature individual ( Shibata et al. 2011). This compares with the nearly complete right dentary of Sirindhorna khoratensis (NRRU3001-167), which is about 38 cm in length with 20 alveoli ( Shibata et al. 2015). In iguanodontians, tooth number increases during growth and single teeth also becoming relatively wider, as in Dysalotosaurus lettowvorbecki and Zalmoxes robustus , and there is a slight ontogenetic increase of dentary tooth positions from 10 to 13 and 8 to 10, respectively ( Weishampel et al. 2003; Hübner and Rauhut 2010). In hadrosauroids, there are many ontogenetic changes occurring in the skull and mandible, and the dentary experiences an elongation of the mandibular ramus during growth ( Bell 2011; Campione and Evans 2011; Prieto-Marquez and Guenther 2018). However, the ratio of length/height of the dentary of R. suranareae is 6.9, and approximately 5 in S. khoratensis (NRRU3001-167), contradicting the ontogenetic tend of hadrosauroids mentioned above, if R. suranareae is a younger stage of S. khoratensis .

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).