Sirindhorna khoratensis Shibata et al., 2015

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

Sirindhorna khoratensis Shibata et al., 2015

Material.

holotype NRRU3001-166, an articulated braincase including referred skull elements: a braincase articulating with a left postorbital (NRRU-A2035), dorsal half of a braincase (NRRU3001-65), caudal portion of a braincase (NRRU3001-179), a right premaxilla (NRRU-A3623), a left maxilla (NRRU-A2048) (Fig. 3K ), a right maxilla (NRRU-A2047), a right jugal (NRRU3001-7), a right quadrate (NRRU3001-175), a predentary (NRRU3001-169), a left dentary (NRRU3001-14), a right dentary (NRRU3001-167) (Fig. 3L ), a right surangular (NRRU3001-137), isolated maxillary teeth (NRRU-A1956, A3630, A3649, NRRU3001-157, 163), an isolated dentary tooth (NRRU3001-28).

Locality and age.

Ban Saphan Hin (a different site from the S. nimngami was found), Suranaree Subdistrict, Nakhon Ratchasima Province; late Early Cretaceous Khok Kruat Formation (Aptian).

Previous study.

It is known from the presence of several braincases and dentaries that at least four individuals are known. The holotype material, a braincase, shows an autapomorphy: a sagittal crest extending along the entire dorsal surface of the parietal and reaching the frontoparietal suture ( Shibata et al. 2015). Referred materials display a unique combination of characters, such as antorbital fossa of the maxilla not visible, a slightly rostrally deepening dentary ramus, and dentary teeth with primary and secondary ridges, but no accessory ridges ( Shibata et al. 2015). It was recovered in the basal position of non-hadrosaurid hadrosauroids ( Shibata et al. 2015). However, later analysis recovered it near the base of Hadrosauromorpha, more advanced than R. suranareae ( Madzia et al. 2020). The S. khoratensis maxilla (NRRU-A2048) has a low-angled triangular shape and the caudally positioned lacrimal process, and its 24 alveoli are rostrocaudally arranged and slightly curved caudolaterally. It is different from the isosceles triangular shape with a dorsal process positioned at the middle of the maxilla in S. nimngami ( Shibata et al. 2015). R. suranareae has a low and elongated dentary ramus and a robust coronoid process that differs from the robust and straight dentary ramus with a subvertical coronoid process seen in S. khoratensis ( Shibata et al. 2015).

Comment.

S. khoratensis is considered to be the best-preserved iguanodontian ornithopod in Southeast Asia ( Shibata et al. 2015). In addition to the published cranial material, the Ban Saphan Hin locality has also yielded a postcranial skeleton that is assumed to belong to S. khoratensis because the five discovered braincases show no features to imply the presence of different taxa ( Shibata et al. 2018). CT-scanning revealed the brain morphology of S. khoratensis , which has general endocast features resembling those of non-hadrosaurid hadrosauroids ( Shibata et al. 2018).

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).