Ratchasimasaurus suranareae Iguanodontian indet. ('Khok Pha Suam iguanodontian' )

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 Iguanodontian indet. ('Khok Pha Suam iguanodontian')

Material.

Isolated teeth and postcranial material including: cervical vertebra ( PRC 155); dorsal vertebra (SM2021-1-113) (Fig. 3G ); proximal caudal vertebra (SM2021-1-114); distal caudal vertebrae ( PRC 156 and SM2021-1-115); chevron ( PRC 157); metacarpal (SM2021-1-116); left femur (SM2021-1-117) (Fig. 3H ); right femur (SM2021-1-118); tibia (SM2021-1-119); fibula ( PRC 158); and phalanx (SM2021-1-120).

Locality and age.

Khok Pha Suam, Na Kham Subdistrict, Si Muang Mai District, Ubon Ratchathani Province; late Early Cretaceous Khok Kruat Formation (Aptian-Albian).

Previous study.

Teeth of iguanodontians are common at Khok Pha Suam, but fragmentary ( Manitkoon et al. 2022). Some isolated postcranial bones belonging to iguanodontians were discovered, including vertebrae and limb bones from different individuals, based on the great discrepancy in size of the left and right femur ( Manitkoon et al. 2022; Samathi and Suteethorn 2022). Samathi and Suteethorn assumed that most of the Khok Pha Suam iguanodontian material belonged to a single taxon, and found its phylogenetic position to be a non-hadrosauriform styracosternan ( Samathi and Suteethorn 2022).

Comment.

The teeth of Thai iguanodontians exhibit a robust primary ridge displaced distally relative to the crown apicobasal axis, which is a derived feature of iguanodontians amongst ornithopods ( Norman 2004, 2014). They also possess mammillate marginal denticles, which is a synapomorphy of taxa closer to hadrosaurids than basal ankylopollexians, such as Camptosaurus ( Fanti et al. 2016).

The maxillary teeth of Thai forms, including S. khoratensis (Fig. 8A ), S. nimngami (Fig. 8C ), and the Khok Pha Suam taxon (Fig. 8E ), are diagnostic of the level of ankylopollexian iguanodontian by displaying the prominent primary ridge, accessory ridges, and the vertical channels marking the positions occupied by successional tooth crowns ( Norman 2014). The different maxillary teeth characteristics are as follows: S. khoratensis : lanceolate-shaped crown, primary ridge separates the labial surface unevenly, distal portion of the labial surface bears weak subsidiary ridges and is slightly broader than the mesial portion ( Shibata et al. 2015); S. nimngami : diamond-shaped crown, primary ridge in a median position, no/one short weak accessory ridge is present in what is presumably the mesial half of the crown, in the apical part. ( Buffetaut and Suteethorn 2011); Khok Pha Suam iguanodontian: possibly diamond-shaped crown, primary ridge divides the crown into two asymmetrical halves, at least four weak accessory ridges in what is presumably the mesial portion along the apicobasal axis. The more derived hadrosauroids usually lose the accessory ridges on the crowns of maxillary teeth, and show a shifting of the primary ridge on the maxillary tooth crown to the mid-line ( You et al. 2003).

The dentary teeth of the Thai forms, including S. khoratensis (Fig. 8B ), S. nimngami (Fig. 8D ), and Khok Pha Suam taxon (Fig. 8F ) (not preserved in R. suranareae ) possess a prominent primary ridge. The crowns allowed the teeth to interlock, resulting in the more elaborate structure of the dental battery. The different characteristics are as follow: S. khoratensis : wide with leaf-shaped crown, the secondary ridge is positioned mesial to and is less prominent than the primary ridge, with no other accessory ridges ( Shibata et al. 2015); S. nimngami : leaf-shaped crown, the secondary ridge is positioned mesial to and is less prominent than the primary ridge, at least one faint accessory ridge is present on the mesial side, crown appears to be curved apicobasally; Khok Pha Suam iguanodontian: leaf-shaped crown, the less prominent secondary ridge is positioned mesial to and is less prominent than the primary ridge and at least two weak accessory ridges are present on the mesial and the distal portion. The dentary teeth of S. nimngami and the Khok Pha Suam iguanodontian show accessory ridges, which are absent in S. khoratensis . This character appeared in basal hadrosauroids ( Prieto-Márquez et al. 2016). In this respect, S. khoratensis is probably more advanced than S. nimngami and the Khok Pha Suam iguanodontian.

So far, three taxa of styracosternan iguanodontians, including S. ninngami , R. suranareae , and S. khoratensis , have been described from the Khok Kruat Formation in Nakhon Ratchasima Province plus one Laotian taxon " M. laosensis " from the Grès Supérieurs Formation of Laos. If Khok Pha Suam iguanodontian is one of the previously-named taxa from Nakhon Ratchasima, this would provide a geographic distribution of about 400 km to the far east (Fig. 1 ). However, the Khok Pha Suam locality is closer to Savannakhet than Nakhon Ratchasima. The comparison between them has to be very careful, and overlapping elements are required. It would be significant if the Khok Pha Suam iguanodont is a new taxon, as it would mean that there was a diversity of up to five to six species in the region. It is necessary to compare the postcranial material between the Khok Pha Suam taxon and S. khoratensis .

If a high diversity in iguanodontians is present in Southeast Asia, then careful consideration and more materials will be required. This may be similar to the case of Edmontosaurus , the duck-billed edmontosaurine that was widely distributed in the Late Cretaceous (Campanian-Maastrichtian) ranging from Colorado to Alaska of North America, where several genera were consolidated into two species under a single genus, based on ontogenetic variation, morphometrics and several other factors ( Campione and Evans 2011; Takasaki et al. 2020).

Figures

Figure 1. Southeast Asia map showing the distribution of Jurassic-Cretaceous non-marine sediments in Southeast Asia (modified from CCOP's 1: 2 million Geologic map of East and Southeast Asia), Number in circles = rock formations: 1, Lower Phu Kradung; 2, Upper Phu Kradung; 3, Phra Wihan; 4,? Tembling Group; 5, Gagau Group; 6, Khok Kruat; 7, Gres Superieurs; 8, Xinlong (Thanh and Khuc 2006; Department of Mineral Resources 2014; Uchida et al. 2017; Teng et al. 2019; Yan et al. 2019).

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 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)).