Ratchasimasaurus suranareae Basal neornithischian indet. ('Dan Luang neornithischian' )

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 Basal neornithischian indet. ('Dan Luang neornithischian')

Material.

SM2016-1-081, a left femur ( Buffetaut et al. 2003, 2006, 2014) (Fig. 5 ).

Locality and age.

Dan Luang locality, Kamcha-I District, Mukdahan Province; upper Phu Kradung Formation,?Early Cretaceous.

Previous study.

This is the first basal neornithischian specimen to have been discovered in Thailand, excavated in 1996, but it has not yet been described ( Buffetaut and Suteethorn 1998a; Buffetaut et al. 2001, 2002, 2003, 2006). Buffetaut and Suteethorn considered that it is generally similar to Yandusaurus (= Hexinlusaurus multidens ) from China ( Buffetaut and Suteethorn 1998a).

Description.

The left femur is robust and almost complete, except the distal end is eroded. It is 12.08 cm in length, and has a transverse mid-shaft diameter of 1.57 cm. The shaft of the femur is bowed in the lateral view resembling that of those early ornithopods and basal neornithischians, such as Hexinlusaurus multidens ( He and Cai 1984), Agilisaurus louderbacki ( Peng 1992), Hypsilophodon foxii ( Galton 2009) and the Phu Noi neornithischian ( Manitkoon et al. 2019). The femoral head is mostly intact, but the finished, articular surface is unpreserved. The anterior end of the greater trochanter is slightly convex, while the posterior end is strongly convex. The greater trochanter lies upon the same plane as the femoral head. The lesser trochanter is distinguished from the greater trochanter by a deep groove. The fourth trochanter, located on the medial margin, is incomplete, its position being in the proximal half of the femur as in PRC 150. An oval fossa occurs between the base of the fourth trochanter and the femoral shaft for muscle insertion. The distal portion of the femoral shaft shows mediolaterally expansion towards the distal condyles and, although it was damaged, posteriorly the distal condyles are separated by a caudal intercondylar groove.

Comments.

The Dan Luang locality has yielded mamenchisaurid teeth ( Suteethorn et al. 2013), a possible mamenchisaurid rib, theropod teeth, crocodyliform osteoderms and large teeth resembling the pholidosaurid Chalawan thailandicus ( Buffetaut and Ingavat 1980; Martin et al. 2014), shell fragments of the basal trionychoid turtle Basilochelys macrobios ( Tong et al. 2009), petrified wood, and amber. The site belongs to the upper part of the Phu Kradung Formation (?basal Cretaceous), based on the appearance of large pholidosaurid crocodylomorphs and large trionychoid turtles. This contrasts with the semi-aquatic fauna, such as the small xinjiangchelyid turtles Phunoichelys thirakhupti , and Kalasinemys prasarttongosothi ( Tong et al. 2015, 2019b), and teleosaurid Indosinosuchus potamosiamensis ( Martin et al. 2019), from the lower Phu Kradung localities, such as Phu Noi. We suggest that SM2016-1-081 belongs in a basal position in Neornithischia, as with the older Phu Noi taxon, but more specimens are needed to increase our understanding of the basal neornithischians from the upper Phu Kradung Formation.

Figures

Figure 5. Left femur of the ' Dan Luang neornithischian' (SM 2016 - 1 - 081) in anterior (A), posterior (B), lateral (C) medial (D) proximal (E) and distal (F) views; Reconstruction showing the bone in left lateral view (G); Abbreviations: fh, femoral head; ft, fourth trochanter; gt, greater trochanter; mc, medial condyle; lc, lateral condyle; lt, lesser trochanter; pg, posterior intercondyle groove. Scale bar: 5 cm.