Tropical Natural History 19(1): 8–20, April 2019 2019 by Chulalongkorn University Spinosaurid theropod teeth from the Red Beds of the Khok Kruat Formation (Early Cretaceous) in Northeastern Thailand KAMONLAK WONGKO1,4, ERIC BUFFETAUT2,3, SUCHADA KHAMHA1 AND KOMSORN LAUPRASERT1* 1 Department of Biology, Faculty of Science, Mahasarakham University, Maha Sarakham, 44150, THAILAND 2 CNRS, UMR 8538, Laboratoire de Géologie de l’Ecole Normale Supérieure, PSL Research University, 24 rue Lhomond, 75231 Paris Cedex 05, FRANCE 3 Palaeontological Research and Education Centre, Mahasarakham University, Maha Sarakham, 44150, THAILAND| 4 Department of Mineral Resources, Khon Kaen, 40150, THAILAND *Corresponding author. Komsorn Lauprasert ([email protected]) Received: 8 August 2018; Accepted: 14 March 2019 ABSTRACT.– Teeth from the Aptian – Albian Khok Kruat Formation of the Khorat Group in Northeastern Thailand, are described as belonging to spinosaurine dinosaurs. They consist of two morphotypes, i.e., the Khok Kruat morphotype and Siamosaurus morphotype. The Khok Kruat morphotype shows fine ridges and a smooth enamel surface of the crown while the Siamosaurus morphotype shows coarse ridges and a wrinkled enamel surface. Comparisons of the Thai material with the teeth of Asian and other spinosaurids indicate that the Khok Kruat morphotype is similar to other Asian spinosaurid teeth and more closely to those of Siamosaurus suteethorni or Siamosaurus morphotype from the older Sao Khua Formation of Northeastern Thailand. The differences in their morphology indicate a greater diversity of spinosaurids in Thailand and Asia. KEY WORDS: Spinosauridae, Early Cretaceous, Northeastern Thailand, Khorat Group, Asia INTRODUCTION the dinosaurs from Thailand, including isolated teeth similar to those of The first Thai spinosaurid dinosaur, Siamosuarus found in the Khok Kruat Siamosaurus suteethorni Buffetaut and Formation, which is younger than the Sao Ingavat, 1986, was described on the basis of Khua Formation. Two years later, in 2005, isolated teeth without bone material Buffetaut and his colleagues also insist the (Buffetaut & Ingavat, 1986). It is only existence of spinosaurid in the Khok Kruat known by the peculiar isolated teeth which Formation by discovery of the teeth with a have tall and slightly compressed crowns number of blade-like teeth and serrations on with a ribbed enamel and very faint or non– both margins at Khok Pha Suam, Ubon existent serrations. Based on the isolated Ratchathani Province, as well as cervical teeth, this dinosaur strongly evoked a and dorsal vertebrae of a spinosaurid from spinosaurid theropod (Buffetaut et al., Khon Kaen Province which had been found 2003). It came from the Sao Khua by staff of the Department of Mineral Formation, which is referred to the Resources. However, these Khok Kruat Barremian age based on pollen, molluscs teeth have not yet been described in detail and vertebrate microremains (Racey, 2009; and differences in their size and morphology Tumpeesuwan, 2010; Khamha, 2017). In strongly suggest that there are several 2003, Buffetaut and his colleagues reviewed spinosaur taxa in Thailand. Asian WONGKO ET AL. — SPINOSAURID THEROPOD TEETH FROM THE KHOK KRUAT FORMATION 9 FIGURE 1. Geological map of the Khorat Plateau showing the three studied localities (red stars) spinosaurid dinosaurs was revealed when address the presence of spinosaurid taxa in the teeth have been found from Sao Khua Southeast Asia. Formation (Buffetaut et al., 2018). Since then, fieldwork in northeastern Thailand as Geological setting part of Thai–French research projects on the The Khok Kruat Formation is well Mesozoic vertebrates of Thailand during distributed in the outer parts of the Phu Phan 2003–2008 has also yielded more teeth from Range along the outer rims of the Phu Phan many localities in the Khok Kruat Formation with conformable contacts. The Formation including the Sam Ran, Khok sharp contact with the basal anhydrite of the Pha Suam and Lam Pao Dam sites. These overlying Maha Sarakham Formation was spinosaurid specimens can be separated by observed (Hite 1974; Hite and Jappakasetr, the number of ridges and the enamel surface 1979) and was reported on seismic profiles of a crown. Here, we compare Khok Kruat (Sattayarak et al., 1991). Generally, the tooth material with other spinosaurid teeth Khok Kruat Formation consists of reddish from Asia, Europe and South America in brown, fine- to medium-grained sandstones; order to allow us understand the diversity siltstones, mudstones and conglomerates are and geographical distribution of the also present. Its distribution on the Khorat Spinosauridae as well as the biogeography Plateau is shown in Figure 1. of Thai Spinosaurid which can help to 10 TROPICAL NATURAL HISTORY 19(1), APRIL 2019 Three significant fossil assemblages of current being observed in the Khok Pha the Khok Kruat Formation in northeastern Suam Unit. Thailand i.e., Sam Ran, Khok Pha Suam and Lam Pao Dam, are discussed in the present MATERIAL AND METHODS paper. The geological settings of these localities are described below: Almost sixty spinosaurid teeth were - The deposits exposed at Sam Ran collected by Staffs of the Department of locality consist of reddish brown siltstone Mineral Resources during the last decade interbedded with fine- to medium- grained from 3 localities of the Khok Kruat sandstone. The beds of sandstone show Formation, i.e., Sam Ran site, Khon Kaen load-casts, rip-up casts and large-scale Province; Khok Pa Suam site, horizontal cross-bedding. Ubonratchathani Province and Lam Pao - The deposits exposed at Khok Pha Dam site Kalasin Province. All the material Suam locality are of reddish brown color, is housed in the Phu Wiang Fossil Research very thin to thin bedded claystone grading up Center and Dinosaur Museum, Khon Kaen to thin bedded siltstone interbedded with fine- Province where tooth material was to medium- grained sandstone. Sandstone subsequently prepared with scalpel (blades beds are micro-crossbedded. A calcrete number 10 and 11) and pneumatic air pen. horizon (paleosol) has been found at the top Only eight specimens, with completely of the succession. preserved base and crown of the teeth, were - The Lam Pao Dam sedimentary described and compared with spinosaurid sequence consists of fining-upward teeth from Asia including Siamosaurus sequences of channelized conglomerates suteethorni, GMNH – PV –999 from Japan, grading up to reddish brown, thin to Chinese spinosaurid from Guangxi IVPP– medium bedded, medium- to coarse- 4793 from China, spinosaurids from the grained sandstone interbedded with greenish Kem Kem Beds of Morocco, spinosaurine gray, thin bedded siltstone. Irritator from Brazil, baryonychines The lithology and sedimentary structures Suchomimus from Niger and Baryonyx from of these three localities suggest that they England. Characters of the teeth were were formed in a meandering system studied using the technical terms of Smith et deposit. The Sam Ran Unit is composed of al (2005) as follows: the crown base length sandstone and siltstone channel deposits (CBL) is the mesiodistal length of the crown corresponding to a crevasse splay sequence. at the level of the cervix; the crown base The Khok Pha Suam Unit consists of width (CBW) is the labiolingual width of siltstone and claystone channel deposits the crown at the cervix level, perpendicular classical of flood plain sequences (Einsele, to CBL; the crown base ratio (CBR) is the 1992). The Lam Pao Dam Unit shows ratio of CBW to CBL and shows the conglomerate channel deposits, characteri- labiolingual compression; crown height zing a point bar sequence (Einsele, 1992). It (CH) is the basoapical extent of the distal seems that the Lam Pao Dam Unit was margin of the crown, from the most distal deposited in a high energy river current. The point of the cervix to the most apical point Sam Ran Unit was deposited in a low of the apex; the crown height ratio (CHR) is energy current, an even lower energy the ratio of CH to CBL and shows the WONGKO ET AL. — SPINOSAURID THEROPOD TEETH FROM THE KHOK KRUAT FORMATION 11 FIGURE 2. Dorsal view of an upper jaw of spinosaurid skull (modified from Smith et al., 2005) in A, technical terms of spinosaurid tooth in B (lateral view) and C (cross-section) (photo by Ployphan Chittarach, 2017) crown elongation; the apical length (AL) is Institutional Abbreviations: PM stands the basoapical extent of the mesial margin of for Phu Wiang fossil research center and the crown; the mesial serration density is the Dinosaur Museum; GMNH stands for the number of denticles per mm on the mesial Gunma Museum of Natural History, margin; initials change according to the Tomioka, Gunma, Japan; IVPP stand for position of the denticles (MA refers to Institute of Vertebrate Paleontology and mesioapical denticles per mm, MC refers to Paleoanthropology, Beijing, China mesiocentral denticles per mm, MB refers to mesiobasal denticles per mm); the distal RESULTS serration density is the number of denticles per mm on the distal margin (DA refers to Systematic palaeontology distoapical denticles per mm, DC refers to distocentral denticles per mm, DB refers to Dinosauria Owen, 1842 distobasal denticles per mm); the average Theropoda Marsh, 1881 mesial serration density (MAVG) is the Tetanurae Gauthier, 1986 arithmetric mean of denticles per mm along Spinosauridae Stromer, 1915 the mesial carina (MA+MC+MB/3); the Spinosaurinae Stromer, 1915 average distal serration density (DAVG) is Spinosaurinae indet. the arithmetic mean of denticles per mm along the distal carina (DA+DC+DB/3) (Figure 2). 12 TROPICAL NATURAL HISTORY 19(1), APRIL 2019 The described material in this paper and slightly serrated. The crown surface is belongs to the collections of the Department covered with distinctive striations. Each of Mineral Resources. In this study, there striation runs almost along the entire length are 8 specimens (Figure 3) from 3 localities, of the crown. The enamel surface of the i.e., Sam Ran site (n=2), Khon Kaen crown of PM2016–1–003 and PM2016–1– Province, Khok Pa Suam site (n=3), 004 is smooth while the base of the crown Ubonratchathani Province and Lam Pao enamel surface show wrinkles.