Siamraptor suwati: A new species of Allosauroid Dinosaur from the Early Cretaceous of Thailand.

The Tetanurans are a large subgroup of the Theropod Dinosaurs, including the Spinosaurs, Tyranosaurs, Ornithomimids, Maniraptors, Therizinosaurs, Oviraptors, Birds, Troodontids, Dromeosaurs, Allosaurs, Raptors and other groups. They appeared in the Middle Jurassic and different groups have flourished ever since, though only one group, the Birds, survived the end-Cretaceous extinction. The first of these groups to come to prominence were the Allosauroids and Megalosauroids, groups which began to produce large-bodied species in the Middle Jurassic, and dominated the top-predator roles in most terrestrial ecosystems until the early Late Cretaceous.

In a paper published in the journal PLoS One on 9 October 2019, Duangsuda Chokchaloemwong of the Northeastern Research Institute of Petrified Wood and Mineral Resources at Nakhon Ratchasima Rajabhat University, Soki Hattori of the Institute of Dinosaur Research at Fukui Prefectural University, and the Fukui Prefectural Dinosaur Museum, Elena Cuesta, also of the Institute of Dinosaur Research at Fukui Prefectural University, Pratueng Jintasakul, also of the Northeastern Research Institute of Petrified Wood and Mineral Resources at Nakhon Ratchasima Rajabhat University, and Masateru Shibata and Yoichi Azuma, again of the Institute of Dinosaur Research at Fukui Prefectural University, and the Fukui Prefectural Dinosaur Museum, describe a new species of Allosauroid Dinosaur from the Early Cretaceous Khok Kruat Formation of Nakhon Ratchashima Province in northeastern Thailand.

The Khok Kruat Formation forms the uppermost unit of the Khorat Group and is widely distributed in the Khorat Basin of northeastern Thailand. The formation is 430–700m thick and consists mainly of reddish-brown siltstones and sandstones, laid down in a fluvial (river) environment. The formation is of Aptian age, between 125 and 113 million years old. The Khok Kruat Formation has produced a number of Dinosaur fossils, most of them from the Ban Saphan Hin (Saphan Hin Village) site in the northwest of the Muang District in Nakhon Ratchashima Province, which has been excavated by the Japan-Thailand Dinosaur Project.

Locality map of new Theropod material and stratigraphy of Khorat Group. (A) map of Nakhon Ratchasima Province, Thailand; (B) distribution map of the Khok Kruat Formation in Nakhon Ratchasima Province; (C) enlarged locality map of Suranaree and Khok Kruat subdistricts with the subdistrict boundaries; (D) a photograph of the excavation site; (E) stratigraphic column of the Khorat Group. A red-coloured star indicates the new Theropod locality, the dotted lines indicate the subdistrict boundaries, and the grey-colored lines indicate the roads in (C) respectively. Chokchaloemwong et al. (2019).

The new species is named Siamraptor suwati, where 'Siamraptor' means 'Thailand thief' and 'suwati' honours Suwat Liptapanlop, who supports and promotes the work of the Northeastern Research Institute of Petrified Wood and Mineral Resources. The new species is described from disarticulated cranial and postcranial elements from at least three individuals. This material comprises three right premaxillae, a right and a left maxillae, a left jugal, two posterior parts of the left mandible comprising the surangular, prearticular, and articular, a posterior part of the left mandible comprising the surangular and prearticular, three anterior cervical vertebrae, three posterior dorsal vertebrae, a middle caudal vertebra, a manual ungual, a right ischium, a distal part of the right tibia, and a left pedal phalanx IV-1. All of these are materials were found in a small area (125 m x 160 m) of a single layer of a single locality, and the overlapping materials exhibit the same diagnostic features.

Skeletal reconstruction of Siamraptor suwati. Cranial elements were scaled to fit in with the holotype (surangular). Scale bar equals 1 m. Chokchaloemwong et al. (2019).

Siamraptor suwati is adjudged to be an Allosauroid on the basis of a straight ventral margin on the jugal (cheekbone), a dorsoventrally deep anterior process below the orbit, a surangular (bone at the back of the jaw) with a deep oval concavity at the posterior end of the lateral shelf and four posterior
surangular foramina (openings), a long and narrow groove along the suture between the surangular and prearticular bones, an articular bone with a foramen at the notch of the suture with prearticular bone, an anterior cervical vertebra (formost vertebra in the neck) with an additional pneumatic foramen in the  parapophysis (transverse processe that projects from the centrum of the vertebra), and cervical and posterior dorsal vertebrae penetrated by a pair of small foramina bilaterally at the base of neural spine. 

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