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| Siamraptor suwati
Chokchaloemwong, Hattori, Cuesta, Jintasakul, Shibata & Azuma, 2019
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Abstract
The isolated fossil remains of an allosauroid theropod from the Lower Cretaceous Khok Kruat Formation of Khorat, Thailand, are described in this study. Detailed observations support the establishment of a new allosauroid, Siamraptor suwati gen. et sp. nov. This new taxon is based on a composite cranial and postcranial skeleton comprising premaxilla, maxilla, jugal, surangular, prearticular, articular, vertebrae, manual ungual, ischium, tibia, and pedal phalanx. It is distinguished from other allosauroids by characters such as a jugal with straight ventral margin and dorsoventrally deep anterior process below the orbit, a surangular with a deep oval concavity at the posterior end of the lateral shelf and four posterior surangular foramina, a long and narrow groove along the suture between the surangular and the prearticular, an articular with a foramen at the notch of the suture with the prearticular, an anterior cervical vertebra with a pneumatic foramen (so-called ‘pleurocoel’) excavating parapophysis, and cervical and posterior dorsal vertebrae penetrated by a pair of small foramina bilaterally at the base of the neural spine. The presence of a huge number of camerae and pneumatopores in cranial and axial elements reveals a remarkable skeletal pneumatic system in this new taxon. Moreover, the phylogenetic analyses revealed that Siamraptor is a basal taxon of Carcharodontosauria, involving a new sight of the paleobiogeographical context of this group. Siamraptor is the best preserved carcharodontosaurian theropod in Southeast Asia, and it sheds new light on the early evolutionary history of Carcharodontosauria.
The isolated fossil remains of an allosauroid theropod from the Lower Cretaceous Khok Kruat Formation of Khorat, Thailand, are described in this study. Detailed observations support the establishment of a new allosauroid, Siamraptor suwati gen. et sp. nov. This new taxon is based on a composite cranial and postcranial skeleton comprising premaxilla, maxilla, jugal, surangular, prearticular, articular, vertebrae, manual ungual, ischium, tibia, and pedal phalanx. It is distinguished from other allosauroids by characters such as a jugal with straight ventral margin and dorsoventrally deep anterior process below the orbit, a surangular with a deep oval concavity at the posterior end of the lateral shelf and four posterior surangular foramina, a long and narrow groove along the suture between the surangular and the prearticular, an articular with a foramen at the notch of the suture with the prearticular, an anterior cervical vertebra with a pneumatic foramen (so-called ‘pleurocoel’) excavating parapophysis, and cervical and posterior dorsal vertebrae penetrated by a pair of small foramina bilaterally at the base of the neural spine. The presence of a huge number of camerae and pneumatopores in cranial and axial elements reveals a remarkable skeletal pneumatic system in this new taxon. Moreover, the phylogenetic analyses revealed that Siamraptor is a basal taxon of Carcharodontosauria, involving a new sight of the paleobiogeographical context of this group. Siamraptor is the best preserved carcharodontosaurian theropod in Southeast Asia, and it sheds new light on the early evolutionary history of Carcharodontosauria.
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| Fig 1. 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 (modified from the Geological map of Thailand, Department of Mineral Resources); C, enlarged locality map of Suranaree and Khok Kruat subdistricts with the subdistrict boundaries; E, a photograph of the excavation site; D, stratigraphic column of the Khorat Group.
A red-colored star indicates the new theropod locality, the dotted lines indicate the subdistrict boundaries, and the grey-colored lines indicate the roads in C, respectively.
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| Fig 10. Dorsal vertebrae NRRU-F01020014 (A–D), F01020015 (E, F), and F01020016 (G, H) and a caudal vertebra NRRU-F01020017 (I–M) in posterior (A, F, L), right lateral (B, E), anterior (C, G, J), left lateral (H, K), dorsal (I), and ventral (F) views.
Abbreviations: cc, chevron contact; cdf; centrodiapophyseal fossa; cprl, centroprezygapophyseal lamina; fo, foramen; hpa, hypantrum; hpo, hyposphene; mr, median ridge; ms, metaplastic scars; nc, neural canal; ns, neural spine; pcdf, prezygocentrodiapophyseal fossa; pocdf, postzygocentrodiapophyseal fossa; pnf, pneumatic foramen; poz, postzygapophysis; prz, prezygapophysis; spof, spinopostzygapophyseal fossa; sprf, spinoprezygapophyseal fossa; tp, transverse process. Scale bar equals 100 mm for A–H and 60 mm for I–M.
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| Fig 2. Skeletal reconstruction of Siamraptor suwati. Cranial elements were scaled to fit in with the holotype (surangular). Scale bar equals 1 m. |
Systematic paleontology:
Dinosauria Owen, 1842
Theropoda Marsh, 1881
Tetanurae Gauthier, 1986
Allosauroidea Marsh, 1878
Carcharodontosauria Benson, Carrano and Brusatte, 2010
Siamraptor suwati gen. et sp. nov.
“สยามแรปเตอร์ สุวัจน์ติ”
Diagnosis: Allosauroid theropod with the following autapomorphies among allosauroids: Jugal with straight ventral margin, and dorsoventrally deep anterior process below the orbit; surangular with a deep oval concavity at the posterior end of the lateral shelf and four posterior surangular foramina; long and narrow groove along the suture between surangular and prearticular; articular with a foramen at the notch of the suture with prearticular; anterior cervical vertebra with an additional pneumatic foramen excavating parapophysis; cervical and posterior dorsal vertebra penetrated by a pair of small foramina bilaterally at the base of neural spine.
Holotype: An articulated posterior half of the right mandible comprising the surangular, prearticular, and articular (NRRU-F01020008).
Referred materials: Disarticulated cranial and postcranial elements from at least three individuals; three right premaxillae (NRRU-F01020001–F01020003), a right (NRRU-F01020004) and a left (NRRU-F01020005) maxillae, a left jugal (NRRU-F01020006), two posterior parts of the left mandible comprising the surangular, prearticular, and articular (NRRU-F01020007, F01020009), a posterior part of the left mandible comprising the surangular and prearticular (NRRU-F01020010), three anterior cervical vertebrae (NRRU-F01020011–F01020013), three posterior dorsal vertebrae (NRRU-F01020014–F01020016), a middle caudal vertebra (NRRU-F01020017), a manual ungual (NRRU-F01020018), a right ischium (NRRU-F01020019 and F01020020), a distal part of the right tibia (NRRU-F01020021), and a left pedal phalanx IV-1 (NRRU-F01020022). All of these are materials comparable to Allosauroidea that 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.
Locality and horizon: In Ban (meaning “village”) Saphan Hin, Suranaree Subdistrict, Muaeng Nakhon Ratchasima District, Nakhon Ratchasima Province, Thailand. Lower Cretaceous (Aptian) Khok Kruat Formation.
Etymology: Siam (Latin): in reference to Thailand; raptor (Latin): meaning a robber; suwati: in honour of Mr. Suwat Liptapanlop [สุวัจน์ ลิปตพัลลภ], who supports and promotes the work of the Northeastern Research Institute of Petrified Wood and Mineral Resources.
Conclusions:
A new carcharodontosaurian theropod, Siamraptor suwati, is described based on isolated cranial and postcranial remains from the Lower Cretaceous Khok Kruat Formation. Siamraptor is diagnosed by eight characters in cranial and axial elements, and it also exhibits several carcharodontosaurian synapomorphies such as two pneumatic foramina oriented anteroventral–posterodorsally in cervical vertebrae, a parallel and sheet-like hyposphene lamina, and a reduced and oblique ridge of suprastragalar buttress for the astragalus in the tibia. Siamraptor is also characterized by remarkable pneumaticity in cranial and axial bones, which is comparable with those observed in several other carcharodontosaurians, although those taxa could have another phylogenetical interpretation as tyrannosaurids. Both phylogenetic analyses using two independent datasets locate Siamraptor as the most basal member of Carcharodontosauria, which also means that this taxon is the first definitive carcharodontosaurian theropod from Southeast Asia. The presence of Siamraptor in this area indicates an extension of the record in the Laurasian landmasses during the earliest stage of the evolutionary history of Carcharodontosauria.
Duangsuda Chokchaloemwong, Soki Hattori, Elena Cuesta, Pratueng Jintasakul, Masateru Shibata and Yoichi Azuma. 2019. A New Carcharodontosaurian Theropod (Dinosauria: Saurischia) from the Lower Cretaceous of Thailand. PLoS ONE. 14(10): e0222489. DOI: 10.1371/journal.pone.0222489
ดวงสุดา โชคเฉลิมวงศ์
