Siamotyrannus isanensis is a large-bodied theropod from the Lower Cretaceous Sao Khua Formation o... more Siamotyrannus isanensis is a large-bodied theropod from the Lower Cretaceous Sao Khua Formation of northeastern Thailand. It was named in 1996 by Buffetaut et al., who concluded it to be a basal member of the Tyrannosauridae. It has subsequently been held to be assignable to the Metriacanthosauridae by Rauhut (in 2003) and Carrano et al. (in 2012), and suggested to be a carcharodontosaurian by Brusatte and Sereno (in 2008). In the original publication, the dorsal vertebrae of Siamotyrannus were only cursorily described, in spite of the importance of these elements in the phylogenetic analysis of the taxon. The objective of the present study is to describe the morphology of these vertebrae in detail. The dorsal vertebrae of Siamotyrannus could be anterior to mid-dorsals, as based on the laminae on the dorsal series. The characters are: dorsal vertebrae with long and backward diapophyses; long and stalk-like parapophyses; hourglass-shaped and amphiplatyan centra; many fossae and laminae on transverse processes; hyposphene lamina is possibly parallel and sheet-like; no pleurocoel or pneumatic foramen on the centra. A preliminary phylogenetic analysis, including dorsal vertebrae, pelvis and tail elements, suggested a 'basal' coelurosaur position with 324 most parsimonious trees, each of 1,031 steps from 351 characters of 61 dinosaur taxa, CI = 0.419 and RI = 0.69. The revision of the pelvic girdle, sacral and caudal vertebrae is still in process. In combination with new data on dorsal vertebrae, the proper phylogenetic position of this taxon is expected to be well understood in future.
Siamotyrannus isanensis is a large-bodied and informative theropod dinosaur from the Early Cretac... more Siamotyrannus isanensis is a large-bodied and informative theropod dinosaur from the Early Cretaceous Sao Khua Formation of Thailand. It was firstly interpreted as a basal member of the tyrannosaur lineage. This conclusion has been debated by various authors, for example it was assigned to belong to Metriacanthosauridae (formerly known as Sinraptoridae) by Rauhut (2003) and Carrano et al. (2012), and suggested to be a carcharodontosaurian by Brusatte and Sereno (2008). Here we revised Siamotyrannus in details. The dorsal vertebrae of Siamotyrannus could be referred to anterior to mid-dorsals based on the laminae on the dorsal series. The hyposphene-hypantrum articulation is possibly parallel and sheet-like. There is no pleurocoel or pneumatic foramen on the centra. Siamotyrannus also shows some unique autapomorphies: double, low and broad vertical ridges on central part of the lateral iliac blade; a notch on the dorso-posterior part of the postacetabular blade of the ilium; parapophyses on the anterior to mid-dorsal vertebrae with long and stalk-like pedicels. The preliminary phylogenetic study suggested that Siamotyrannus might be a basal coelurosaurian (324 most parsimonious trees, 1031 steps. The CI = 0.419, RI = 0.69), but the alternative possibility that it might belong to basal Allosauria or Metriacanthosauridae cannot be rejected (each of 1034 steps). The ongoing re-description and re-scoring of this taxon will provide further information. This knowledge will help us understand the evolution, distribution and paleobiogeography of large-bodied theropods in the Early Cretaceous of Southeast Asia and global scale.
Megaraptora is a clade of medium to large-sized theropods found mainly from the Early Cretaceous ... more Megaraptora is a clade of medium to large-sized theropods found mainly from the Early Cretaceous to the early Late Cretaceous of Gondwana, but their basal member was reported from the Early Cretaceous of Japan (Fukuiraptor). Despite their many discoveries and studies, the phylogenetic status of this group still be debated, whether they belong to carcharodontosaurian allosauroids or basal tyrannosauroids. We report here a new specimen referred to this clade. It consists of a left tibia with associated astragalocalcanium found from the Sao Khua Formation of Wiangkao District, Khon Kaen Province, northeastern Thailand. The tibia clearly shows tetanuran synapomorphies: fibular condyle offset from the cnemial crest by a strongly developed incisura tibialis; fibular flange clearly separated from the proximal articular surface of the tibia. It shows a megaraptoran and ceolurosaurian synapomorphy: tibial length more than 12 times its anteroposterior width at mid-length. The Thai specimen also shows characters that found in megaraptorans and some other theropods: distal tibia flat with medial ridge, as in Australovenator, Aerosteon, and Chilantaisaurus; anteroproximal expansion of astragalar lateral condyle, as in Australovenator and Fukuiraptor; the present of cranio-proximal process at the base of astragalus, as in Australovenator, possibly in Fukuiraptor; shape of ascending process of astragalus as in Australovenator, Fukuiraptor, Aerosteon, Qianzhousaurus, Raptorex, and Alioramus; The ascending process of the astragalus is 1.7 times the height of its body as in Fukuiraptor. Preliminary study suggests this specimen belongs to the clade Megaraptora. The ongoing description and phylogenetic analysis of Thai specimen will provide information on the evolution and paleobiogeography of this clade.
2019. Two new basal coelurosaurian theropod dinosaurs from the Lower Cretaceous Sao Khua Formatio... more 2019. Two new basal coelurosaurian theropod dinosaurs from the Lower Cretaceous Sao Khua Formation of Thailand. Acta Palaeontologica Polonica 64 (2): 239-260. Megaraptora is a clade of mid to large-sized theropods that are long-snouted, large-clawed, highly pneumatized, and have long and gracile metatarsals. The basal member was reported from the Barremian of Japan. A more derived clade, the Megaraptoridae, is known from the Cenomanian to Santonian of Gondwana. Here two new basal coelurosaurs from the Lower Cretaceous Sao Khua Formation of Thailand are described and named as Phuwiangvenator yaemniyomi gen. et sp. nov. and Vayuraptor nongbualamphuensis gen. et sp. nov. Phuwiangvenator is a megaraptoran coelurosaur and diagnosed by the ventrally flat sacral vertebrae with sulci in the anterior and posterior region of the centra and the anterior rim of metatarsal IV sloping proximolaterally to distomedially and being much lower than that of metatarsal III anteriorly. Vayuraptor is a basal coelurosaur and diagnosed by its astragalus which has two horizontal grooves, two fossae at the base of the ascending process, the ascending process being straight laterally and straight and parallel medially with the medial rim sloping to the tip laterally, and a long and slender astragalar ascending process. Although the position of the basal coelurosaur Vayuraptor remains unclear and must await further discovery, megaraptoran affinities are likely. The Early Cretaceous megaraptoran fossil record has been recovered from the Barremian to Aptian of Asia. All Asian megaraptorans might be a monophyletic clade or a paraphyletic series relative to the Megaraptoridae. Several specimens have been reported from the Aptian to mid-Cretaceous of Australia, and one report from the Albian of South America. These fossils show a high diversity of the Early Cretaceous megaraptorans and a wide distribution during that time. The clade then became more provincial in the Late Cretaceous.
Siamotyrannus isanensis is a large-bodied theropod from the Lower Cretaceous Sao Khua Formation o... more Siamotyrannus isanensis is a large-bodied theropod from the Lower Cretaceous Sao Khua Formation of northeastern Thailand. It was named in 1996 by Buffetaut et al., who concluded it to be a basal member of the Tyrannosauridae. It has subsequently been held to be assignable to the Metriacanthosauridae by Rauhut (in 2003) and Carrano et al. (in 2012), and suggested to be a carcharodontosaurian by Brusatte and Sereno (in 2008). In the original publication, the dorsal vertebrae of Siamotyrannus were only cursorily described, in spite of the importance of these elements in the phylogenetic analysis of the taxon. The objective of the present study is to describe the morphology of these vertebrae in detail. The dorsal vertebrae of Siamotyrannus could be anterior to mid-dorsals, as based on the laminae on the dorsal series. The characters are: dorsal vertebrae with long and backward diapophyses; long and stalk-like parapophyses; hourglass-shaped and amphiplatyan centra; many fossae and laminae on transverse processes; hyposphene lamina is possibly parallel and sheet-like; no pleurocoel or pneumatic foramen on the centra. A preliminary phylogenetic analysis, including dorsal vertebrae, pelvis and tail elements, suggested a 'basal' coelurosaur position with 324 most parsimonious trees, each of 1,031 steps from 351 characters of 61 dinosaur taxa, CI = 0.419 and RI = 0.69. The revision of the pelvic girdle, sacral and caudal vertebrae is still in process. In combination with new data on dorsal vertebrae, the proper phylogenetic position of this taxon is expected to be well understood in future.
Siamotyrannus isanensis is a large-bodied and informative theropod dinosaur from the Early Cretac... more Siamotyrannus isanensis is a large-bodied and informative theropod dinosaur from the Early Cretaceous Sao Khua Formation of Thailand. It was firstly interpreted as a basal member of the tyrannosaur lineage. This conclusion has been debated by various authors, for example it was assigned to belong to Metriacanthosauridae (formerly known as Sinraptoridae) by Rauhut (2003) and Carrano et al. (2012), and suggested to be a carcharodontosaurian by Brusatte and Sereno (2008). Here we revised Siamotyrannus in details. The dorsal vertebrae of Siamotyrannus could be referred to anterior to mid-dorsals based on the laminae on the dorsal series. The hyposphene-hypantrum articulation is possibly parallel and sheet-like. There is no pleurocoel or pneumatic foramen on the centra. Siamotyrannus also shows some unique autapomorphies: double, low and broad vertical ridges on central part of the lateral iliac blade; a notch on the dorso-posterior part of the postacetabular blade of the ilium; parapophyses on the anterior to mid-dorsal vertebrae with long and stalk-like pedicels. The preliminary phylogenetic study suggested that Siamotyrannus might be a basal coelurosaurian (324 most parsimonious trees, 1031 steps. The CI = 0.419, RI = 0.69), but the alternative possibility that it might belong to basal Allosauria or Metriacanthosauridae cannot be rejected (each of 1034 steps). The ongoing re-description and re-scoring of this taxon will provide further information. This knowledge will help us understand the evolution, distribution and paleobiogeography of large-bodied theropods in the Early Cretaceous of Southeast Asia and global scale.
Megaraptora is a clade of medium to large-sized theropods found mainly from the Early Cretaceous ... more Megaraptora is a clade of medium to large-sized theropods found mainly from the Early Cretaceous to the early Late Cretaceous of Gondwana, but their basal member was reported from the Early Cretaceous of Japan (Fukuiraptor). Despite their many discoveries and studies, the phylogenetic status of this group still be debated, whether they belong to carcharodontosaurian allosauroids or basal tyrannosauroids. We report here a new specimen referred to this clade. It consists of a left tibia with associated astragalocalcanium found from the Sao Khua Formation of Wiangkao District, Khon Kaen Province, northeastern Thailand. The tibia clearly shows tetanuran synapomorphies: fibular condyle offset from the cnemial crest by a strongly developed incisura tibialis; fibular flange clearly separated from the proximal articular surface of the tibia. It shows a megaraptoran and ceolurosaurian synapomorphy: tibial length more than 12 times its anteroposterior width at mid-length. The Thai specimen also shows characters that found in megaraptorans and some other theropods: distal tibia flat with medial ridge, as in Australovenator, Aerosteon, and Chilantaisaurus; anteroproximal expansion of astragalar lateral condyle, as in Australovenator and Fukuiraptor; the present of cranio-proximal process at the base of astragalus, as in Australovenator, possibly in Fukuiraptor; shape of ascending process of astragalus as in Australovenator, Fukuiraptor, Aerosteon, Qianzhousaurus, Raptorex, and Alioramus; The ascending process of the astragalus is 1.7 times the height of its body as in Fukuiraptor. Preliminary study suggests this specimen belongs to the clade Megaraptora. The ongoing description and phylogenetic analysis of Thai specimen will provide information on the evolution and paleobiogeography of this clade.
2019. Two new basal coelurosaurian theropod dinosaurs from the Lower Cretaceous Sao Khua Formatio... more 2019. Two new basal coelurosaurian theropod dinosaurs from the Lower Cretaceous Sao Khua Formation of Thailand. Acta Palaeontologica Polonica 64 (2): 239-260. Megaraptora is a clade of mid to large-sized theropods that are long-snouted, large-clawed, highly pneumatized, and have long and gracile metatarsals. The basal member was reported from the Barremian of Japan. A more derived clade, the Megaraptoridae, is known from the Cenomanian to Santonian of Gondwana. Here two new basal coelurosaurs from the Lower Cretaceous Sao Khua Formation of Thailand are described and named as Phuwiangvenator yaemniyomi gen. et sp. nov. and Vayuraptor nongbualamphuensis gen. et sp. nov. Phuwiangvenator is a megaraptoran coelurosaur and diagnosed by the ventrally flat sacral vertebrae with sulci in the anterior and posterior region of the centra and the anterior rim of metatarsal IV sloping proximolaterally to distomedially and being much lower than that of metatarsal III anteriorly. Vayuraptor is a basal coelurosaur and diagnosed by its astragalus which has two horizontal grooves, two fossae at the base of the ascending process, the ascending process being straight laterally and straight and parallel medially with the medial rim sloping to the tip laterally, and a long and slender astragalar ascending process. Although the position of the basal coelurosaur Vayuraptor remains unclear and must await further discovery, megaraptoran affinities are likely. The Early Cretaceous megaraptoran fossil record has been recovered from the Barremian to Aptian of Asia. All Asian megaraptorans might be a monophyletic clade or a paraphyletic series relative to the Megaraptoridae. Several specimens have been reported from the Aptian to mid-Cretaceous of Australia, and one report from the Albian of South America. These fossils show a high diversity of the Early Cretaceous megaraptorans and a wide distribution during that time. The clade then became more provincial in the Late Cretaceous.
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