Gigantoraptor

Manual ungual of LH V0011.

In a quarry at Saihangaobi, Iren Dabasu Formation, Erlian basin, Sonid Left Banner (Inner Mongolia), numerous remains of the sauropod Sonidosaurus have been uncovered since 2001. Chinese paleontologist Xu Xing was asked to reenact the discovery of Sonidosaurus in April 2005 for a Japanese documentary. Xu obliged them by digging out a thighbone. As he wiped the bone clean, he suddenly realized it was not from a sauropod, but from an unidentified theropod in the size class of Albertosaurus. He then stopped the filming to secure the serendipitous find. This way, the discovery of the Gigantoraptor holotype fossil was documented on film.[1] That holotype, LH V0011, consists of the incomplete and disassociated remains of a single subadult individual, preserving a nearly complete mandible, a partial isolated cervical vertebra, dorsal vertebrae, caudal vertebrae, right scapula, right humerus, right radius and ulna, nearly complete right manus, partial ilium with a nearly complete pubis and hind limbs, including both femur, tibia and fibula with virtually complete pes.[1]

In 2007, the type species Gigantoraptor erlianensis was named and described by Xu, Tan Qingwei, Wang Jianmin, Zhao Xijin and Tan Lin. The generic name, Gigantoraptor, is derived from the Latin gigas, gigantis, meaning "giant" and raptor, meaning "seizer". The specific name, erlianensis, refers to the Erlian Basin.[1]

Size of the holotype and a potential egg compared to a human.

Life restoration.

Hind view of reconstructed skeleton

It was clear to Xu et al. that Gigantoraptor belonged to the Oviraptorosauria, a group named after Oviraptor, but compared to other known members, Gigantoraptor was much larger, approximately three times as long and 35 times more massive than the heaviest earlier discovered oviraptorosaurid Citipati. Xu et al. estimated its length at 8 m (26 ft), a height of 5 m (16 ft) and the weight at 1.4 t (1,400 kg), but Xu et al. infer that Gigantoraptor reached its young adult size within seven years and was still at relatively early young adult stage at the time of death, and estimate that a full-sized Gigantoraptor is considerably heavier than 1.4 t (1,400 kg).[1] In 2010, Gregory S. Paul even gave an estimate of 2 t (2,000 kg).[2] However, the paleontologist Thomas Holtz has estimated its size at 8.6 m (28 ft) with a weight between 0.9 to 3.6 t (900.0 to 3,600.0 kg) in 2012.[3]

The preserved mandible of Gigantoraptor is fused into a broad shovel-like shape, indicating that the unknown skull was over half a metre long and toothless also, probably equipped with a horny beak. The anterior caudal vertebrae have very long neural spines and are heavily pneumaticised with deep pleurocoels. The middle section of the relatively short tail is somewhat stiffened by long prezygapophyses. The caudal vertebrae are lightened by spongeous bone. The forelimbs are rather long because of an elongated slender manus. The humerus is bowed outwards to an exceptionally large extent and has a very rounded head. The first metacarpal is very short and carries a strongly diverging thumb. The hindlimbs are relatively elongated, a trait also mentioned by Holtz.[3] The femur is slender with a distinct head and neck, measuring 110 cm (43 in). The pes is robust with large and strongly curved pedal unguals.[1]

Comparison between the manus of Gigantoraptor and other caenangnathoids

In 2007, Xu et al. assigned Gigantoraptor to the Oviraptoridae, in a basal position. The anatomy of Gigantoraptor includes the diagnostic features of the Oviraptorosaurs. However, it also includes several features found in more derived eumaniraptoran dinosaurs, such as a forelimb/hindlimb ratio of 60%, a lack of expansion of the distal scapula and the lack of a fourth trochanter on the femur. Despite its size, Gigantoraptor would thus have been more bird-like than its smaller oviraptorosaurian relatives.[1]

In 2010, a second analysis of Gigantoraptor relationships found it to be a member of the Caenagnathidae rather than an oviraptorid.[4] Phylogenetic analysis conducted by Lamanna et al. (2014), supported that Gigantoraptor was a basal caenagnathid.[5]

The cladogram below follows the phylogenetic analysis conducted by Funston & Currie in 2016, which found Gigantoraptor to be a Caenagnathid.[6]

Life-sized nest of Gigantoraptor

Embryo nicknamed "Baby Louie", belonging to the related genus Beibeilong, Children's Museum of Indianapolis

The diet of Gigantoraptor is uncertain. Although some oviraptorosaurs, such as Caudipteryx and Incisivosaurus, are thought to have been mostly herbivorous, because Gigantoraptor has a small head and long neck like many herbivores. Gigantoraptor had long hind legs with proportions that allowed for fast movement (it was probably more nimble than the heavier and less agile Tarbosaurus), and large claws, a combination that is not usually found in herbivores of this size.[1] Paul suggested that Gigantoraptor was also a herbivore and used its speed to escape predators.[2] The preserved jaw for Gigantoraptor also indicates that the theropod had a shearing bite, possibly for cutting through plants (and potentially meat). This is comparable to other caenagnathids and contrasting with the jaws of oviraptorids, whose jaws seem better suited for crushing food.[7]

The specimen has advanced ossification and growth rings in the fibula that indicate it was likely 11 years old when it died. It appears to have reached an early young adult stage at age seven, and probably would have grown much larger when it reached the adult stage. This indicates a growth rate that is faster than in most large non-avian theropods.[1]

The existence of giant oviraptorosaurians, such as Gigantoraptor, explains several earlier Asian finds of very large, up to 53 centimetres long, oviraptorosaurian eggs, assigned to the oospecies Macroelongatoolithus carlylensis. These were laid in enormous rings with a diameter of three metres.[2] The presence of Macroelongatoolithus in North America indicates that gigantic oviraptorosaurs were present there as well, though no fossil skeletal remains have been found.[8]

Gigantoraptor was unearthed from the Iren Dabasu Formation, with a existence between 95 million to 80 million years ago.[3] During the times of the Late Cretaceous, in this area existed a large braided river valley with floodplain and fluvial environments. These environments had extensive vegetation, evidenced in the palaeosol development and the numerous herbivorous dinosaurs remains that were found in both the river channel and the floodplain sediments.[9] Here, it lived alongside other Theropods: Alectrosaurus, Archaeornithomimus, Avimimus, Caenagnathasia, Erliansaurus, Mononykus, Neimongosaurus, Saurornithoides and Velociraptor; the sauropod Sonidosaurus, and hadrosaurids: Bactrosaurus and Gilmoreosaurus.[10][11][9][12][13]

Wikispecies has information related to Gigantoraptor

• Timeline of oviraptorosaur research
• 2007 in paleontology
• Xu Xing
• Iren Dabasu Formation
• Oviraptorosauria

• news@nature.com: "Giant bird-like dinosaur found".
• Wired Science Scientists Discover 3,000-Pound Gigantoraptor Dinosaur in Mongolia
• Fleshed-out restoration of a Gigantoraptor erlianensis. Credit: Julius T. Csotonyi
• Yahoo! News: China finds new species of big, bird-like dinosaur
• Description: Factsheet with picture English/German at Dinosaurier-Web