Gigantoraptor (lit. 'giant thief') is a genus of large oviraptorosaurian dinosaur that lived in Asia during the Late Cretaceous period. It is known from the Iren Dabasu Formation of Inner Mongolia, where the first remains were found in 2005. Gigantoraptor was the largest-known oviraptorosaur, reaching 8 or 8.6 metres (26 or 28 ft) in length and 1.4–2 metric tons (1.5–2.2 short tons) in body mass. It had an extensively pneumatized vertebral column and elongated arms and legs. Both femur and tibia measured over 1 m (3.3 ft) in length, an unusual trait among giant theropods. The lower jaws were toothless and ended in a keratinous beak, as seen in other oviraptorosaurs. Though several oviraptorosaur species are known to have developed a full coat of feathers, Gigantoraptor, due to its size, could have lost some of this integument. The genus is classified as an oviraptorosaurian dinosaur, a group of generally small feathered animals. Though it was originally found to represent a basal oviraptorid, subsequent analyses have shown it to be a caenagnathid. It was a giant, ground-dwelling bipedal omnivore or herbivore with a shearing bite as indicated by the preserved mandible. The shape of its beak indicates a generalist diet with a potentially occasional carnivory. The holotype—and only known specimen—has been determined to represent a young adult that died at the age of 11, and it reached a young adulthood around 7 years of life. Such development indicates an accelerated growth compared to other large theropods. The discovery and examination of large oviraptorosaur eggs, Macroelongatoolithus, indicates that large genera such as Gigantoraptor built their nests with the center lacking eggs in order to avoid crushing.
History of discovery
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. However, the footage is lost 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. The holotype, LH V0011, consists of the incomplete and disassociated remains of a single adult 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 hindlimbs, including both femur, tibia and fibula with a very complete pes. It is currently housed at the Longhao Institute of Geology and Paleontology.
In 2015, Takanobu Tsuihiji with team reported a partial—and large—caenagnathid dentary (specimen MPC-D 107/17) from the Bayan Shireh Formation, discovered in 2008 by the Hayashibara Museum of Natural Sciences-Mongolian Paleontological Center Joint Expedition, at the Tsagaan Teg locality. As preserved, the dentary is virtually similar to that of Gigantoraptor in both morphology and size. Despite shared traits between Gigantoraptor and MPC-D 107/17, and their close relationships, Tsuihiji and team regarded it as Caenagnathidae indet. due to its fragmentary nature. Authors Rubén Molina-Pérez and Asier Larramendi in 2019 assigned this partial dentary as cf. Gigantoraptor erlianensis, suggesting that it could represent an additional specimen.
Description
Gigantoraptor is the largest known oviraptorosaur for which skeletal material is available. Approximately three times as long and 35 times heavier than the largest earlier-discovered oviraptorosaur Citipati, the holotype of Gigantoraptor has been estimated at 8 m (26.2 ft) long with a height of 3.5 m (11.5 ft) at the hips, and a ponderous weight of 1.4 metric tons (1.5 short tons). Other studies have estimated it to be 2 t (2.2 short tons) in weight, and one at 8.6 m (28.2 ft) in length. Oviraptorosaurs are fairly known to have developed feathers, as seen on feathered specimens of Caudipteryx and the pygostyle of several oviraptorids. As an oviraptorosaur, Gigantoraptor was likely feathered as well, however, given its massive size it may have been partially naked.
Skull
As preserved, the total length of the lower jaws is 46 cm (460 mm), and they are toothless. This element fused into a broad shovel-like shape, indicating that the unknown skull was over half a metre long and also toothless, likely equipped with a rhamphotheca (horny beak). Compared to the right side, the left side of the jaws is well-preserved with an almost intact shape. The dentary bones are very deep and toothless, and have a pair of sharply-developed shelves on the top edges. These structures are relatively flat. The anterior end of the dentary is rounded, more similar to caenagnathids and different from the chin-shaped one of some oviraptorids. From a top view, the symphyseal (bone union) region at the front of the dentary is U-shaped. Along the lateral surfaces of this bone, some nutrient foramina can be found, which likely supported a rhamphotheca.
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