Gigantoraptor

Holotype mandible in right lateral view

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.[3]

Holotype mandible in lateral, dorsal, anterior and posterior views

Both angular and articular-surangular-coronoid (ASC) complex are comparatively smaller than the dentary. The angular is firmly fused to the ASC complex and gives form to the lower margin of the external mandibular fenestra. The angular has a bowed shape and is mainly forwards extended with a large lateral depression that joints with the posterior lower process (bony projection/extension) of the dentary. The ASC complex is composed by the fusion of the articular and surangular bones with the coronoid process of the mandible. This complex extends forwards to articulate with the posterior upper process of the dentary and gives form to the upper margin of the external mandibular fenestra. The coronoid process prominence is low and distinctively rounded. On the posterior end of the mandibles the articular glenoid can be located. It is relatively large, convex, and bowl-shaped.[3]

Skeletal reconstruction of the holotype, known remains in white

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 heavily pneumatized with a spongeous-like bone filling. The forelimbs are rather long because of an elongated humerus (73.5 cm (2.41 ft) long) and slender manus. The humerus is bowed outwards to an exceptionally large extent and has a very rounded head, similar to some birds. The first metacarpal is very short and carries a strongly diverging thumb. The hindlimbs are well-developed and highly elongated: the femur is slender with a distinct head and neck, measuring 1.10 m (3.6 ft); the tibia has similar top and bottom borders and measures 1.10 m (3.6 ft) long; and the metatarsus is 58.3 cm (1.91 ft) long with the metatarsal III being the largest. The pes is robust with large and strongly curved pedal unguals.[1][10]

Life restoration

Xu and team in 2007 suggested that Gigantoraptor might have been partially "naked" because it is 300 times as heavy as feathered species like Caudipteryx, and large animals tend to rely more on body mass for temperature regulation, leading to the loss of insulating coverings found on their smaller relatives. However, they suggested that at least arm feathers were probably still present on Gigantoraptor, since their primary functions, such as display behaviour and covering the eggs while brooding, are not related to the regulation of body heat.[1]

A histological analysis performed on the holotype fibula by Xu and colleagues in 2007 showed that the specimen had 7 lines of arrested growth (LAGs, or growth rings) as preserved. As several LAGs were missing, the team used inverse calculations to determine them, resulting in a total of 11. The amount of LAGs indicate the holotype was around 11 years old at the time of death and had an average growth of 128–140 kg (282–309 lb) per year. The performed histology also suggests that this individual was an adult, as there was extensive development of the secondary osteons, tightly packed LAGs close to the periphery of the bone, and a poorly vascularized layer of lamellar bone in the outer circumferential layer, which indicates a relatively sloppy growth rate. However, given that the fourth to seventh sets of LAGs were not very tightly packed, Xu and colleagues inferred that the individual reached its young adulthood at the age of 7. Moreover, the specimen was found to still be a relatively young adult and thereby it would have grown larger when it reached the adult stage. This indicates that Gigantoraptor had a growth rate much faster than most large non-avian theropods, such as tyrannosaurids.[1]

Another unusual development in Gigantoraptor are the elongated hindlimbs. Xu and colleagues pointed out the larger the theropod becomes, the shorter and stockier the legs become in order to withstand the increasing mass. This is contrasted by Gigantoraptor with both femur and tibia being over 1 m (3.3 ft), a combination that is not usually found in theropods of this size. The elongation of these limbs indicates that it was one of the most cursorial—an animal adapted to run or maintain certain speed for a large amount of time—large theropods. Furthermore, Gigantoraptor had an extensively pneumatized vertebral column, which may have aided in weight-loss.[1]

Lower jaw of the holotype

In 2017, Waisum Ma and colleagues re-examined the preserved jaws of Gigantoraptor and found that it had the deepest beak among caenagnathids and a relatively different diet from other oviraptorosaurs. For instance, the articular region of the jaws of Gigantoraptor show convergence to those of the modern tuatara. The articular region in this reptile enables propalinal jaw movement (a forward and backward movement) during feeding, and based on the similarities between their jaws, Ma and colleagues suggested that Gigantoraptor could have employed propalinal jaw movement as well. This movement aided with the sharp shelves of the dentary may indicate a shearing bite capable of 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. The depth of the lower jaw indicates the presence of a large tongue, which could have assisted the animal in food proccesing, and possibly improving the feeding efficiency.[3]

As the shape and size of the rostrum in large herbivores is related to dietary habits, the strongly U-shaped dentary of Gigantoraptor may suggest that this oviraptorosaur was a highly non-selective feeder, an adaptation that could have been useful in the variable environmental settings of the Iren Dabasu Formation. The team based this hypothesis on comparisons with several herbivorous animals, including other dinosaurs. Whereas sharply developed and narrow rostra are more correlated with a selective diet, such as the case of some ceratopsians, wide and rounded rostra are more linked with a generalist diet, as seen on the morphology of Gigantoraptor, Euoplocephalus and hadrosaurids. Ma and colleagues also pointed out that its large size, and therefore greater energy requirements, could have influenced its dietary habits as larger animals tend to consume lower quality food since it is available in larger amounts on the surroundings, and has a more stable supply. However, it is not known whether the gut of Gigantoraptor was specialized to process this greater ingestion. The team could not disregard an occasional carnivorous diet as the strong beak together with a propalinal jaw movement may have allowed meat processing.[3]

Fossilized Macroelongatoolithus nest from Zhengzhou, featuring the large center region devoid of eggs

The existence of giant oviraptorosaurians, such as Gigantoraptor, explains several earlier Asian finds of very large, up to ~50 cm (20 in) long, oviraptorosaurian eggs, assigned to the oospecies Macroelongatoolithus carlylensis. These were laid in enormous rings with a diameter of three metres. The presence of Macroelongatoolithus in North America indicates that gigantic oviraptorosaurs were present there as well, though no fossil skeletal remains have been found.[14] The 2017 description and naming of Beibeilong—based on an embryonic individual associated with eggs—further supports that Macroelongatoolithus eggs and nests belong to colossal caenagnathid oviraptorosaurs, such as Gigantoraptor.[15]

In 2018, Kohei Tanaka and team examined the egg clutches of numerous oviraptorosaur specimens, including egg clutches of Macroelongatoolithus, in order to correlate the nest configuration and body size to incubation behaviour. Their results showed that eggshell porosity indicates that the eggs of almost certainly all oviraptorosaurs were exposed in the nest without an external covering. Though most oviraptorosaur nests have eggs arranged in a circular fashion, the morphology of the nest is different in smaller and larger species in that the center of the nest is highly reduced in the former species, and becomes significantly larger in the latter species. This nest configuration suggest that whereas smallest oviraptorosaurs probably sat directly on the eggs, a large, Gigantoraptor-sized animal likely sat on the area devoid of eggs. Tanaka and colleagues pointed out that this adaption was beneficial to avoid egg-crushing and could have allowed some body-contact during incubation in these giant oviraptorosaurs.[16]