Garudimimus (meaning "Garuda mimic") is a genus of ornithomimosaur that lived in Asia during the Late Cretaceous. The genus is known from a single specimen found in 1981 by a Soviet-Mongolian paleontological expedition in the Bayanshiree Formation and formally described in the same year by Rinchen Barsbold; the only species is Garudimimus brevipes. Several interpretations about the anatomical traits of Garudimimus were made in posterior examinations of the specimen, but most of them were criticized during its comprehensive redescription in 2005. Extensive undescribed ornithomimosaur remains at the type locality of Garudimimus may represent additional specimens of the genus.
The only known specimen of Garudimimus was a medium-sized animal measuring nearly in length and weighing about . It was an ornithomimosaur with a mix of basal and derived features; unlike primitive ornithomimosaurs, both upper and lower jaws were toothless, a trait that is often reported in more derived ornithomimids. Garudimimus had relatively short and stocky hindlimbs, robust feet, and a reduced ilia. The foot had four toes with the first one very reduced, whereas ornithomimids were three-toed with the first toe lost. The toothless skull has very straight jaws ending in a more rounded snout tip than that of other genera. It was previously thought that this primitive ornithomimosaurian possessed a lacrimal "horn" at the top of the skull, in front of the eye socket. However, the redescription of the only specimen has shown that this structure was simply the distorted left prefrontal bone. Another early interpretation was the metatarsus reconstructed with an arctometatarsalian condition.
With the first description in 1981, Garudimimus was identified as a primitive ornithomimosaurian within its own family. However, with the description of new specimens of Deinocheirus in 2014, it was found that the latter was the sister taxon of Garudimimus, grouping within the Deinocheiridae—ornithomimosaurs not adapted for running or agile movements. This placement has been disputed, though. The pelvic girdle and hindlimbs of Garudimimus show that the musculature of the legs was not as well-developed as in the fast-running ornithomimids, therefore, indicating poor cursorial capacities. Like other members of Ornithomimosauria, Garudimimus was an omnivore/herbivore with a reduced bite force that was compensated by a horny beak.
History of discovery
In 1981, during a Soviet-Mongolian paleontological expedition to the Gobi Desert, a relatively small theropod skeleton was discovered by the team at Bayshin Tsav in the Late Cretaceous Bayan Shireh Formation, Southeastern Mongolia. The remains are catalogued as MPC-D 100/13 (Mongolian Palaeontological Center; originally GIN 100/13) and represent a rather complete and articulated skeleton. In the same year this specimen was formally and briefly described by the Mongolian paleontologist Rinchen Barsbold as the holotype of the genus and species Garudimimus brevipes. The generic name, Garudimimus, combines a reference to Garuda which are legendary winged creatures from Mongolian Buddhist mythology and Latin mimus (meaning mimic). The specific name is derived from Latin brevis (meaning short) and pes (meaning foot), referring to the short metatarsus. Barsbold identified Garudimimus as an ornithomimosaurian taxon but noted it was more primitive than ornithomimids and gave it its own family, the Garudimimidae. Translated paper Barsbold described additional remains of the holotype specimen in 1983, and later in 1990 with Halszka Osmólska. Translated paper
The holotype consist of the skull in its entirety, 8 cervical vertebrae (including the atlas and axis), 9 dorsal vertebrae, 6 sacral vertebrae and 4 caudal vertebrae, some ribs, both ilia, pubes, femora, tibiae, fibulae, and a virtually complete left pes of a sub-adult individual.
In 1988 the North American author Gregory S. Paul illustrated Garudimimus with a prominent nasal horn unlike any other ornithomimosaur, considering this feature as actually preserved. Another interpretation was made also in 1988 by Philip J. Currie and Dale Alan Russell who referred the metatarsus of the holotype to Oviraptor sp. and reconstructed it with an arctometatarsal—a condition where the upper end of the third metatarsal is narrowed between the surrounding metatarsals—structure. In 1992 Thomas R. Holtz followed this interpretation and suggested the metatarsus could have been arctometatarsalian and was just disarticulated as preserved. Supporting an arctometatarsalian condition, Currie and David A. Eberth in 1993 claimed that part of the Archaeornithomimus (an ornithomimosaur from the nearby Iren Dabasu Formation) material belonged to Garudimimus based on the assumed arctometatarsalian condition, presence of the vestigial digit I and the proportions of metatarsals II, III, and IV. They pointed out that the metatarsals are crushed and metatarsal III is set back from the extensor surface of metatarsus. In 1994 Holtz suggested some similarities between the metatarsus of Garudimimus and Chirostenotes.
In 1994, Bernardino P. Pérez-Moreno with colleagues described the primitive ornithomimosaur Pelecanimimus and identified the presence of a crest in the holotype specimen. They claimed a similar trait in Garudimimus which was represented by a nasal horn. In a comprehensive redescription in 2005 at the hands of Yoshitsugu Kobayashi and Barsbold, numerous of the previous statements were refuted. The supposed orbital horn is actually the disarticulated left prefrontal bone and verified that the metatarsus did not suffer taphonomical (changes during decay and fossilisation) distortion and is non-arctometatarsalian. Moreover, Kobayashi previously showed that in addition to actually lacking an arctometatarsalian condition, the metatarsal ratios are different to Archaeornithomimus.
Although Garudimimus is solely known from MPC-D 100/13, additional specimens may potentially be present among a large bonebed composed of at least five individuals in Bayshin Tsav and several undescribed ornithomimosaurs from other localities in the Bayan Shireh Formation.
Description
Garudimimus (based on the holotype specimen) is estimated to have been around weighing approximately . Several unfused vertebrae indicates that the holotype was likely a still-growing sub-adult animal. Some of the distinguishing traits of this taxon include the jaw articulation positioned more backwards than the postorbital bar (arched bone around the orbit, also referred as the ), depressions on the upper surface of the supraoccipital at the back of the skull, paired depressions on the lateral surfaces of the neural spines of the anterior caudal vertebrae, and a deep groove at the upper end of lateral surface of pedal phalanges III-1 and III-2 (first and second phalanxes of the third digit of the foot).
Based on feather impressions of several specimens of Ornithomimus, Garudimimus likely developed a shaggy feathering mostly similar to that of large paleognath birds such as ratites.
Skull
Garudimimus had a rather small skull in proportion to the overall skeleton, and both jaws were edentulous (devoid of teeth). The skull was about long with a width of . The eye socket was a large cavity with a height of wide. The radius of the scleral ring (bony plates that support the eyeball) was approximately long inside and outside. The breadth of each plate was about . The (an opening behind the eye socket) had an irregular, nearly oval-shaped. In front of the eye socket there was the , a large and rather irregular opening. The was an elongated bone and had a roughened lateral surface, which was common in ornithomimosaurs. Both promaxillary and maxillary fenestrae (small openings in the maxilla) were poorly developed—almost absent—in Garudimimus. The bottom edges of the were thin and sharply-developed. On the lateral sides of this bone, numerous foramina (small holes) were located, a trait more characteristic of derived ornithomimids. The edentulous condition of both premaxilla and dentary with the addition of numerous foramina in their surfaces, indicate that a prominent (horny beak) was present in life. The nostrils were moderately long and turned upwards. They were surrounded and mainly formed by the , which had a narrow end that extended up above the eye sockets.
The was semi T-shaped and located in front of the eye. The was a thin and delicate bone that contributed to the overall shape of the eye socket. In a top view, the were triangular and formed the upper borders of the eye socket. Both frontals together formed a single dome-like structure. The posterior border of the eye socket was formed by the , which was a sharply-developed bone of well-distributed thickness in lateral view. The had a triangular main body with very thin-walled processes (bony extensions); it also gave form to the lower border of the eye socket. The was a large and triangular-shaped bone, contrasting with the L-shaped quadratojugal of most ornithomimosaurs. On its posterior surface, the had a large, oval-shaped depression positioned at its mid-height. A small pneumatic foramen (hole) was also present in the bottom area of the depression. The was the longest mandibular element measuring long and was thickened towards the end. The front of both dentaries was shovel-like and had well-developed cutting edges. Numerous foramina were developed on the lateral surfaces of the dentary tip. The was the second large mandibular element and formed the upper border of the (a hole in the lower jaw). Compared to the two latter bones, the was much smaller and formed the lower border of this fenestra.
Postcranial skeleton
The complete vertebral column of Garudimimus is not known. The intercentrum and of the were downwards-directed. The central length of the was rather short, measuring approximately long and high—probably a primitive condition in ornithomimosaurs. Its posterior articular surface was concave and taller than wide. Its was tall and very rounded, and its neural arch had a pneumatic fossa (depression) at the mid-length of its (body). The neural spines of most were narrow and directed towards the end. The progressively increased in size towards the end and along with the size, the neural spines became progressively taller in posterior dorsals with nearly equal lengths. In a lateral view, all spines were slightly inclined towards the end. The dorsal centra lacked pneumatization and were larger in more posterior vertebrae, having amphicoelous (strongly concave) facets.
The was composed of six sacral vertebrae of which the first and sixth had sacral ribs attached to the inner surface of the ilium. The total length of the sacrum () was nearly equal to that of the ilium. Most sacral centra were roughly equal in length except for the fourth one. Unlike other ornithomimosaurs, the sacral vertebrae lacked pneumatization (air-spaced). The neural spines became progressively shorter and had straight and horizontal top borders. The caudal vertebrae had paired projections on either side of the sulci (fissures) for the chevron articulation. Putative large foramen and deep fossae (depressions) were present on the anterior neural arches, which may indicate caudal pneumaticity in Garudimimus.