Adasaurus ( ; meaning "evil lizard"
Adasaurus was a large dromaeosaurid that was about long weighing . Unlike other dromaeosaurids, Adasaurus developed a rather small and blunt sickle claw that likely had a reduced use, and a recurved lacrimal bone; this latter trait is also shared with Austroraptor. Though reduced, the sickle claw retained the characteristic rounded articulation of most dromaeosaurids.
Adasaurus was originally regarded as a dromaeosaurine by Barsbold, a group that includes robust dromaeosaurs with deep jaws. Revisions made to the specimens have shown that this dromaeosaurid belongs to the Velociraptorinae, composed of more lightly-built animals like Velociraptor.
History of discovery
Adasaurus was first figured in 1977 by the
paleontologist Rinchen Barsbold on a pelvic comparison with other
theropods, but it would remain as an informally named taxon until a proper description.
[ Translated paper] In 1983, Barsbold published a large comparative revision of the known
theropod
taxa at the time where he formally named
Adasaurus and the
type species A. mongoliensis, which was based on two partial specimens. The generic name,
Adasaurus, is taken from the Mongolian word ад (ada, meaning
evil spirit), and the
Ancient Greek word σαῦρος (sauros, meaning lizard). The specific name for the
Monotypic,
mongoliensis, refers to the country of discovery Mongolia. Barsbold briefly described
Adasaurus as a dromaeosaurid and noted that this new taxon possessed a notably reduced second pedal ungual. Given that this trait contrasted to the large, sharply-developed ungual of most members, Barsbold listed it as a
Autapomorphy character for
Adasaurus.
[ Translated paper] However, the authenticity of this unusual reduction was disputed in 2010 by Phil Senter, who claimed that the supposed ungual did not pertain to the specimen.
Nevertheless, in the revised diagnosis conducted by Turner and colleagues in 2012, this character is still considered as authentic,
[ which has been widely followed by other authors.][ Book preview] However, these specimens are actually known from the Shine Us Khuduk and Tel Ulan Chaltsai localities (respectively) of the Bayan Shireh Formation and thus, they are older than the remains of Adasaurus from the younger Nemegt Formation. They represent a different and new taxon that differs from Adasaurus.[
]
Description
Adasaurus was a rather large-sized dromaeosaurid. The holotype has an estimated length of with a weight of .[
]
Skull
On the right side of the skull, the lower portion of the jugal is expanded from the top to the bottom. The quadrate is a large and vertical bone with a large triangular projection on its lateral border. This triangular projection is located on the quadrate shaft and bent to the top. The top surface of the right ectopterygoid—a smalle bone of the palate—is flattened to the palate. As in other dromaeosaurids, the lacrimal has an inverted L-shape, but the thin body of this bone is curved, which is also seen in Austroraptor.[
]
Skeleton
The scapula and coracoid of the holotype are completely fused giving form to the scapulocoracoid, and the suture between them is not present.[ Pneumatic foramina are present in the holotypic anterior sacral vertebrae.][ The femur and tibia of the holotype measure and long, respectively,][ and the fourth trochanter is a prominent and rugose ridge that is located on the posterior inner surface of the upper region of the femoral shaft. The femur itself is very similar to that of the indeterminate dromaeosaur DGBU-78 from the Gugyedong Formation of South Korea.][
The posterior top border of the ilium is proportionally more thickened than that of Achillobator,][ and the anterior border of the anterior blade of the ilium has a similar shape to that of Saurornitholestes. This anterior border has a notched appearance that is characteristic to Adasaurus. As a whole, the top border is straightened in shape. The pubic peduncle—a robust anterior extension that articulates with the pubis—is wide and developed to the bottom. A large supratrochanteric (above the trochanter of the femur) extension is absent on the ilium. Like other dromaeosaurids, the pubis is elongated with an expanded pubic boot (lower end) and features an opisthopubic (backwards directed) condition.][ The digit II ungual is not hypertrophied (elongated) as in most dromaeosaurids,][ and though Adasaurus features a similar metatarsal II-III ratio to that of Balaur bondoc, this is due to the reduced sickle claw of digit II instead of an elongated ungual of digit I. Metatarsal III of the paratype shows that a tubercle is present on the extensor surface and this tuberosity likely originates the insertion of the muscle tibialis cranialis.][ The lower tarsals and upper ends of the metatarsals are somewhat fused.][
]
Classification
Adasaurus is a member of Dromaeosauridae, a group that is closely related to living birds. When erected by Barsbold in 1983, Velociraptorinae was conceived as a group containing Velociraptor and closely related species that were characterized by their smaller size and long-narrow snouts. However, Barsbold did not include Adasaurus in the group; instead, he placed it within the Dromaeosaurinae.[ It was not until 1998 that this group was defined as a clade by Paul Sereno. Sereno defined the group as all dromaeosaurids more closely related to Velociraptor than to Dromaeosaurus.][ The traditional view of the Velociraptorinae commonly included Velociraptor, Tsaagan and Linheraptor, which are known from complete skulls, however, most analyses vary widely regarding which species are actually velociraptorines and which are dromaeosaurines. Turner and colleagues in 2012 supported a traditional, monophyletic composition of Velociraptorinae.][ However, some studies found a very different group of dromaeosaurids in Velociraptorinae, such as Longrich and Currie in 2009, which recovered Deinonychus outside of the Velociraptorine and Dromaeosaurinae.]
Paleobiology
Paleopathology
In 1997, Norell and Makovicky stated that the holotype specimen of Adasaurus represents a largely Pathology (due to injury or disease) individual.[ They reaffirmed this observation in 2004 by claiming the pelvis as pathological.][
]
Sickle claw function
Kubota and Barsbold in 2006 stated that the highly reduced sickle claw of Adasaurus may have been used with less frequency than other deinonychosaurs as the bottom surface of lower heel on the penultimate Phalanx bone has no apparent asymmetrical ridges like other dromaeosaurids and troodontids.[
]
In 2011, Denver Fowler and colleagues suggested a new method by which dromaeosaurids may have taken smaller prey. This predation model, "Raptor Prey Restraint" (RPR), proposes that dromaeosaurids killed their prey by leaping onto their quarry, pinning it under their body weight, and gripping it tightly with the large, sickle claws of the pedal digit II—in a manner very similar to extant Accipitridae birds of prey. Like accipitrids, the dromaeosaurid would then begin to feed on the animal while still alive, until it eventually died from Bleeding and organ failure. This proposal is based primarily on comparisons between the morphology and proportions of the feet and legs of dromaeosaurids to several groups of extant birds of prey with fairly known predatory behaviors. Fowler and colleagues found that the feet and legs of dromaeosaurids most closely resemble those of and , especially in terms of having an enlarged second claw and a similar range of grasping motion, but the short Tarsometatarsus and foot strength would have been more similar to that of . The RPR model would be consistent with other aspects of dromaeosaurid anatomy, such as their unusual dentition and arm morphology. The arms were covered in long feathers and may have been used as flapping stabilizers for balance while atop a struggling prey, along with the stiff counter-balancing tail. Lastly, the comparatively weak jaws would have been useful for eating prey alive but not as useful for forceful dispatch of the prey.[
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Paleoenvironment
Adasaurus is known from the Late Cretaceous Nemegt Formation, the age of which has been considered from the Late Campanian to Early-Middle Maastrichtian stages, about 70 million and 68 million years ago. The environments that were present on the formation included stream and river channels, mudflats, and shallow lakes. Much of the sedimentation also indicates that a rich habitat existed, offering extensive vegetation in abundant amounts that could sustain most herbivorous dinosaurs. Most fluvial systems functioned as oases for oviraptorosaurs.
See also
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Timeline of dromaeosaurid research
External links
