Adasaurus

*Adasaurus mongoliensis*

***Adasaurus*** (/ˌɑːdəˈsɔːrəs/ AH-də-SAWR-əs; meaning "Ada lizard") is a genus of dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous period about 70 million years ago. The genus is known from two partial specimens found in the Nemegt Formation of Mongolia that were partially described in 1983 by the paleontologist Rinchen Barsbold.

It was a medium-sized dromaeosaurid that was about 2.4 m (7.9 ft) long weighing 36.4 kg (80 lb). 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 showed that this dromaeosaurid belongs to the Velociraptorinae which is composed of more lightly build animals like *Velociraptor*.

## History of discovery

*Adasaurus* was first figured in 1977 by the Mongolian paleontologist Rinchen Barsbold on a pelvic comparison with other theropods, but it would remain as an informally named taxon until a proper description. In 1983, Barsbold published a large comparative revision of the known Mongolian 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 evil spirit Ada in the mythology of Mongolia, and the Greek word σαῦρος (sauros, meaning lizard). The specific name for the single species, 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 unique character for *Adasaurus*. 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.

*Adasaurus* is known from the holotype MPC-D 100/20, which represents an adult individual comprising a partial skull missing its anterior region, the right scapulocoracoid and a sternal plate, 8 cervical vertebrae, 11 partial dorsal vertebrae, the sacrum, 7 caudal vertebrae, partial hindlimbs with the right foot, and a nearly complete right pelvic girdle comprising the ilium, ischium and pubis. A second specimen is represented by the less complete paratype MPC-D 100/21 that includes two caudal vertebrae and a nearly complete right foot. Both specimens were unearthed from the Nemegt Formation at the locality of Bügiin Tsav, in the Gobi Desert of Mongolia. In 2004, Philip J. Currie and David J. Varricchio referred another two specimens to *Adasaurus* labelled as IGM 100/22 and IGM 100/23. Supposedly, the specimens were recovered from the same locality of the holotype. 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 medium-sized dromaeosaurid. The holotype has an estimated length of 2.4 m (7.9 ft) with a weight of 36.5 kg (80 lb).[10] The comparatively larger pedal elements of the paratype indicate a gently bigger size in this latter specimen.[5] Aside from the reduced pedal ungual II, *Adasaurus* can be recognised by the following additional traits: expanded projection of the maxillary; recurved lacrimal; lower jaw with a prominent surangular foramen; irregular triangular projection on the quadrate shaft; pleurocoels are present on the anterior sacral vertebrae; and the anterior border of the anterior blade in the ilium is relatively shortened.

### 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*. 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.

### Skeleton

The femur and tibia of the holotype measure 27.3 cm (273 mm) and 30.3 cm (303 mm) 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. The anterior surface of the lower end of the femoral shaft is convex in shape, and the lateral tubercle of the upper end of the tibia-fibula articulation is a compact structure. The presence of a rounded pit on the inner surface of the lower tibiotarsus is often documented in dromaeosaurids, however, this feature is not verifiable in *Adasaurus* since the astragalus region is covered with sediments.

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 appareance that is characterisic 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*, 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. Traditionally, *Adasaurus* was assigned to the Dromaeosaurinae, which includes giant, heavily built animals such as *Achillobator* and *Utahraptor* but several analyses have suggested that it belongs to the Velociraptorinae instead.

Below is a cladogram based on the phylogenetic analysis conducted by Powers and colleagues in 2020.

## Paleobiology

The age of the Nemegt, like most of Mongolia's Late Cretaceous sediments, is not known for certain, but it is commonly thought to belong to the Maastrichtian stage of the Late Cretaceous Period.Therefore the Nemegt was deposited somewhere between 74 and 65 million years ago. Other dinosaurs found in this formation include *Tarbosaurus*, *Anserimimus*, and *Saurolophus*.

## JPInstitute.com Description

Was Adasaurus a dinosaur or was it a bird?

Scientists believe that it had feathers, and that it was small and lightweight. It also had a killer claw like Velociraptor, but the claw was smaller.

What we do know from the few fossils that have been discovered so far, is that Adasaurus was a small, meat-eating dinosaur that was very lightweight and had many bird-like features. It has been placed in the same family as Velociraptor for now, but future discoveries may change that.

This small dinosaur adds fuel to the ever-growing debate surrounding dinosaurs and birds. Adasaurus has been described as being extremely similar to Archeopteryx. It has enough of the dromaeosaur characteristics to be placed in that family too, but the overall view of those types of raptors is evolving into their being referred to as bird-like dinosaurs, complete with feathers. Eventually, some members of that family may be reclassified as birds instead of dinosaurs.

## Links

https://web.archive.org/web/20030802071810/http://www.jpinstitute.com/dinopedia/dinocards/dc\_adasa.html