Adasaurus

Fossil localities in Mongolia. Locality of Adasaurus in Bugin Tsav, at Area A

The holotype of Adasaurus, IGM 100/20, and a second specimen, IGM 100/21, were unearthed from the Nemegt Formation at the locality of Bugin Tsav, in the Gobi Desert, Mongolia. The holotype consists of a partial skull and postcranial remains, while the other specimen is mostly represented by the hindlimbs and additional postcrania.[1] It was first reported on 1977 in a pelvic comparison with other theropods, and informally labelled as Adasaurus.[2] Later on, in 1983 it was shortly described and named by the Mongolian paleontologist Rinchen Barsbold along with other asiatic dinosaur genera. The generic name, Adasaurus, is taken from Ada, an evil spirit 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.[1] Both specimens are housed in the collection of the Mongolian Geological Institute (IGM) in Ulaanbaatar, Mongolia.[1][3]

Currie and Varricchio in 2004 referred another two additional specimens, labelled as IGM 100/22 and IGM 100/23 that were recovered from the same locality of the holotype.[4] However, these specimens are actually known from the Shine Us Khuduk and Tel Ulan Chaltsai localities (respectively) at the Bayan Shireh Formation and thus, they are older than the remains of Adasaurus from the Nemegt Formation. Apparently, they represent a different and new taxa that differs from Adasaurus.[5][3]

Diagram of the right pelvis on IGM 100/20

Life restoration

Adasaurus was a moderate-sized dromaeosaurid. During the description of Mahakala, by using body mass equations to estimate size and length in many paravians, Adasaurus was estimated at 2.39 m (7.8 ft) long with a weight of 36.4 kg (80 lb).[6] In 2012, Thomas R. Holtz estimated its length around 1.8 m (5.9 ft) and a weight around 9.1 to 22.7 kg (20.1 to 50.0 lb).[7] Nevertheless, Gregory S. Paul estimated its length at 2 m (6.6 ft) and gave a weight of 15 kg (33 lb) in 2016.[8] The holotypic remains are fragmentary, consisting of a skeleton with partial skull missing its anterior region, 9 cervical vertebrae, 3 partial dorsal vertebrae, and approximately 6 sacral vertebrae, the hindlimbs, a nearly complete right pelvic girdle compromising the illium, ischium and pubis. The second specimen, also briefly described in the original paper[1], consists of a right hindlimb, including a complete pes and other unspecified fragmented postcrania.[3] The femur of the holotype IGM 100/20 measures 27.3 cm (273 mm) and the fourth trochanter is developed in a ridge-like structure with a rugose texture. The femur itself is very similar to that of the indeterminate dromaeosaur DGBU-78, known from the Gugyedong Formation.[9][6]

In the original description, the only autapomorphy given to Adasaurus was the reduced second pedal ungual.[1] The authenticity of the reduced II pedal ungual has been disputed though, Senter 2010 claimed that the supposed ungual did not pertain to the specimen.[10] However, in the revised diagnosis conducted by Turner et al. 2012, Adasaurus can be recognised by the following traits: maxillary process of the jugal dorsoventrally expanded; the descending process of the lacrimal is strongly curved anteriorly; mandible with large surangular foramen; quadrate shaft with dorsally irregular triangular process along lateral border; only the anterior sacral vertebrae have pleurocoels; second pedal ungual reduced; the anterior margin of the preacetabular blade of ilium is short.[3]

Comparison between members of the Velociraptorinae, featuring Linheraptor, Tsaagan and Velociraptor

Adasaurus is a member of Dromaeosauridae, a group that is closely related to living birds. The anatomical features of Adasaurus seems to indicate a velociraptorine affinity. Traditionally, Adasaurus was assigned to the Dromaeosaurinae, which includes giant, heavily built animals such as Achillobator and Utahraptor[11][12] but several analyses have suggested that it belongs to the Velociraptorinae instead.[13][14][15]

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 included Adasaurus in the group.[1] 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.[16] 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 et al. 2012 supported a traditional, monophyletic composition of Velociraptorinae.[3] 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. Saurornitholestes was recovered within the more basal group which they named Saurornitholestinae.[17]

Below is a cladogram based on the phylogenetic analysis conducted by Currie and Evans in 2019.[18]

The age of the Nemegt Formation has been estimated from the Late Campanian to Early-Middle Maastrichtian stages, about 70 million and 68 million years ago.[19] 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.[20][21][22][23] Other dinosaurs found in this formation include the ornithomimosaurs Anserimimus, Deinocheirus, Gallimimus and a fourth taxon that remains unnamed.[24]; diverse oviraptorosaurs such as Nemegtomaia, Elmisaurus and Gobiraptor[23][22]; the large troodontid Zanabazar[25]; the famous tyrannosaur Tarbosaurus and hadrosaur Saurolophus, among other dinosaurs.[26]

• Timeline of dromaeosaurid research
• Nemegt Basin
• Nemegt Formation
• Rinchen Barsbold