Achillobator ‭(‬Achilles hero‭)

Achillobator (/əˌkɪləˈbeɪtɔːr/ ə-KIL-ə-BAY-tor; meaning “Achilles hero”) is a genus of large dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous period about 96 million to 89 million years ago in what is now the Bayan Shireh Formation.

It is currently monotypic, only including the type species A. giganticus. It was a large, heavy-built, ground-dwelling, bipedal carnivore that would have been an active feathered predator hunting with the enlarged sickle claw on each second toe. Estimated at 5 m (16 ft) in length with a weight about 250-348 kg (551-767 lb), Achillobator is considered to be one of the largest dromaeosaurs, along with Austroraptor, Dakotaraptor and Utahraptor.

Its stocky and short hindlimb ratio indicates that it was not cursorial—an animal that was mainly terrestrial instead of arboreal or amphibious.

In 1989, a large and associated partial theropod skeleton was discovered in sediments of the Burkhant locality, Bayan Shireh Formation, in South Central Mongolia. The specimen was found preserving a left maxilla with nine teeth and two empty alveoli, four cervical vertebrae, three dorsal vertebrae and eight caudal vertebrae. It was collected and prepared by the assistant paleontologist Namsarai Batulseen and stored as MNUFR-15.

Ten years later, the specimen was formally described in 1999 and became the holotype for the new genus and species Achillobator giganticus. It was identified as a dromaeosaurid taxon and was derived from the Latin word “Achillis” (genitive singular of Achilles) and the old Mongolian word “бaaтap” (baatar, meaning hero). However, the description was published in a very preliminary format, being not complete at all, having issues with preserved elements and numerous typographical errors.

In 1993, a large dromaeosaur claw was found at the Burkhant locality of the Bayan Shireh Formation by a Japanese-Mongolian joint paleontological expedition. In 2010, additional postcranial elements remains were found. In 2007, Rinchen Barsbold and team reported new dinosaur fossil findings at the Shine Us Khuduk locality.

In 2005 and 2006, an isolated pedal phalange II-2 (second phalanx of the second digit of the foot) was discovered during excavations of the Mongolian Academy of Sciences. The pelvic girdle of Achillobator features plesiomorphic (primitive) saurischian characteristics, the pubis is aligned vertically and has a relatively large pubic boot.

The preserved vertebrae are very robust and features a series of pleurocoels. Burnham and team in 2000 suggested that the holotype was a paleontological chimera, but given that the specimen was found in semiarticulation, the assignment of remains to a single individual is supported. Despite its unusual and primitive characteristics, it is commonly recovered as a taxon falling within Dromaeosaursauridae in cladistic analyses.