Achillobator ( ; meaning "Achilles hero") is a genus of large dromaeosaurid dinosaurs 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. The genus is currently monotypic, only including the type species A. giganticus. The first remains were found in 1989 during a Mongolian-Russian field expedition in Mongolia and later described in 1999. Remains at the type locality of Achillobator may represent additional specimens. It represents the first and largest dromaeosaurid known from the Bayan Shireh Formation. 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 in length with a body mass about or more, Achillobator is considered to be one of the largest dromaeosaurs, along with Austroraptor, Dakotaraptor and Utahraptor. Achillobator was a deep-bodied and relatively short-armed dromaeosaurid with stocky and robust hindlimbs. Some of the most notable features consisted in the robustly built skeleton—an unusual trait in dromaeosaur dinosaurs, which were generally lightly built animals—such as the deep maxilla and femur, and the primitive pelvis, having a vertically oriented pubis that differs from most other dromaeosaurids. Achillobator is classified as a dromaeosaurid taxon, more specifically within the Eudromaeosauria, a group of hypercarnivore dromaeosaurids that were mainly terrestrial instead of arboreal or amphibious. In most cladistic analyses, Achillobator is recovered as a close relative of Dromaeosaurus and Utahraptor, although it is often considered to be the sister taxon of the latter. The stocky and short hindlimb ratio of Achillobator indicates that it was not cursorial—an animal adapted for speed or to maintain high speeds. Moreover, the robust morphology of the maxilla suggests a predatory behavior based on large-sized prey. History of discovery In 1989 during a field exploration examining the outcrops at the Khongil locality in South Central Mongolia, conducted by the Mongolian and Russian Paleontological Expedition in the Gobi Desert, many dinosaur fossil discoveries were made. About away from this locality, a large and associated, but mostly disarticulated partial theropod skeleton was discovered in sediments of the Burkhant locality, Bayan Shireh Formation; no other findings were made by the expedition at this locality. It was found in fine-grained, medium sandstone/gray mudstone that was deposited dating back to the Late Cretaceous epoch. The specimen was found preserving a left maxilla with nine teeth and