Achillobator (Achilles hero)
Achillobator (Achilles hero)
Named By : Altangerel Perle, Mark A. Norell & Jim Clark - 1999
Diet : Carnivore
Size : Estimated 6 meters long
Type of Dinosaur : Large Theropod
Type Species : A. giganticus (type)
Found in : Mongolia, Dornogovi Province - Bayan Shireh Formation
When it Lived : Late Cretaceous, 99-84 million years ago
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. 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 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. 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 the rest of 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.
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 5.6 km (5,600 m) 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 two empty alveoli, four cervical vertebrae, three dorsal vertebrae and eight caudal vertebrae, a nearly complete pelvic girdle compromising both pubes, right illium and right ischium, both femora and left tibia, left metatarsals III and IV, manual and pedal phalanges with some unguals, right scapulocoracoid, an isolated radius, two ribs and caudal chevrons. 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. The description was performed by the Mongolian paleontologist Altangerel Perle, and North American paleontologists Mark A. Norell and James M. Clark. In terms of etymology, the generic name, Achillobator, is derived from the Latin word “Achillis” (genitive singular of Achilles) in reference to the large Achilles tendon that supported the second pedal ungual (known as “sickle claw”) of most dromaeosaurids, and the old Mongolian word “баатар” (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. Due to a misinterpretation, the pedal ungual II (or sickle claw) was claimed to be preserved and to articulate with the pedal phalanx II, however, this was corrected by Senter in 2007 and this ungual actually represents a manual one. Turner and colleagues in 2012 during their large revision of the Dromaeosauridae stated that the describing paper of Achillobator was likely published without the knowledge of the two latter paleontologists as indicated by a draft left in Mongolia in 1997.
In 1993 on August 13 a large dromaeosaur claw was found at the Burkhant locality—which is the type locality of Achillobator—by a Japanese-Mongolian joint paleontological expedition. Later in 2010, paleontologist Mahito Watabe and colleagues reported that additional postcranial elements remains were found, all of them belonging to a large-sized dromaeosaurid. In 2007 Mongolian paleontologist Rinchen Barsbold and team reported new dinosaur fossil findings at the Shine Us Khuduk locality of the Bayan Shireh Formation. Among elements, an isolated pedal phalange II-2 (second phalanx of the second digit of the foot) shares similar traits to that of Achillobator and “Troodon”. The remains were discovered during excavations of the Mongolian Academy of Sciences in 2005 and 2006.
he pelvic girdle of Achillobator features plesiomorphic (primitive) saurischian characteristics compared to other dromaeosaurids. For instance, the pubis is aligned vertically and has a relatively large pubic boot (a wide expansion at the end), unlike most other dromaeosaurids, where there is generally a much smaller boot. The preserved vertebrae are very robust and features a series of pleurocoels. The above differences led Burnham and team in 2000 to suggest that the holotype of Achillobator in fact, represents a paleontological chimera, and only the pedal unguals may have come from a dromaeosaurid-grade dinosaur.
However, given that the specimen was actually found in semiarticulation, and all elements have the same color and preservation quality the assignment of remains to a single individual is supported. Despite the fact that Achillobator features unusual and primitive characteristics compared to other dromaeosaurids, it is commonly recovered as a taxon falling within Dromaeosauridae in cladistic analyses.