Achillobator ( ; meaning "Achilles hero") is a genus of large dromaeosaurid theropod dinosaur that lived during the Late Cretaceous period about 96 million to 89 million years ago in what is now the Bayan Shireh Formation of Mongolia. 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 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, heavily-built, ground-dwelling, bipedal carnivore that would have been an active feathered predator hunting with the enlarged sickle claw on each second toe. Measuring around long and weighing between , Achillobator is considered to be one of the largest dromaeosaurs, along with Austroraptor and Utahraptor. Achillobator was a deep-bodied and relatively short-armed dromaeosaurid with stocky and robust legs. 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, along with the primitive pelvis, having a vertically oriented pubis that differs from most other dromaeosaurids.
Achillobator is classified as a dromaeosaurid taxon, more specifically within Eudromaeosauria, a group of hypercarnivorous 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 leg ratio of Achillobator indicates that it was not cursorial—an animal adapted for high speed or to maintain said high speeds. Moreover, the robust morphology of the maxilla suggests a predatory behavior based around hunting large prey.
History of discovery
During a field exploration examining the outcrops at the Khongil locality in South Central Mongolia in 1989, 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. The specimen was found preserving a left maxilla with nine teeth and two empty alveoli, four cervical vertebrae, three dorsal vertebrae, eight caudal vertebrae, a nearly complete pelvic girdle compromising both pubes, the right ilium, the right ischium, both femora, the left tibia, left metatarsals III and IV, manual and pedal phalanges with some unguals, the 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 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 the "sickle claw") of most dromaeosaurids, and the old Mongolian word "баатар" (baatar, meaning hero).
However, the description was published in a very preliminary format, as it was incomplete, as well as 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, but 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 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.
On August 13, 1993, a large dromaeosaur claw was found at the Burkhant locality, which is the type locality of Achillobator, by a Japanese-Mongolian joint paleontological expedition. In 2010, paleontologist Mahito Watabe and colleagues reported that additional postcranial elements remains were found, all of which 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 various 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.
Chimera hypothesis The 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 a generally a much smaller boot is seen. The preserved vertebrae are very robust and feature a series of pleurocoels. The above differences led Burnham and team in 2000 to suggest that the holotype of Achillobator 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, as well as all the elements being 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.
Description
Achillobator represents one of the largest described dromaeosaurid taxa, reaching up to long and weighing around in body mass. Supporting Online Material Although the holotype was found lacking traces of feather integument, strong evidence coming from other relatives suggests the likely presence of plumage on Achillobator.
According to the revised diagnosis by Turner and colleagues in 2012, Achillobator can be distinguished based on the following combination of characteristics and autapomorphies: The promaxillary fenestra is completely exposed; the promaxillary and maxillary fenestrae are elongated and vertically oriented at same level in the maxilla; metatarsal III is wide on the upper end; the femur is longer than the tibia; the pelvis is propubic (pubis extends forward towards the head of the animal); the obturator process on the ischium is large and triangular situated on the upper half of ischial shaft; and the boot at distal symphysis of the pubis is developed in a cranial and caudal aspect.
Skull The maxilla was deep and robust, and measured about in length. Its lateral side was smooth compared to the dorsal areas, but had a robust constitution. It had approximately 11 alveoli (sockets that held the roots of teeth). The teeth of Achillobator displayed marked homodonty (teeth of similar shape and size) and they were serrated and recurved with the posterior serrations being slightly larger than the anterior serrations. Along with the posterior serrations were 15 or 18 denticles per near the center of the teeth, although the anterior serrations had 17 to 20 denticles per . The dimensions of the maxilla suggest that Achillobator had a relatively large skull compared to those of carnosaurs.
Postcranial skeleton
The vertebral column was composed of large and robust cervical, dorsal, and caudal vertebrae. The anterior series of cervical vertebrae were sharply angled in development, with the anterior articulation facet of the centrum developed over the posterior facet. These characteristics are interpreted as indicators of an S-shaped neck, as seen in most maniraptorans. The dorsal vertebrae were stocky and had pleurocoels at the lateral surfaces, increasing in posterior vertebrae. Their central articulation facets were rounded and concave in shape. The neural processes of the dorsal vertebrae show resemblance to those of large ratites such as the extinct moa and extant emu, ostrich, and cassowary, and they had protruding interspinous ligaments scars that indicates a robust and similar back to those birds. In all , the centrum was amphiplatian (flat on both ends) with the posterior articulation facet being more concave than the anterior one. As indicated by a transitional caudal, the anterior neural spines were elongated and progressively disappeared on posterior caudals. The posterior series of caudals was articulated with long chevrons and had very elongated (projections of the vertebral arch that connect adjacent vertebrae).
The scapulocoracoid was formed by the fusion of the scapula and coracoid. In the posterior area of the acromion process, the scapula had a small tubercle that attached the area for the muscle m. scapulohumeralis in life. The glenoid was located at the bottom area facing to the lateral side. The scapula was relatively elongated and flattened at the scapular blade. The blade is thickened and slightly curved to the inner side following the rounded shape of the rib cage. The coracoid is robust and broad, measuring long and in height. The anterior edge had a coarse texture, likely for the articulation with cartilaginous tissue. A large foramen was located from the front to the back, probably acting as a channel for blood vessels and nerves. The radius measured long.
The femur was highly robust and longer than the tibia, a rare trait in dromaeosaurids, measuring in length. On the anterior inner surface, there was a large, sculptured and concave surface that likely worked as the insertion for the m. iliofemoralis internus in life, and the lesser trochanter had the insertion for the m. pubo ischio femoralis on the lower edge. On the upper lateral surface of the femur, bottom-oriented to the greater trochanter, there was a tubercle that formed the attachment for the m. iliofemoralis. Insertions for m. ilio trochantericus and ischio trochantericus were located on the posterolateral surface at the proximal end of the femur. In the inner side, on the proximal third part of the shaft, a coarse, concave area likely attached m. pubo ischio femoralis externus-3. Below this area, a small, convex tubercle attended the insertion for m. pubo ischio femoralis-2. The tibia was less robust than the femur, but straighter, measuring in length. The surface of the shaft center was smooth, without traces of muscles. In front and towards the back surface of the cnemial crest, however, a coarse and microsculptured area worked as the attachment for m. ilio tibialis and ambiens. Additional tissue traces in the tibia were located on the lower surface and towards the posterior aspect. Here, a depression with a somewhat coarse surface likely attached prominent aponeurosis of the connected ligaments to the tibia and fibular head in life.