Achillobator

***Achillobator*** (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.

## Description

The holotype is estimated to have reached a length of 5 m (16 ft) and weighed 250–348 kg (551–767 lb). *Achillobator* represents the second largest described dromaeosaurid taxon (after *Utahraptor*) with a tibial length of 49 cm (490 mm). The gigantic size of *Achillobator* was only approached or surpassed by the also paravians *Austroraptor*, *Dakotaraptor* and *Utahraptor*. The holotype was found lacking traces of feather integument, however, there are strong evidence coming from other relatives that suggest 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 elongate 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 pubis is developed in a cranial and caudal aspect.

### Skull

The maxilla was deep and robust, and measured about 29 cm (290 mm) 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, there were 15 or 18 denticles per 5 mm (0.50 cm) near the center of the teeth, although the anterior serrations had 17 to 20 denticles per 5 mm (0.50 cm). The dimensions of the maxilla suggest that *Achillobator* had a relatively large skull competing 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 developed, with the anterior articulation facet of the centrum developed over the posterior facet. These characteristics are interpreted as indicators of a 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 caudal vertebrae 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 were articulated with long chevrons and had very elongated prezygapophyses (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 *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 16.7 cm (167 mm) long and 14.8 cm (148 mm) 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 26 cm (260 mm) long.

The femur was highly robust and longer than the tibia, a rare trait in dromaeosaurids, measuring 50.5 cm (505 mm) 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 more straight, measuring 49 cm (490 mm) 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, there was a coarse and microsculptured area that 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.

Metatarsals III and IV were stocky, measuring 23.4 cm (234 mm) and 20.9 cm (209 mm) respectively, indicating that the length of the metatarsus was under the 50% of the tibial length. As in other maniraptorans, the metatarsals were closely united. The top end of metatarsal III was side to side flattened and not pinched at the upper end, thus lacking an arctometatarsalian (pinched upper end) condition. The lower end of metatarsal IV had a joint where motion was restricted to one plane, and the lateral condyle was short. The pelvis was formed by the pubis, ilium and ischium bones. The ilium was prominently tall measuring 51.3 cm (513 mm) in length and 28.8 cm (288 mm) in height with the preacetabular process situated from top to bottom. Both insertions for the m. *caudofemoralis* were located at the inner surface of the ilium. On the anterior edge were microstriations that likely gave form to the m. *iliofemoralis* in life, behind to this area, traces of the m. *ilio tibialis-2* were also present. The pubis measured 54.8 cm (548 mm) long and is very straight having a large pubic boot (a wide expansion at the end). While other dromaeosaurids had a very opisthopubic pelvic configuration, *Achillobator* had a primitive propubic configuration. The large pubic boot had coarse areas on the dorsal surface that served as attachment sites for the m. *pubo ischio femoralis internus ventralis*. Its lateral surface was very flat and had numerous microstriations that probably originated the m. *pubo ischio femoralis externus*. The ischium was shorter than the ilium and pubis, measuring 37.8 cm (378 mm) in length. A large ridge was formed on its shaft and expanded to the anterior edge, likely forming the m. *adductor femoris* in life. This ridge-like structure was heavy-built compared to other dromaeosaurids. At the lateral sides the surface was very rough, possibly attaching m. *flexor tibialis internus-1* in life.

## *Achillobator* in The Amazing Dinosaur Adventure

### Characters

Amanda and Jeff, a pair of subordinate *Achillobator* from the TV series
### Anonymous

*Achillobator* from *The Stone of Cold Fire**Achillobator* from *Invasion of the Tinysauruses*Mama *Achillobator* and her 3 hatchlings from *The Great Egg Adventure*