Achillobator

Achillobator (meaning "Achilles' warrior/hero") is a genus of dromaeosaurid theropod dinosaur from the late Cretaceous Period of what is now Mongolia, about 90 million years ago. It was probably an active bipedal predator, hunting with the large sickle-shaped claw on the second toe of each hind foot. It was a large dromaeosaurid, between 15 and 20 feet (4.6 to 6 meters) from snout to tail.

The generic name comes from Achilles, a famous ancient Greek warrior of the Trojan War, and the Mongolian word bator ("warrior" or "hero"). It refers to the large Achilles tendon needed to use the sickle claw on the foot, which was the major combat organ of dromaeosaurids. The type and only species is named A. giganticus because it is much larger than most other dromaeosaurids.

Discovery and species
The fossil remains of Achillobator were first discovered during a Mongolian and Russian field expedition in 1989, though it was not described and named until ten years later, in 1999, by Mongolian paleontologist Altangerel Perle, and Americans Mark Norell and Jim Clark, although the description was not complete and was actually published without the knowledge of the latter two authors.

Fossil bones of Achillobator were found mostly disarticulated, but associated, including a fragment of the upper jaw with teeth, as well as vertebrae from all sections of the spinal column, ribs, and bones from the shoulder, pelvis, forelimbs and hindlimbs. These remains were found in the Bayan Shireh Formation of Dornogovi Province, Mongolia, which dates to the Late Cretaceous epoch. The exact age is uncertain, with two competing hypotheses; based on comparisons with other formations, the Bayan Shireh fauna seems to correspond best with the Turonian through early Campanian stages of the Late Cretaceous, about 93 to 80 million years ago. However, examination of the magnetostratigraphy of the formation seems to confirm that the entire Bayan Shireh lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a possible Cenomanian through Santonian age, or between 98 and 83 million years ago. Other dinosaurs found in the Bayan Shireh include Alectrosaurus, Segnosaurus, Talarurus, and Bactrosaurus.

Classification
Achillobator is probably a dromaeosaurid, a family of dinosaurs currently thought to be very closely related to birds. While the relationship of dromaeosaurids to other theropods (including birds) is relatively well-understood, the phylogeny within the family itself is not. The most recent analysis shows Achillobator as a member of the subfamily Dromaeosaurinae, most closely related to North American forms like Utahraptor and Dromaeosaurus. Deinonychus and Velociraptor are also dromaeosaurids, but appear to represent a different branch of the family.

Chimera hypothesis
The pelvis of Achillobator seems to show plesiomorphic ("primitive") saurischian characteristics compared to other dromaeosaurids. For instance, the pubis is aligned vertically and has a large pubic boot (a wide expansion at the end), unlike most other dromaeosaurids, where there is a much smaller boot, if any, and the pubis points backwards in the same direction as the ischium (a condition called opisthopuby, which is also seen in the unrelated therizinosaurs and ornithischians, as well as in birds).

The above differences and others have led to suggestions that Achillobator represents a paleontological chimera. However, other studies have attempted to refute this, noting that many pieces were found semi-articulated, and that Achillobator routinely comes out as a dromaeosaurid in cladistic analyses, even taking into account the differences.