Achillobator

*Achillobator giganticus*

***Achillobator*** ( /əˌkɪlɵˈbeɪtɔr/; "Achilles' warrior/hero") is a genus of large 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: the holotype and only known individual of *Achillobator* is estimated at 6 metres (20 ft) long, around the same length as the closely related *Utahraptor.*

## Etymology

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 one 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 90 and 83 million years ago.Other dinosaurs found in the Bayan Shireh include *Alectrosaurus*, *Segnosaurus*, *Talarurus*, and *Bactrosaurus*.

### 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.

## Description

*Achillobator* was large, heavy bipedal dinosaur with a large sickle claw on each foot. It is one of the largest dromaeosaurs, along with *Utahraptor*, *Dakotaraptor* and *Austroraptor*. The most notable feature of *Achillobator* is its pelvis. In all other know dromaeosaurs, the pubis points backwards, while in contrast the pubis of *Achillobator* points vertically downwards. Overall the animal had a heavy build and would not have been fast moving like other dromaeosaurs.

## 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.

## Paleoecology

The Bayan Shireh Formation was a semi arid environment with large bodies of water which supported extensive plant growth. The most common dinosaurs in this formation are therizinosaurids, such as *Segnosaurus* and *Erlikosaurus*. Additionally there were ankylosaurids, sauropods, hadrosaurs and small primitive marginocephalia. *Achillobator* was most likely the apex predator and used its strength to tackle large, slow moving prey.

## JPInstitute.com Description

Achillobator was a giant raptor dinosaur that lived in Asia. It was discovered in 1999 and scientists are still studying this huge meat-eater. This dinosaur was similar in size to the North American Utahraptor and was probably one of the most vicious hunters of its time. It has made scientists rethink their ideas about the raptor family in Asia.

Further study is needed to firmly assign Achillobator to the dromaeosaur family. Its initial assignment to this group is based on a number of shared characteristics, but these findings are preliminary.

## Links

https://web.archive.org/web/20031006054522fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_achil.html

## References

1. ↑ Burnham, D.A., Derstler, K.L., Currie, P.J., Bakker, R.T., Zhou, Z., and Ostrom, J.H. (2000). "Remarkable new birdlike dinosaur (Theropoda: Maniraptora) from the Upper Cretaceous of Montana." *University of Kansas Paleontological Contributions*, **13**: 1-14.
2. ↑ Norell, M.A. and Makovicky, J.A. (2004). "Dromaeosauridae." In Weishampel, D.B., P. Dodson, and H. Osmolska (eds.). *The Dinosauria* (2nd Edition). Berkeley: University of California Press. Pp. 196-209.
3. ↑ Makovicky, J.A., Apesteguía, S., and Agnolín, F.L. (2005). "The earliest dromaeosaurid theropod from South America." *Nature*, **437**: 1007-1011.