Achillobator

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Achillobator (a-KIL-o-BAH-tor; "Achilles' warrior/hero") is a genus of dromaeosaurid theropod dinosaur from the Late Cretaceous Period of Central Asia. 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 one species is named A. giganticus because it is much larger than most other dromaeosaurids. This dinosaur was first described in 1999 by Mongolian paleontologist Altangerel Perle, and Americans Mark Norell and Jim Clark, although the description is not complete and was actually published without the knowledge of the latter two authors. A more complete description may be forthcoming.

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 back to the Late Cretaceous Period. The exact age is unknown, with two main 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 (Jerzykiewicz & Russell 1991). 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, giving a possible Cenomanian through Santonian age, or between 98 and 83 million years ago (Hicks et al. 1999). Some other dinosaurs found in the Bayan Shireh are Alectrosaurus, Segnosaurus, Talarurus, and Bactrosaurus.

Achillobator is probably a dromaeosaurid, a family of dinosaurs currently thought to be very closely related to birds. While the relationships 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 animals like Adasaurus, also from Mongolia, as well as North American forms like Utahraptor, Dromaeosaurus, and Saurornitholestes. Deinonychus and Velociraptor are also dromaeosaurines, but appear to represent a different branch of the subfamily (Makovicky et al. 2005).

Possible Chimera?

The pelvis 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 (Burnham et al., 2000). However, other studies 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 (Norell & Makovicky 2004).

References

Burnham, D.A., Derstler, K.L., Currie, P.J., Bakker, R.T., Zhou Z., & Ostrom, J.H. 2000. Remarkable new birdlike dinosaur (Theropoda: Maniraptora) from the Upper Cretaceous of Montana. University of Kansas Paleontological Contributions 13: 1-14.
Hicks, J.F., Brinkman, D.L., Nichols, D.J. & Watabe, M. 1999. Paleomagnetic and palynological analyses of Albian to Santonian strata at Bayn Shireh, Burkhant, and Khuren Dukh, eastern Gobi Desert, Mongolia. Cretaceous Research 20(6): 829-850.
Jerzykiewicz, T. & Russell, D.A. 1991. Late Mesozoic stratigraphy and vertebrates of the Gobi Basin. Cretaceous Research 12(4): 345-377.
Makovicky, J.A., Apesteguía, S., & Agnolín, F.L. 2005. The earliest dromaeosaurid theropod from South America. Nature 437: 1007-1011.
Norell, M.A. & Makovicky, J.A. 2004. Dromaeosauridae. In Weishampel, D.B., P. Dodson, & H. Osmolska (eds.). The Dinosauria (2nd Edition). Berkeley: University of California Press. Pp. 196-209.
Perle, A., Norell, M.A. & Clark, J. 1999. A new maniraptoran theropod - Achillobator giganticus (Dromaeosauridae) - from the Upper Cretaceous of Burkhant, Mongolia. Contributions of the Mongolian-American Paleontological Project 101: 1–105.

External links

[Achillobator] at [DinoData]

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