Achillobator

Achillobator ( ) is a dromaeosaurid theropod dinosaur that lived roughly 98 to 83 million years ago during the Late Cretaceous in what is now Mongolia, in Asia. It was among the largest dromaeosaurs; the holotype and only known individual of Achillobator is estimated at long.Holtz, Thomas R. Jr. (2011) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2010 Appendix. Achillobator was a moderately-built, ground-dwelling, bipedal carnivore. It would have been an active predator, hunting with the enlarged, sickle-shaped claw on the second toe.
==Etymology==
The genus name Achillobator means 'Achilles hero' and is derived from Achilles, a famous ancient Greek warrior who fought in the Trojan War, and the Mongolian word baatar, anciently bagatur, which means 'hero'. The generic name refers to the large Achilles tendon that connects to the sickle claw on the foot, which was the major combat weapon of dromaeosaurids. The specific name giganticus, is derived from Greek word gigantas (γίγαντας) meaning 'giant', which is in reference to Achillobator's size, which exceeds that of most other dromaeosaurids.
==Discovery and species==
 
Fossils of Achillobator were first discovered during a Mongolian and Russian field expedition, and collected by Burkhant in 1989, but the specimen was not described and named until 1999,Perle, A., Norell, M.A., and 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. by Mongolian paleontologist Altangerel Perle, and American paleontologists Mark Norell and Jim Clark, although the description was not complete and was actually published without the knowledge of the latter two paleontologists.Poling, Jeff (1996). "Dinosauria Translation and Pronunciation Guide" Last accessed 2008-07-07.
The fossils of the type specimen of Achillobator, FR.MNUFR 15, were found associated but mostly disarticulated, and include a left maxilla with teeth, two cervical vertebrae, two dorsal vertebrae, rib fragments, seven caudal vertebrae, a scapula and coracoid, a pelvis with a right ilium, pubis and ischium, a radius, an incomplete manus, a left femur and tibia, and an incomplete pes. Smith at al. (2012) noted that this genus represents the second largest of the known dromaeosaurid taxa with a tibial length of 490 mm. Its femur, which is 3% longer than the tibia, a rare trait in Dromaeosaurs, measures 505 mm in length. Estimates suggest that Achillobator weighed at most. The teeth are serrated and recurved, and the posterior serrations are slightly larger than the anterior serrations.
===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 led Burnham et al. (2000) to suggest that Achillobator represents a paleontological chimera.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. However, other studies have attempted to refute this, noting that many pieces were found to be semi-articulated, all of the elements are the same color and preservation, and that Achillobator routinely comes out as a dromaeosaurid in cladistic analyses, even taking into account the differences.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.
==Classification==
Eudromaeosauria cladogram based on the phylogenetic analysis, conducted by Phil Senter, James I. Kirkland, Donald D. DeBlieux, Scott Madsen and Natalie Toth., in 2012.
Achillobator is a dromaeosaurid, a family of dinosaurs currently thought to be very closely related to birds.P. Godefroit, P. J. Currie, H. Li, C. Y. Shang, and Z.-M. Dong. 2008. A new species of Velociraptor (Dinosauria: Dromaeosauridae) from the Upper Cretaceous of northern China. Journal of Vertebrate Paleontology 28(2):432-438 While the relationship of dromaeosaurids to other theropods (including birds) is relatively well-understood, the phylogeny within the family itself is not. The more recent phylogenetic analyses, conducted by Longrich and Currie (2009) and Senter et al. (2012) show that Achillobator is a member of the subfamily Dromaeosaurinae, most closely related to North American forms like Utahraptor and Dromaeosaurus.N. R. Longrich and P. J. Currie. 2009. A microraptorine (Dinosauria–Dromaeosauridae) from the Late Cretaceous of North America. Proceedings of the National Academy of Sciences Deinonychus and Velociraptor are also dromaeosaurids, but appear to represent a different branch of the family.Senter, P., Kirkland, J.I., DeBlieux, D.D., Madsen, S., and Toth, N. (2012). "New Dromaeosaurids (Dinosauria: Theropoda) from the Lower Cretaceous of Utah, and the Evolution of the Dromaeosaurid Tail." PLoS ONE, 7(5): e36790. Makovicky, J.A., Apesteguía, S., and Agnolín, F.L. (2005). "The earliest dromaeosaurid theropod from South America." Nature, 437: 1007-1011.
===Distinguishing anatomical features===
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.
According to the revised diagnosis by Turner et al. (2012), Achillobator can be distinguished based on the following combination of characteristics and autapomorphies:
* the promaxillary fenestra is completely exposed
* the promaxillary and maxillary fenestra are elongate and vertically oriented at same level in the maxilla*
* metatarsal III is wide proximally
* the femur is longer than the tibia
* the pelvis is propubic
* the obturator process on the ischium is large and triangular, and is situated on the proximal half of ischial shaft
* the boot at distal symphysis of pubis is both cranially and caudally developed
==Paleoecology==
===Provenance and occurrence===
The remains of Achillobator were recovered in the Bayan Shireh Formation of Dornogovi Province, Mongolia, in fine-grained, medium sandstone/gray mudstone that was deposited during 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.Jerzykiewicz, T. and Russell, D.A. (1991). "Late Mesozoic stratigraphy and vertebrates of the Gobi Basin." Cretaceous Research, 12(4): 345-377. 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.Hicks, J.F., Brinkman, D.L., Nichols, D.J., and 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.
===Fauna and habitat===
The paleofauna of the Bayan Shireh Formation included the tyrannosaurid Alectrosaurus, the therizinosaurid Segnosaurus, the ankylosaurid Talarurus, the hadrosaurid Bactrosaurus, as well as a genus of sauropod, and an azhdarchid pterosaur.
==See also==
* Timeline of dromaeosaurid research
==References==
==External links==
*Achillobator at Dinodata
* in the Dino Directory
Category:Cretaceous dinosaurs
Category:Dinosaurs of Asia
Category:Eudromaeosaurs
Category:Fossil taxa described in 1999