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250px-Achillobator giganticus skeleton
Achillobator

Temporal range: Late Cretaceous, 89 MaPreЄ

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Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: †Dromaeosauridae
Clade: †Eudromaeosauria
Subfamily: †Dromaeosaurinae
Genus: Achillobator

Perle, Norell, &Clark, 1999

Species: A. giganticus
Binomial name
Achillobator giganticus

Perle, Norell, & Clark, 1999

Achillobator (/əˌkɪləˈbaɪtɔːr/ a-kill-ə-bay-tohr) 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 5 to 6 m (16.4 to 19.7 ft) long.[1][2] Achillobator was a moderately-built, ground-dwelling, bipedal carnivore. It would have been an active predator, hunting with the enlarged, sickle-shapedclaw on the second toe.

Discovery and species

220px-Achillobator scale

Size comparison between Achillobatorand a human

Fossils of Achillobator were first discovered during a Mongolian andRussian field expedition, and collected by Burkhant in 1989, but the specimen was not described and named until 1999,[3] by Mongolian paleontologistAltangerel Perle, and Americanpaleontologists Mark Norell and Jim Clark, although the description was not complete and was actually published without the knowledge of the latter two paleontologists.

The fossils of the type specimen ofAchillobator, 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 thatAchillobator weighed 350 kg (771.6 lb) at most.[4] The teeth are serrated and recurved, and the posterior serrations are slightly larger than the anterior serrations.

Restoration

The genus name Achillobator means 'Achilles hero' and is derived fromAchilles, a famous ancient Greekwarrior who fought in the Trojan War, and the Mongolian word baatar, anciently bagatur, which means 'hero'. The generic name refers to the largeAchilles tendon that connects to the sickle claw on the foot, which was the major combat weapon of dromaeosaurids. The specific namegiganticus, is derived from Greek wordgigantas (γίγαντας) meaning 'giant', which is in reference to Achillobator's size, which exceeds that of most other dromaeosaurids.

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 calledopisthopuby, which is also seen in the unrelated therizinosaurs andornithischians, as well as in birds).

The above differences and others led Burnham et al. (2000) to suggest thatAchillobator represents a paleontological chimera.[5] 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 Achillobatorroutinely comes out as a dromaeosaurid in cladistic analyses, even taking into account the differences.[6]

Classification

Eudromaeosauria


Bambiraptor



Velociraptorinae

Adasaurus



Tsaagan


Velociraptor



Dromaeosaurinae

Deinonychus



Achillobator


Dromaeosaurus


Utahraptor


Yurgovuchia





Eudromaeosauria cladogram based on thephylogenetic analysis, conducted by Phil Senter, James I. Kirkland, Donald D. DeBlieux, Scott Madsen and Natalie Toth., in 2012.[7]

Achillobator is a dromaeosaurid, a familyof dinosaurs currently thought to be very closely related to birds.[8] 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 subfamilyDromaeosaurinae, most closely related to North American forms like Utahraptorand Dromaeosaurus.[9] Deinonychus andVelociraptor are also dromaeosaurids, but appear to represent a different branch of the family.[7][10]

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 ischiumis large and triangular, and is situated on the proximal half of ischial shaft
  • the boot at distal symphysis of pubisis 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 Cretaceousepoch. 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 Turonianthrough early Campanian stages of the Late Cretaceous, about 93 to 80 million years ago.[11] However, examination of the magnetostratigraphy of the formation seems to confirm that the entire Bayan Shireh lies within theCretaceous Long Normal, which lasted only until the end of the Santonianstage, giving a possible Cenomanianthrough Santonian age, or between 98 and 83 million years ago.[12]

Fauna and habitat

The paleofauna of the Bayan Shireh Formation included, the therizinosauridSegnosaurus, the deinocheiridGarudimimus, and the ankylosauridTalarurus, as well as a genus ofsauropod, a tyrannosauroid formerly referred to Alectrosaurus, and anazhdarchid pterosaur.

220px-Achillobator by durbed
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