Achillobator

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Achillobator
Temporal range: Late Cretaceous, 99–85 Ma
Achillobator giganticus skeleton.jpg
Skeletal restoration by Jaime Headden
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
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 (/əˌkɪləˈbtɔːr/ ə-KIL-ə-BAY-tor) is a genus of dromaeosaurid theropod dinosaur that lived during the Late Cretaceous in what is now Mongolia, Asia. Achillobator was a heavy-built, ground-dwelling, bipedal carnivore. It would have been an active predator, hunting with the enlarged, sickle-shaped claw on the second toe.[1]

Discovery and naming

Fossils of Achillobator were first discovered during a Mongolian and Russian field expedition in the Bayan Shireh Formation, and collected by Burkhant in 1989, but the specimen was not described and named until 1999, 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.[1]

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 most 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.[1]

Description

Size comparison between Achillobator (5 meters estimate) and a human
Life restoration

The holotype of Achillobator, FR.MNUFR-15, was 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. The teeth are serrated and recurved, and the posterior serrations are slightly larger than the anterior serrations.[1] It was among the largest dromaeosaurids; the holotype and only known individual of Achillobator is estimated at 5 to 6 m (16.4 to 19.7 ft) long.[1][2] Smith et al. 2012 noted that this genus represents the second largest of the known dromaeosaurid taxa with a tibial length of 490 mm.[citation needed] Its femur, which is 3% longer than the tibia, a rare trait in dromaeosaurids, measures 505 mm in length.[3] Estimates suggest that Achillobator weighed 350 kg (771.6 lb) at most.[2][4]

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:[5]

  • 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

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).[1]

The above differences and others led Burnham et al. 2000 to suggest that Achillobator represents a paleontological chimera.[6] 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.[7]

Classification

Achillobator is a dromaeosaurid, a family of dinosaurs currently thought to be very closely related to birds. The more recent phylogenetic analyses, conducted by Hartman et al. 2019 shows that Achillobator is a member of the Dromaeosaurinae, most closely related to Yixianosaurus.[8]

Comparison between Achillobator and other giant Dromaeosaurids

During the description of Dakotaraptor in 2015, it was proposed a new cladistic analisys using data from the Theropod Working Group. Achillobator was found to be a relative of Utahraptor:[9]

Eudromaeosauria

Saurornitholestes

Velociraptor

Dromaeosaurinae

Deinonychus

Atrociraptor

Achillobator

Utahraptor

Dakotaraptor

Dromaeosaurus

Below is an Eudromaeosauria cladogram based on the phylogenetic analysis, conducted by Andrea Cau et al. in 2017:[10]

Eudromaeosauria

Bambiraptor

Tianyuraptor

Dromaeosaurinae

Achillobator

Utahraptor

Dromaeosaurus

Velociraptorinae

Adasaurus

Deinonychus

Saurornitholestes

Velociraptor

Tsaagan

Linheraptor

Paleoecology

Restoration of Achillobator surrounding Talarurus

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.[1] 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.[11] 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.[12] However, Dr. Thomas R. Holtz, JR. has suggested that Achillobator lived between 99.6-85.8 million years ago.[2]

Contemporaneous paleofauna of the Bayan Shireh Formation, in which, Achillobator was unearthed, included: [11][13]

Fauna of Bayan Shireh (Achillobator in pale yellow)

See also

References

  1. ^ a b c d e f g 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).
  2. ^ a b c Holtz, T. R.; Rey, L. V. (2007). Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. Random House. Supplementary Information 2012
  3. ^ "Dromaeosaurs". The Theropod Database. Archived from the original on 2012-10-24. Retrieved 2010-02-04.
  4. ^ "ACHILLOBATOR". DinoChecker.com. Retrieved 24 August 2013.
  5. ^ Turner, A. H.; Makovicky, P. J.; Norell, M. A. (2012). "A Review of Dromaeosaurid Systematics and Paravian Phylogeny". Bulletin of the American Museum of Natural History. 371 (371): 1–206. doi:10.1206/748.1.
  6. ^ 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". The University of Kansas Paleontological Contributions. 13: 1–14. doi:10.17161/PCNS.1808.3761. ISSN 1046-8390.
  7. ^ Weishampel, D. B.; Dodson, P.; Osmolska, H. (2007). The Dinosauria, Second Edition. University of California Press. p. 196-209.
  8. ^ Hartman, Scott; Mortimer, Mickey; Wahl, William R.; Lomax, Dean R.; Lippincott, Jessica; Lovelace, David M. (2019). "A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight". PeerJ. 7: e7247. doi:10.7717/peerj.7247. PMC 6626525. PMID 31333906.
  9. ^ DePalma, Robert A.; Burnham, David A.; Martin, Larry D.; Larson, Peter L.; Bakker, Robert T. (2015). "The First Giant Raptor (Theropoda: Dromaeosauridae) from the Hell Creek Formation". Paleontological Contributions (14). doi:10.17161/paleo.1808.18764.
  10. ^ Cau, A.; Beyrand, V.; Voeten, D.; Fernandez, V.; Tafforeau, P.; Stein, K.; Barsbold, R.; Tsogtbaatar, K.; Currie, P.; Godrfroit, P. (6 December 2017). "Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs". Nature. 552 (7685): 395–399. Bibcode:2017Natur.552..395C. doi:10.1038/nature24679. PMID 29211712.
  11. ^ a b Jerzykiewicz, T.; Russell, D. A. (1991). "Late Mesozoic stratigraphy and vertebrates of the Gobi Basin". Cretaceous Research. 12 (4): 345–377. doi:10.1016/0195-6671(91)90015-5. ISSN 0195-6671.
  12. ^ Hicks, J. F.; Brinkman, D. L.; Nichols, D. J.; Watabe, M. (1999). "Paleomagnetic and palynologic analyses of Albian to Santonian strata at Bayn Shireh, Burkhant, and Khuren Dukh, eastern Gobi Desert, Mongolia". Cretaceous Research. 20 (6): 829–850. doi:10.1006/cres.1999.0188.
  13. ^ Weishampel, D. B.; Dodson, P.; Osmolska, H. (2007). The Dinosauria, Second Edition. University of California Press. p. 596.

External links

  • Achillobator in the Dino Directory
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