Achillobator

Source: Wikipedia, the free encyclopedia.

Achillobator
Temporal range:
Ma
Skeletal diagram showing the size and preserved elements from holotype
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Dromaeosauridae
Clade: †Eudromaeosauria
Subfamily: †Dromaeosaurinae
Genus: †Achillobator
Perle et al. 1999
Type species
†Achillobator giganticus
Perle et al. 1999

Achillobator (

monotypic, only including the type species
A. giganticus. The first remains were found in 1989 during a Mongolian-Russian field expedition and later described in 1999. Remains at the type locality of Achillobator may represent additional specimens. It represents the first and largest dromaeosaurid known from the Bayan Shireh Formation.

It was a large, heavily-built, ground-dwelling,

predator hunting with the enlarged sickle claw on each second toe. Measuring around 5 m (16 ft) long and weighing between 250–350 kg (550–770 lb), Achillobator is considered to be one of the largest dromaeosaurs, along with Austroraptor, Dakotaraptor and Utahraptor
. Achillobator was a deep-bodied and relatively short-armed dromaeosaurid with stocky and robust legs. Some of the most notable features consisted in the robustly built skeleton—an unusual trait in dromaeosaur dinosaurs, which were generally lightly built animals—such as the deep maxilla and femur, along with the primitive pelvis, having a vertically oriented pubis that differs from most other dromaeosaurids.

Achillobator is classified as a dromaeosaurid taxon, more specifically within Eudromaeosauria, a group of hypercarnivorous dromaeosaurids that were mainly terrestrial instead of arboreal or amphibious. In most cladistic analyses, Achillobator is recovered as a close relative of Dromaeosaurus and Utahraptor, although it is often considered to be the sister taxon of the latter. The stocky and short leg ratio of Achillobator indicates that it was not cursorial—an animal adapted for high speed or to maintain said high speeds. Moreover, the robust morphology of the maxilla suggests a predatory behavior based around hunting large prey.

History of discovery

Fossil localities in Mongolia. Achillobator fossils have been collected at Burkhant (area D)

During a field exploration examining the

genitive singular of Achilles), in reference to the large Achilles tendon that supported the second pedal ungual (known as the "sickle claw") of most dromaeosaurids, and the old Mongolian word "баатар" (baatar, meaning hero).[1]

Illustration of the pelvis

However, the description was published in a very preliminary format, as it was incomplete, as well as having issues with preserved elements and numerous

typographical errors.[1] Due to a misinterpretation, the pedal ungual II (or sickle claw) was claimed to be preserved and to articulate with the pedal phalanx II, but this was corrected by Senter in 2007 and this ungual actually represents a manual one.[2] Turner and colleagues, in 2012, during their large revision of Dromaeosauridae, stated that the describing paper of Achillobator was likely published without the knowledge of the two latter paleontologists, as indicated by a draft left in Mongolia in 1997.[3]

On August 13, 1993, a large dromaeosaur claw was found at the Burkhant locality, which is the type locality of Achillobator, by a Japanese-Mongolian joint paleontological expedition.[4] In 2010, paleontologist Mahito Watabe and colleagues reported that additional postcranial elements remains were found, all of which belonging to a large-sized dromaeosaurid.[5] In 2007, Mongolian paleontologist Rinchen Barsbold and team reported new dinosaur fossil findings at the Shine Us Khuduk locality of the Bayan Shireh Formation. Among various elements, an isolated pedal phalange II-2 (second phalanx of the second digit of the foot) shares similar traits to that of Achillobator and "Troodon". The remains were discovered during excavations of the Mongolian Academy of Sciences in 2005 and 2006.[6]

Chimera hypothesis

The pelvic girdle of Achillobator features

saurischian characteristics compared to other dromaeosaurids. For instance, the pubis is aligned vertically and has a relatively large pubic boot (a wide expansion at the end), unlike most other dromaeosaurids, where a generally a much smaller boot is seen. The preserved vertebrae are very robust and feature a series of pleurocoels. The above differences led Burnham and team in 2000 to suggest that the holotype of Achillobator represents a paleontological chimera and only the pedal unguals may have come from a dromaeosaurid-grade dinosaur.[7]

However, given that the specimen was actually found in semiarticulation,

cladistic analyses.[3]

Description

Size comparison of the holotype and an 1.8-m-tall human

Achillobator represents one of the largest described dromaeosaurid taxa, reaching up to 4–5 m (13–16 ft) long and weighing around 250–350 kg (550–770 lb) in body mass.[9][10][11][12] Although the holotype was found lacking traces of feather integument,[1] strong evidence coming from other relatives suggests the likely presence of plumage on Achillobator.[13][14]

According to the revised diagnosis by Turner and colleagues in 2012, Achillobator can be distinguished based on the following combination of characteristics and

metatarsal III is wide on the upper end; the femur is longer than the tibia; the pelvis is propubic (pubis extends forward towards the head of the animal); the obturator process on the ischium is large and triangular situated on the upper half of ischial shaft; and the boot at distal symphysis of the pubis is developed in a cranial and caudal aspect.[3]

Skull

The maxilla was deep and robust, and measured about 29 cm (290 mm) in length. Its lateral side was smooth compared to the dorsal areas, but had a robust constitution. It had approximately 11

carnosaurs.[1]

Postcranial skeleton

Life restoration

The

chevrons and had very elongated prezygapophyses (projections of the vertebral arch that connect adjacent vertebrae).[1]

The

nerves. The radius measured 26 cm (260 mm) long.[1]

Diagram featuring the preserved tissue traces on femur and tibia

The

ligaments to the tibia and fibular head in life.[1]

arctometatarsalian (pinched upper end) condition. The lower end of metatarsal IV had a joint where motion was restricted to one plane, and the lateral condyle was short. The pelvis was formed by the pubis, ilium, and ischium bones. The ilium was prominently tall, measuring 51.3 cm (513 mm) in length and 28.8 cm (288 mm) in height with the preacetabular process situated from top to bottom. Both insertions for the m. caudofemoralis were located at the inner surface of the ilium. On the anterior edge were microstriations that likely gave form to the m. iliofemoralis in life, behind to this area, traces of the m. ilio tibialis-2 were also present. The pubis measured 54.8 cm (548 mm) long and is very straight, having a large pubic boot (a wide expansion at the end). While other dromaeosaurids had a very opisthopubic pelvic configuration, Achillobator had a primitive propubic configuration. The large pubic boot had coarse areas on the dorsal surface that served as attachment sites for the m. pubo ischio femoralis internus ventralis. Its lateral surface was very flat and had numerous microstriations that probably originated the m. pubo ischio femoralis externus. The ischium was shorter than the ilium and pubis, measuring 37.8 cm (378 mm) in length. A large ridge was formed on its shaft and expanded to the anterior edge, likely forming the m. adductor femoris in life. This ridge-like structure was heavily built compared to other dromaeosaurids. At the lateral sides, the surface was very rough, possibly attaching m. flexor tibialis internus-1 in life.[1]

Classification

Many genera of the Dromaeosauridae

In its original description, Achillobator was placed as a close relative of Dromaeosaurus with an ambiguous position in the family.[1] In more recent and solid research Achillobator is classified within the Dromaeosauridae, a group of very bird-like, maniraptoran dinosaurs, being placed in the Eudromaeosauria, a group of dromaeosaurids that were obligate terrestrial and hypercarnivore animals, better known as the "true dromaeosaurids".[15] They strongly differ from other dromaeosaurs, such as the arboreal microraptorians[16] or amphibious halszkaraptorines.[17] Eudromaeosauria was first defined as a node-based clade by Nicholas R. Longrich and Philip J. Currie in 2009 as the most inclusive natural group containing Dromaeosaurus, Velociraptor, Deinonychus, and Saurornitholestes, their most recent common ancestor and all of its other descendants. The various "subfamilies" have also been re-defined as clades, usually defined as all species closer to the groups namesake than to Dromaeosaurus or any namesakes of other sub-clades.[15]

Comparison between giant dromaeosaurids including Achillobator

Most phylogenetic analyses recover Achillobator as a relative of Utahraptor.

anchiornithid.[17]

The left cladogram follows Cau et al. 2017,[17] while the right cladogram follows Jasinski et al. 2020:[22]

Eudromaeosauria
Eudromaeosauria

Paleobiology

Perle and team pointed out in 1999 that the structure of the hindlimbs and pelvic region of Achillobator indicates that the animal had massive

thighs and robustly built legs suited for moderate fast-running. In addition, the prominent pelvis of Achillobator has its own specialized femoral muscle retractors that may indicate a strong ability for leaping.[1]

In 2016, Scott Persons IV and Currie examined the limb proportion of numerous theropods and found that

tyrannosauroids were cursorial animals with many taxa recovered with relatively high CLP (cursorial-limb-proportion) scores. Dromaeosaurids however, were recovered with low CLP scores, with Achillobator scoring −5.3 suggesting that it was not adapted to maintain high speeds for extended amounts of time.[24]

Predatory behavior

A leaping Deinonychus employing RPR on Zephyrosaurus

In

predators of often medium to large-sized prey.[15]

Manning and team in 2009 tested the function of the sickle claw of dromaeosaurids by analyzing the biomechanics of how stresses and strains would be distributed along the claws and into the limbs, and using comparisons within the curvature of the dromaeosaurid sickle claw on the foot with curvature in modern birds and mammals. They found that they were ideal for climbing and for a ground-dwelling life style.[25] Peter Mackovicky stated that the analysis might be correct on primitive dromaeosaurids (such as Microraptor) being tree-climbers, however, this does not explain why giant animals like Achillobator or Utahraptor retained sickle claws as they were likely far too large to have climbed trees with any great faculty. Mackovicky suggested that larger dromaeosaurids adapted the claw to be used exclusively for a more aggressive predatory behavior.[26]

The striking resemblance between the feet and legs of dromaeosaurids and those of

niche partitioned as large and small prey predators, respectively.[27]

In 2020, Powers and colleagues re-examined the

maxillae of eudromaeosaur taxa concluding that Asian and North American eudromaeosaurs were separated by snout morphology and ecological strategies. They found the maxilla to be a reliable reference when inferring the shape of the premaxilla and overall snout. For instance, most Asian species have elongated snouts based on the maxilla (animals like Velociraptor are known from complete skulls), indicating a selective feeding, such as picking up small, fast prey. Achillobator however, is an exception for Asian eudromaeosaurs, featuring a robust and deep maxillar morphology similar to North American eudromaeosaurs which also have stocky and deep snouts, and thus indicating relationships with these members. The adaptations of Achillobator and North American eudromaeosaurs indicate a diet based on large-sized prey.[28]

Paleoenvironment

dinosaurs of the Bayan Shireh Formation
(Achillobator in dark yellow, second from right)

The remains of Achillobator were unearthed from the Burkhant locality of the

Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a Cenomanian-Santonian age.[34] The recent calcite U–Pb analyses performed by Kurudama and colleagues in 2020 have confirm the age of the Bayan Shireh Formation from 95.9 ± 6.0 million to 89.6 ± 4.0 million years ago, also supporting a Cenomanian-Santonian age.[35] Over the time, a strong correlation with the Iren Dabasu Formation has been proposed by numerous authors mainly based on the similar fossil assemblages.[36][37][32] This hypothesis may be also supported by the similar ages.[38]

The

Angiosperms were largely present in the formation, based on fossils of cornaceans and fossilized fruits at several localities.[29][39]

Contemporary paleofauna

Restoration of an Achillobator pack surrounding an adult Talarurus

Achillobator shared its surroundings in the Bayan Shireh Formation with other

osteichthyans at various localities.[32] Mammals were extremely rare around the formation.[49]

therizinosaurids Erlikosaurus and Segnosaurus,[40] or the grazer Talarurus and browser Tsagantegia.[43] Erketu may have been the tallest herbivore of the paleofauna.[10]

See also

References

  1. ^
    OCLC 69865262
    .
  2. ISSN 1342-4092
    .
  3. ^
    S2CID 83572446
    .
  4. ^ Watabe, M.; Suzuki, S. (2000). "Report on the Japan - Mongolia Joint Paleontological Expedition to the Gobi desert, 1993". Hayashibara Museum of Natural Sciences Research Bulletin. 1: 17−29.
  5. ^ Watabe, M.; Tsogtbaatar, K.; Suzuki, S.; Saneyoshi, M. (2010). "Geology of dinosaur-fossil-bearing localities (Jurassic and Cretaceous: Mesozoic) in the Gobi Desert: Results of the HMNS-MPC Joint Paleontological Expedition". Hayashibara Museum of Natural Sciences Research Bulletin. 3: 41−118.
  6. S2CID 220411226
    .
  7. ISSN 1046-8390
    .
  8. ISBN 9780520941434
    .
  9. ISSN 0016-7568
    .
  10. ^
    ISBN 9780691167664
    .
  11. PMID 17823350. Supporting Online Material
  12. PMID 34527277
    .
  13. PMID 17885130
    .
  14. PMID 26181289
    .
  15. ^
    PMID 19289829
    .
  16. PMID 25025742
    .
  17. ^
    S2CID 4471941
    .
  18. ^ Senter, P.; Barsbold, R.; Britt, B. B; Burnham, D. A. (2004). "Systematics and evolution of Dromaeosauridae (Dinosauria, Theropoda)". Bulletin of Gunma Natural History Museum. 8: 1–20.
  19. PMID 22615813
    .
  20. S2CID 220414868
    .
  21. S2CID 202002676
    .
  22. ^
    PMID 32218481
    .
  23. PMID 31333906
    .
  24. PMID 26813782
    .
  25. S2CID 12902686
    .
  26. doi:10.1016/s0262-4079(09)62381-2
    .
  27. PMID 22194962
    .
  28. S2CID 216499705
    .
  29. ^
    ISBN 9780444502766
    .
  30. ISBN 9780521545822
    .
  31. S2CID 197540125
    .
  32. ^
    S2CID 54210424. Archived from the original
    (PDF) on 2019-03-07.
  33. ^
    PMID 30995236
    .
  34. ^
    doi:10.1006/cres.1999.0188
    .
  35. doi:10.1111/ter.12456
    .
  36. ^
    doi:10.1016/0195-6671(91)90015-5
    .
  37. doi:10.1016/j.cretres.2015.03.007
    .
  38. doi:10.1016/j.jseaes.2017.12.007. Archived from the original
    (PDF) on 2020-09-19. Retrieved 2020-06-02.
  39. ^
    S2CID 86032547
    .
  40. ^
    PMID 27069815
    .
  41. hdl:2115/14579
    .
  42. ^ Perle, A. (1977). "O pervoy nakhodke Alektrozavra (Tyrannosauridae, Theropoda) iz pozdnego Mela Mongolii" [On the first discovery of Alectrosaurus (Tyrannosauridae, Theropoda) in the Late Cretaceous of Mongolia]. Shinzhlekh Ukhaany Akademi Geologiin Khureelen (in Russian). 3 (3): 104–113.
  43. ^
    S2CID 212423361
    .
  44. ^ Watabe, M.; Tsogtbaatar, K.; Sullivan, R.M. (2011). "A new pachycephalosaurid from the Baynshire Formation (Cenomanian-late Santonian), Gobi Desert, Mongolia" (PDF). Fossil Record 3. New Mexico Museum of Natural History and Science, Bulletin. 53: 489–497.
  45. ^ Sereno, P.C. (2000). "The fossil record, systematics and evolution of pachycephalosaurs and ceratopsians from Asia" (PDF). The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press. pp. 489–491.
  46. doi:10.4202/app.2008.0405
    .
  47. S2CID 86304259
    .
  48. PMID 25714338
    .
  49. doi:10.1016/j.cretres.2014.09.009
    .

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Achillobator&oldid=1196850990"