| Achillobator | |
|---|---|
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| Skeletal diagram showing the size and preserved elements from holotype | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Dromaeosauridae |
| Clade: | †Eudromaeosauria |
| Subfamily: | †Dromaeosaurinae |
| Genus: | †Achillobator Perleet al.1999 |
| Type species | |
| †Achillobator giganticus Perleet al. 1999 | |
Achillobator (/əˌkɪləˈbeɪtɔːr/ə-KIL-ə-BAY-tor; meaning "Achilles hero") is a genus of largedromaeosauridtheropoddinosaur that lived during theLate Cretaceous period about 96 million to 89 million years ago in what is now theBayan Shireh Formation ofMongolia. The genus is currentlymonotypic, only including thetype speciesA. giganticus. The first remains were found in 1989 during a Mongolian-Russian field expedition and later described in 1999. Remains at the type locality ofAchillobator 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,bipedalcarnivore that would have been an active featheredpredator hunting with the enlarged sickleclaw on each secondtoe. Measuring around 4.5–5 m (15–16 ft) long and weighing between 250–350 kg (550–770 lb),Achillobator is considered to be one of the largest dromaeosaurs, along withAustroraptor,Dakotaraptor andUtahraptor.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 withinEudromaeosauria, 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 ofDromaeosaurus andUtahraptor, although it is often considered to be the sister taxon of the latter. The stocky and short leg ratio ofAchillobator indicates that it was notcursorial—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.
During a field exploration examining theoutcrops at the Khongil locality in South CentralMongolia in 1989, conducted by the Mongolian and Russian Paleontological Expedition in theGobi Desert, many dinosaur fossil discoveries were made. About 5.6 km (5,600 m) away from this locality, a large and associated, but mostly disarticulated partialtheropod skeleton was discovered insediments of the Burkhant locality,Bayan Shireh Formation. No other findings were made by the expedition at this locality. It was found in fine-grained, mediumsandstone/graymudstone that was deposited dating back to theLate Cretaceous. The specimen was found preserving a leftmaxilla with nineteeth and two emptyalveoli, fourcervical vertebrae, threedorsal vertebrae, eightcaudal vertebrae, a nearly completepelvic girdle compromising bothpubes, the rightilium, the rightischium, bothfemora, the lefttibia, leftmetatarsals III and IV, manual and pedalphalanges with someunguals, the rightscapulocoracoid, an isolatedradius, tworibs, and caudalchevrons. It was collected andprepared by the assistant paleontologist,Namsarai Batulseen, and stored asMNUFR-15. Ten years later, the specimen was formally described in1999 and became the holotype for the new genus and speciesAchillobator giganticus. It was identified as adromaeosaurid taxon. The description was performed by Mongolian paleontologistAltangerel Perle and North American paleontologistsMark A. Norell and James M. Clark. In terms ofetymology, thegeneric name,Achillobator, is derived from theLatin word "Achillis" (genitive singular ofAchilles), in reference to the largeAchilles tendon that supported the second pedal ungual (known as the "sickle claw") of most dromaeosaurids, and the oldMongolian word "баатар" (baatar, meaning hero).[1]
However, the description was published in a very preliminary format, as it was incomplete, as well as having issues with preserved elements and numeroustypographical 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 ofAchillobator 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 ofAchillobator, by aJapanese-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 paleontologistRinchen 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 seconddigit of the foot) shares similar traits to that ofAchillobator and"Troodon". The remains were discovered during excavations of theMongolian Academy of Sciences in 2005 and 2006.[6]
The pelvic girdle ofAchillobator featuresplesiomorphic (primitive)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 in2000 to suggest that the holotype ofAchillobator represents a paleontologicalchimera and only the pedal unguals may have come from a dromaeosaurid-grade dinosaur.[7]
However, given that the specimen was actually found in semiarticulation,[8] as well as all the elements being the same color and preservation quality, the assignment of remains to a single individual is supported.[8] Despite the fact thatAchillobator features unusual and primitive characteristics compared to other dromaeosaurids, it is commonly recovered as a taxon falling within Dromaeosauridae incladistic analyses.[3]
Achillobator represents one of the largest described dromaeosaurid taxa, reaching up to 4.5–5 m (15–16 ft) long and weighing around 250–350 kg (550–770 lb) in body mass.[9][10][11] Although the holotype was found lacking traces offeather integument,[1] strong evidence coming from other relatives suggests thelikely presence ofplumage onAchillobator.[12][13]
According to the revised diagnosis by Turner and colleagues in 2012,Achillobator can be distinguished based on the following combination of characteristics andautapomorphies: The promaxillary fenestra is completely exposed; the promaxillary and maxillary fenestrae are elongated and vertically oriented at same level in themaxilla;metatarsal III is wide on the upper end; thefemur is longer than thetibia; 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 distalsymphysis of the pubis is developed in a cranial and caudal aspect.[3]
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 11alveoli (sockets that held the roots of teeth). The teeth ofAchillobator displayed markedhomodonty (teeth of similar shape and size) and they were serrated and recurved with the posteriorserrations being slightly larger than the anterior serrations. Along with the posterior serrations were 15 or 18 denticles per 5 mm (0.50 cm) near the center of the teeth, although the anterior serrations had 17 to 20 denticles per 5 mm (0.50 cm). The dimensions of the maxilla suggest thatAchillobator had a relatively largeskull compared to those ofcarnosaurs.[1]
Thevertebral column was composed of large and robust cervical, dorsal, and caudal vertebrae. The anterior series ofcervical vertebrae were sharply angled in development, with the anteriorarticulation facet of thecentrum developed over the posterior facet. These characteristics are interpreted as indicators of an S-shaped neck, as seen in mostmaniraptorans. Thedorsal vertebrae were stocky and hadpleurocoels at the lateral surfaces, increasing in posterior vertebrae. Their central articulation facets were rounded and concave in shape. The neuralprocesses of the dorsal vertebrae show resemblance to those of largeratites such as the extinctmoa and extantemu,ostrich, andcassowary, and they had protrudinginterspinous ligaments scars that indicates a robust and similar back to those birds. In allcaudal vertebrae, the centrum was amphiplatian (flat on both ends) with the posterior articulation facet being more concave than the anterior one. As indicated by a transitional caudal, the anteriorneural spines were elongated and progressively disappeared on posterior caudals. The posterior series of caudals was articulated with longchevrons and had very elongatedprezygapophyses (projections of the vertebral arch that connect adjacent vertebrae).[1]
Thescapulocoracoid was formed by the fusion of thescapula andcoracoid. In the posterior area of theacromion process, the scapula had a small tubercle that attached the area for themusclem. scapulohumeralis in life. Theglenoid was located at the bottom area facing to the lateral side. The scapula was relatively elongated and flattened at the scapular blade. The blade is thickened and slightly curved to the inner side following the rounded shape of therib cage. The coracoid is robust and broad, measuring 16.7 cm (167 mm) long and 14.8 cm (148 mm) in height. The anterior edge had a coarse texture, likely for the articulation withcartilaginous tissue. A largeforamen was located from the front to the back, probably acting as a channel forblood vessels andnerves. Theradius measured 26 cm (260 mm) long.[1]
Thefemur was highly robust and longer than the tibia, a rare trait in dromaeosaurids, measuring 50.5 cm (505 mm) in length. On the anterior inner surface, there was a large, sculptured and concave surface that likely worked as the insertion for the m.iliofemoralis internus in life, and thelesser trochanter had the insertion for the m.pubo ischio femoralis on the lower edge. On the upper lateral surface of the femur, bottom-oriented to thegreater trochanter, there was a tubercle that formed the attachment for the m.iliofemoralis. Insertions for m.ilio trochantericus andischio trochantericus were located on the posterolateral surface at the proximal end of the femur. In the inner side, on the proximal third part of the shaft, a coarse, concave area likely attached m. pubo ischio femoralis externus-3. Below this area, a small, convextubercle attended the insertion for m. pubo ischio femoralis-2. Thetibia was less robust than the femur, but straighter, measuring 49 cm (490 mm) in length. The surface of the shaft center was smooth, without traces of muscles. In front and towards the back surface of thecnemial crest, however, a coarse and microsculptured area worked as the attachment for m. ilio tibialis and ambiens. Additionaltissue traces in the tibia were located on the lower surface and towards the posterior aspect. Here, a depression with a somewhat coarse surface likely attached prominentaponeurosis of the connectedligaments to the tibia andfibular head in life.[1]
Metatarsals III and IV were stocky, measuring 23.4 cm (234 mm) and 20.9 cm (209 mm), respectively, indicating that the length of the metatarsus was under the 50% of the tibial length. As in other maniraptorans, the metatarsals were closely united. The top end of metatarsal III was side-to-side flattened and not pinched at the upper end, thus lacking anarctometatarsalian (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. Thepelvis was formed by the pubis, ilium, and ischium bones. Theilium 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 weremicrostriations that likely gave form to the m. iliofemoralis in life, behind to this area, traces of the m. ilio tibialis-2 were also present. Thepubis 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 veryopisthopubic pelvic configuration,Achillobator had a primitivepropubic 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. Theischium 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]
In its original description,Achillobator was placed as a close relative ofDromaeosaurus with an ambiguous position in the family.[1] In more recent and solid researchAchillobator is classified within theDromaeosauridae, 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".[14] They strongly differ from other dromaeosaurs, such as the arborealmicroraptorians[15] or amphibioushalszkaraptorines.[16] Eudromaeosauria was first defined as a node-basedclade by Nicholas R. Longrich andPhilip J. Currie in2009 as the most inclusive natural group containingDromaeosaurus,Velociraptor,Deinonychus, andSaurornitholestes, 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 toDromaeosaurus or any namesakes of other sub-clades.[14]
Most phylogenetic analyses recoverAchillobator as a relative ofUtahraptor.[17][3][18][16] Similarities betweenAchillobator andUtahraptor were found by osteological reexamination ofUtahraptor specimens, supporting their close relationship.[19] The exact position of these two vary; during the description ofHalszkaraptor in2017,Achillobator andUtahraptor were recovered as close relatives ofDromaeosaurus in theDromaeosaurinae.[16] However, other studies place the two in theVelociraptorinae, such as Currie and Evans (2019)[20] and Jasinskiet al. (2020).[21] An unusual result was found by Hartman and colleagues in 2019, where bothUtahraptor andAchillobator were found to be outside Dromaeosaurinae and Velociraptorinae, joined byYixianosaurus,[22] which is usually considered ananchiornithid.[16]
The leftcladogram follows Cauet al. 2017,[16] while the right cladogram follows Jasinskiet al. 2020:[21]
Perle and team pointed out in 1999 that the structure of the hindlimbs and pelvic region ofAchillobator indicates that the animal had massivethighs and robustly built legs suited for moderate fast-running. In addition, the prominent pelvis ofAchillobator has its own specialized femoralmuscle 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 thatcompsognathids,troodontids andtyrannosauroids were cursorial animals with many taxa recovered with relatively high CLP (cursorial-limb-proportion) scores. Dromaeosaurids however, were recovered with low CLP scores, withAchillobator scoring −5.3 suggesting that it was not adapted tomaintain high speeds for extended amounts of time.[23]
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In2009 Longrich and Currie suggested that while other dromaeosaurids filled a variety of specializedecological niches like the slenderunenlagiines, eudromaeosaurs, such asAchillobator, retained a conservative life-style and filled the niche of large-bodiedpredators of often medium to large-sized prey.[14]
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.[24] Peter Mackovicky stated that the analysis might be correct on primitive dromaeosaurids (such asMicroraptor) being tree-climbers, however, this does not explain why giant animals likeAchillobator orUtahraptor 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.[25]
The striking resemblance between the feet and legs of dromaeosaurids and those ofaccipitridbirds of prey, led Fowler et al. 2011 to propose that dromaeosaurids hunted in a similar way to those ofraptorial birds. They found that the feet and legs of dromaeosaurs resemble those ofeagles andhawks by having an enlarged second claw and a similar range of flexion, but themetatarsals share more resemblance to those ofowls. The model RPR (Raptor Prey Restraint), proposes that dromaeosaurs leaped into theirprey, immobilizing it with their body weight, and then hold it tightly with the large, sickle-shaped claws; afterwards, the dromaeosaur would start to feed on the animal while it's still alive and the death will eventually come fromblood loss andorgan failure. The arms or "wings", that werelikely covered in long feathers, may have been flapped by the dromaeosaur in order to stabilize its balance while restraining prey, along with this, the long, feathered tail probably worked as a counter-balance to the main body. Lastly, the snout would have been useful for finishing off its prey. With these observations, they established that dromaeosaurids and troodontids wereniche partitioned as large and small prey predators, respectively.[26]
In 2020, Powers and colleagues re-examined themaxillae 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 thepremaxilla and overallsnout. For instance, most Asian species have elongated snouts based on the maxilla (animals likeVelociraptor 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 ofAchillobator and North American eudromaeosaurs indicate a diet based on large-sized prey.[27]
The remains ofAchillobator were unearthed from the Burkhant locality of theBayan Shireh Formation. This formation has been divided into upper and lower parts, with some localities representing Lower and Upper Cretaceous boundary.[28][29][30][31] Burkhant, the type locality ofAchillobator, has been identified as an Upper Cretaceous-boundary locality.[32] Analyses on themagnetostratigraphy of the formation indicate that the entire Bayan Shireh lies within theCretaceous Long Normal, which lasted only until the end of theSantonian stage, giving aCenomanian-Santonian age.[33] The recentcalciteU–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.[34] Over the time, a strong correlation with theIren Dabasu Formation has been proposed by numerous authors mainly based on the similar fossil assemblages.[35][36][31] This hypothesis may be also supported by the similar ages.[37]
Theenvironments that were present in the Bayan Shireh Formation andAchillobator inhabited were relativelyhumid and had extensivefluvial andlacustrinefacies, that is, a well-watered region dominated by prominentmeanders,rivers,lakes andstreams. Theclimate of the formation was slightlysemi-arid, as seen oncaliche-based sediments.[35][33]Angiosperms were largely present in the formation, based on fossils ofcornaceans and fossilizedfruits at several localities.[28][38]
Achillobator shared its surroundings in the Bayan Shireh Formation with otherpaleofauna such as the medium-sizedtheropodsErlikosaurus andSegnosaurus,[39]Garudimimus,[40] and possiblyAlectrosaurus;[41] the heavy-builtankylosaursTalarurus andTsagantegia;[42] the smallmarginocephaliansAmtocephale[43] andGraciliceratops;[44] thehadrosauroidGobihadros;[32] and the large, long-neckedsauropodErketu.[38] Non-dinosaur taxa was present as well, mostly compromising semiaquatic and terrestrialreptiles like theturtlesLindholmemys,Gobiapalone and"Trionyx";[45][46] and thecrocodylomorphParalligator.[47]Fishes are also known from the formation, such as thesharkHybodus and the remains ofosteichthyans at various localities.[31]Mammals were extremely rare around the formation.[48]
Niche partitioning has been reported among Bayan Shireh species, such is the case of high browsertherizinosauridsErlikosaurus andSegnosaurus,[39] or thegrazerTalarurus andbrowserTsagantegia.[42]Erketu may have been the tallest herbivore of the paleofauna.[10]
Media related toAchillobator at Wikimedia Commons
Data related toAchillobator at Wikispecies
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