page_title: Achillobator
section_title: Paleoecology
hierarchical_section_title: Achillobator / Paleoecology
caption_attribution_description: Talarurus surrounded by Achillobator.
context_page_description: Achillobator is a genus of large dromaeosaurid theropod dinosaurs that lived in Asia during the Late Cretaceous period about 96 million to 89 million years ago in what is now the Bayan Shireh Formation. The first remains were found in 1989 during a mongolian-russian field expedition in Mongolia and later described in 1999. The genus is currently monotypic, only including the type species A. giganticus. Since the first discovery and description, no more specimens have been found. It represents the first and largest dromaeosaurid taxon known from the Bayan Shireh Formation.
  They were large, heavy-built, ground-dwelling, bipedal carnivores that would have been active feathered predators, hunting with the enlarged, sickle claw on each second toe. Achillobator are considered to be some of the largest dromaeoaurs, estimated to have cover 5 to 6 m in length and weighed 250 kg. Along with Austroraptor, Dakotaraptor and Utahraptor, they are the largest known dromaeosaurs. The most characteristic feature of Achillobator is its primitive pelvis, having a vertically oriented pubis that differs from the rest of dromaeosaurids.
context_section_description: The remains of Achillobator were unearthed from the Burkhant locality at the Bayan Shireh Formation. This formation has been divided into upper and lower parts, with some localities representing Lower and Upper Cretaceous boundary. Burkhant, the type locality of Achillobator, has been identified as a Upper Cretaceous-boundary locality, meaning that is part of the upper Bayan Shireh. Analyses on the magnetostratigraphy of the formation indicate that the entire Bayan Shireh lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a Cenomanian-Santonian age. 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.
  Over the time, a strong correlation with the Iren Dabasu Formation has been proposed by numerous authors mainly based on the similar fossil assemblages. This hypothesis may be also supported by the similar ages: based on U–Pb, paleomagnetic and biostratigraphic analyses, the Iren Dabasu Formation has been dated about 95.8 ± 6.2 million years ago, very similar to Bayan Shireh.
  Therizinosaurs were some of the most abundant theropods in terms of biodiversity across the formation, as evidenced on the remains of Enigmosaurus, Erlikosaurus, Segnosaurus and an unnamed two-fingered therizinosaur that remains undescribed. Hadrosauroids appear to have been pretty abundant too, with remains discovered at numerous localities of the formation, particularly at Baynshire. Furthermore, the relatively new genus Gobihadros is known from at least three localities. Most ecological niches appear to have been well established in the formation. While other dromaeosaurids filled a variety of specialized ecological niches like the slender unenlagiines, eudromaeosaurs, such as the ponderous Achillobator, retained a conservative life-style and filled the niche of large-bodied predators of often medium to large-sized prey. Achillobator may have been predators of large-sized prey. Apparently, there was niche partitioning among species, such is the case of high browsers Erlikosaurus and Segnosaurus, or the grazers Talarurus and browsers Tsagantegia. Erketu, a genus of long necked sauropods from the formation, may have been the tallest herbivores.
  The re-examination of the maxillae in eudromaeosaur taxa indicates that Asian and  North American eudromaeosaurs were separated by snout morphology and ecological strategies. The maxilla is apparently 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 though), indicating a selective feeding, such as picking up small, fast prey. Achillobator however, are an exception for Asian eudromaeosaurs, featuring robust and deep maxillar morphology and thus indicating a strong connection and relationships to North American members, which have stocky and deep snouts. The adaptations of Achillobator and North American eudromaeosaurs demonstrate a conservative ecology mainly based on large-sized prey.
caption: Restoration of an Achillobator pack surrounding an adult Talarurus
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