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 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. Classification In its original description, Achillobator was placed as a close relative of Dromaeosaurus with an ambiguous position in the family. 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". They strongly differ from other dromaeosaurs, such as the arboreal microraptorians or amphibious halszkaraptorines. 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. Most phylogenetic analyses recover Achillobator as a relative of Utahraptor. Similarities between Achillobator and Utahraptor were found by osteological reexamination of Utahraptor specimens, supporting their close relationship. The exact position of these two vary; during the description of Halszkaraptor in 2017, Achillobator and Utahraptor were recovered as close relatives of Dromaeosaurus in the Dromaeosaurinae. However, other studies place the two in the Velociraptorinae, such as Currie and Evans (2019) and Jasinski et al. (2020). An unusual result was found by Hartman and colleagues in 2019, where both Utahraptor and Achillobator were found to be outside Dromaeosaurinae and Velociraptorinae, joined by Yixianosaurus, which is usually considered an anchiornithid. The left cladogram follows Cau et al. 2017, while the right cladogram follows Jasinski et al. 2020: Paleobiology Perle and team pointed out in 1999 that the structure of the hindlimbs and pelvic region of Achillobator indicates that the animal had