Close Cousins?

Utahraptor on top, Achillobator below. Used with permission as noted on the image.
Taken from A Field Guide to Mesozoic Birds and Other Winged Dinosaurs

Utahraptor and Achillobator were related, somehow. They were the largest raptors in their homelands. They were also strangely "backwards" in terms of the evolutionary lines of their family; some of their character traits seem rather basal for somewhat advanced dinosaurs. These animals were both large terrestrial theropods built with squat bodies (compared to other raptors) that housed large muscle masses more likely to wrestle opponents and overwhelm them with sheer strength than lightning quick attacks. That is not to say that neither of these animals may have been able to execute quick attacks at opponents with jaws or claws; both of which would be devastating weapons. Two hypotheses have been circulated as to the use of giant toe claws of raptors but I think that I would be correct in saying that the more popular hypothesis still is that the claw was used to stab and slice through the arc of motion of the leg. The claws that both of these predators possessed, though through extrapolation of bone shards alone for Achillobator at this point, would have cause massive damage with such behavior. The less popular hypothesis is that the toe claw was used as an anchor for grasping onto prey while teeth and hands shredded the vital arteries and soft paunches of large herbivores. There is a bit of evidence behind that sort of behavior but there is also support for the other behavior hypothesis. However, with animals such as Utahraptor and Achillobator the second hypothesis, that of a grappling animal rather than a quick striking animal, has quite a bit of merit.

Achillobator was an Asian animal, in modern context, and had a different landscape for a home. The Mongolia of 90 Mya was a semiarid desert, kind of like some of it is today. The Utah of Utahraptor, which is considerably older at 125 Mya, was a marshy mud flat with open tracts of land. The squatness of both animals was most likely an adaptation that favored the reduction in length of hindlimb to produce the ability to hide closer to the ground in the low scrub environments in which they lived. Utahraptor lived in a time devoid of large predators; allosaurids, ceratosaurids, and megalosaurids had all disappeared from the landscape during the existence of Utahraptor. Large carnosaur reintroduction into the landscape likely led to a loss of niche and subsequent dwindling of population and eventual extinction of Utahraptor. Achillobator lived alongside many other carnivores, though. Until we have a better range of existence data for Achillobator we cannot really surmise the events leading to its disappearance from the fossil record.

Reference of the day:

Martyniuk, M. P. (2012). A Field Guide to Mesozoic Birds and Other Winged Dinosaurs. Pan Aves.