2009 study estimated that ankylosaurids could swing their tails at 100 degrees laterally, and the mainly cancellous clubs would have had a lowered moment of inertia and been effective weapons. The study also found that while adult ankylosaurid tail clubs were capable of breaking bones, those of juveniles were not. Despite the feasibility of tail-swinging, the researchers could not determine whether ankylosaurids used their clubs for defense against potential predators, in intraspecific combat, or both. Other studies have found evidence of ankylosaurids using their tail clubs for intraspecific combat. One specimen of Tarchia showed signs of injury on both the pelvic and tail area and the club was found to be asymmetrical, a sign of being worn down by the strikes. In 1993 Tony Thulborn proposed that the tail club of ankylosaurids primarily acted as a decoy for the head, as he thought the tail too short and inflexible to have an effective reach; the "dummy head" would lure a predator close to the tail, where it could be struck. Carpenter has rejected this idea, as tail club shape is highly variable among ankylosaurids, even in the same genus. Paleoenvironment Ankylosaurus existed between 68 and 66 million years ago, in the final, or Maastrichtian, stage of the Late Cretaceous Period. It was among the last dinosaur genera that appeared before the Cretaceous–Paleogene extinction event. The type specimen is from the Hell Creek Formation of Montana, while other specimens have been found in the Lance and Ferris Formations in Wyoming, the Scollard Formation in Alberta, and the Frenchman Formation in Saskatchewan, all of which date to the end of the Cretaceous. Fossils of Ankylosaurus are rare in the sediments it is known from, and the distribution of its remains suggests that it was ecologically rare, or restricted to the uplands of the formations, where it would have been less likely to fossilize, rather than the coastal lowlands. Another ankylosaur, a nodosaur referred to as Edmontonia sp., is also found in the same formations, but according to Carpenter, the range of the two genera does not seem to have overlapped. Their remains have so far not been found in the same localities, and the nodosaur appears to have inhabited the lowlands. The narrower muzzle of the nodosaur suggests it had a more selective diet than Ankylosaurus, further indicating ecological separation, whether their range overlapped or not. With its low center of gravity,