from a partial sub-adult individual consisting of a nearly complete skull with the lower jaws missing all teeth (lost during fossilization), 9 cervical vertebrae, 3 dorsal vertebrae, 18 caudal vertebrae, both scapulocoracoids, partial ilia, and the left hindlimb compromising the femur, tibia, fibula, astragalus with calcaneum, and metatarsals III and IV. Lythronax, the oldest known member of Tyrannosauridae, was described in 2013 by Mark A. Loewen and colleagues from a nearly complete specimen that was uncovered in 2009 in the Wahweap Formation of the Grand Staircase–Escalante National Monument. Natural history Description In comparison to the albertosaurines, tyrannosaurines were more heavily built and larger. The alioramin genera of Qianzhousaurus and Alioramus, however, were the exception, as they were more comparable in built to albertosaurines and have longirostrine snouts. Like albertosaurines, tyrannosaurines also had heterodont dentition, large heads design to catch and kill their prey, and short didactyl arms. Based on the growth stages of Tyrannosaurus (and possibly Tarbosaurus), tyrannosaurines undergone ontogenetic changes from gracile or slender, semi-longirostrine immatures to robust, heavy-headed adults. This implies that these animals occupy different ecological niches as they developed. While there is fossil evidence of earlier tyrannosauroids having feathers, the evidence of such structures in tyrannosaurids is controversial as a study in 2017 from Bell and colleagues found no support in feathered integument in tyrannosaurids. The study used skin impressions which are small, found widely dispersed across the post-cranium at different regions of the body with a pattern similar to crocodiles. Further the croc analogy Thomas Carr and colleagues in 2017 by studying the snout of Daspletosaurus suggested they have large scales with sensory sensory neuron pits under the skin This notion has been challenged from other authors who suggested a more lip-covering of the teeth. Distribution The temporal range for tyrannosaurines went from almost 80.6 million years ago in the Campanian stage of the Late Cretaceous epoch to 66 million years ago in the Maastrichtian stage. Fossils have been found in different formations in what is now east Asia and western North America. While the Asian alioramins are the basal most group of the tyrannosaurines, the geographic placement of albertosaurines and other eutyrannosaurian tyrannosauroids found in North America suggests greatly the tyrannosaurines are North American in origin. Systematics Prior to the 2010s, the relationships of the tyrannosaurines was best understood as Tyrannosaurus being a sister taxon to Tarbosaurus. These two genera in turn were the