A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism generating species diversity amongst dinosaurs, and perhaps beyond. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis. Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. However, given their close phylogenetic relationship, geographic proximity, and temporal succession, where D. torosus (~76.7–75.2 Ma) precedes the younger new species (~75.1–74.4 Ma), we argue that the two forms most likely represent a single anagenetic lineage. Daspletosaurus was an important apex predator in the late Campanian dinosaur faunas of Laramidia; its absence from later units indicates it was extinct before Tyrannosaurus rex dispersed into Laramidia from Asia. In addition to its evolutionary implications, the texture of the facial bones of the new taxon, and other derived tyrannosauroids, indicates a scaly integument with high tactile sensitivity. Most significantly, the lower jaw shows evidence for neurovasculature that is also seen in birds.
Tyrannosaurinae is a diverse clade of advanced theropods from the Late Cretaceous of Asia and the American West that culminated in the giant T. rex at the end of the Mesozoic1,2,3,4. The clade has a high diversity in body size and skull shape, including medium-sized taxa with long and low skulls5, large taxa with short snouts2, and a dwarf species from the Arctic Circle6. Tyrannosaurines are geographically widespread; fossils have been collected in China and Mongolia, and throughout western North America, from Alaska to Texas. The evolutionary success of Tyrannosaurinae is also reflected in its 14 million-year duration, which includes the geologically oldest species of tyrannosaurid4.