Dale Edward Malinzak

Tyrannosaurid dinosaurs dominated as predators in the Late Cretaceous of Laurasia, culminating in the evolution of the giant Tyrannosaurus rex, both the last and largest tyrannosaurid. Where and when Tyrannosaurini (T. rex and kin) originated remains unclear. Competing hypotheses place tyrannosaurin origins in Asia, or western North America (Laramidia). We report a new tyrannosaurin, Tyrannosaurus mcraeensis, from the Campanian–Maastrichtian Hall Lake Formation of New Mexico, based on a fossil previously referred to T. rex. T. mcraeensis predates T. rex by ~ 6–7 million years, yet rivaled it in size. Phylogenetic analysis recovers T. mcraeensis as sister to T. rex and suggests Tyrannosaurini originated
in southern Laramidia. Evolution of giant tyrannosaurs in southern North America, alongside giant ceratopsians, hadrosaurs, and titanosaurs suggests large-bodied dinosaurs evolved at low latitudes in North America.

Although the fossil record of non-avian dinosaurs from the Cretaceous of Antarctica is the poorest of any continent, fossils representing at least five major taxonomic groups (Ankylosauria, early-diverging Ornithopoda, Hadrosauridae, Titanosauria, and Theropoda) have been recovered. All come from Upper Cretaceous (Coniacian–Maastrichtian) marine and nearshore deposits belonging to the Gustav and Marambio groups of the James Ross Basin at the northern tip of the Antarctic Peninsula. The majority of these finds have come from the Campanian–Maastrichtian Snow Hill Island and Lopez de Bertodano formations of James Ross and Vega islands. Given the rarity of Antarctic Cretaceous non-avian dinosaurs, discoveries of any fossils of these archosaurs, no matter how meager, are of significance. Here we describe fragmentary new ornithischian (ankylosaur and ornithopod) material from the upper Campanian–lower Maastrichtian Cape Lamb Member of the Snow Hill Island Formation and the Maastrichtian S...