High-precision U−Pb zircon geochronology establishes a mid-Maastrichtian age for the Hall Lake tyrannosaur of southern New Mexico

Ages of Late Cretaceous North American dinosaurs typically are inferred from radioisotopic ages that require stratigraphic correlation to fossil sites because tuffs suitable for precise geochronology are rarely located at the sites themselves. A dinosaur fossil from the Upper Cretaceous Hall Lake Formation of the McRae Group in southern New Mexico, USA, originally identified as Tyrannosaurus rex, was subsequently assigned to a new species, T. mcraeensis, on the basis of inferred morphological differences and a presumed late Campanian age, which would make it older than all extant T. rex occurrences. However, the age constraint is not robust because a dated tuff on which it was based occupies a different stratigraphic section than the fossil. We present a precise U-Pb zircon age of 69.422 ± 0.032 Ma from a newly discovered tuff directly beneath the tyrannosaur fossil site and conclude that the Hall Lake tyrannosaur, now one of the best-dated dinosaurs in the western interior province of North America, is unequivocally mid-Maastrichtian, not Campanian. We also show that a titanosaur fossil previously identified as Alamosaurus in a different section of the Hall Lake Formation was found in strata with a maximum depositional age of 69.9 ± 0.6 Ma, making it also Maastrichtian.