A new species of tyrannosaur from southern North America that may the closest known relative of Tyrannosaurus rex is described in a study published in Scientific Reports.
Sebastian Dalman and colleagues identified the new species — which they have named Tyrannosaurus mcraeensis — by examining a fossilised partial skull, which was previously discovered in the Hall Lake Formation, New Mexico, USA. Although these remains were initially assigned to T. rex and are comparable in size to those of T. rex (which was up to 12 metres long), the authors propose that they belong to a new species due to the presence of multiple subtle differences in the shape of, and joins between, the skull bones of the specimen and T. rex. Based on the locations of the remains in relation to rocks and other dinosaur remains that have been previously dated to between 66 and 75 million years ago, the authors suggest that T. mcraeensis may have lived between 71 and 73 million years ago — between five and seven million years before T. rex.
Analysis of the relationships between T. mcraeensis and other theropod dinosaur species indicates that T. mcraeensis may have been a sister species to T. rex, making it the closest known relative of T. rex. Based its discovery in New Mexico and its relationship to T. rex, the authors suggest that the Tyranosaurus lineage, Tyrannosaurini, may have originated in southern Laramidia — an island continent that existed between 100 and 66 million years ago and stretched from modern-day Alaska to Mexico. Additionally, they propose that Tyrannosaurini may have evolved a giant body size by approximately 72 million years ago, alongside other giant dinosaurs from southern Laramidia such as ceratopsians, hadrosaurs, and titanosaurs.
The authors speculate that the evolution of giant tyrannosaurins may have been driven by the giant body sizes of herbivores that they preyed on in southern Laramidia.
