A new study published in Scientific Reports by Sebastian G. Dalman et al. has reclassified fossil skull fragments excavated in New Mexico in 1983 as a new species in the genus Tyrannosaurus, known as T. mcraensis. The new species is closely related to and predates the famous T. rex, yet is of a similar size to its well-known cousin and could push back the date of the evolution of giant theropod dinosaurs by several million years.
Subtle Differences, Significant Discoveries
The head of the proposed T. mcraensis closely resembles that of T. rex, with only a few differences in the width of the skull and the size of the ridges upon it. However, the scientists studying the fossil said this is enough to classify it as a new species of Tyrannosaurid theropod dinosaur, the second ever found after T. rex in 1905.
The skull was found over 40 years ago in New Mexico’s Hall Lake Formation, a layer of sandstone, shale, and mudstone dating back to the Campanian and Maastrichtian ages of the late Cretaceous period, 83.5 to 66 million years ago. In that era, the Formation was in a coastal region near the Western Interior Seaway, a large body of water that once bisected North America into the continents of Laramidia (the Formation’s location) in the west and Appalachia in the east. In addition to Tyrannosaurs, paleontologists have uncovered fossils of Triceratops, its relative Torosaurus, and the giant sauropod Alamosaurus.
T. mcraensis dates to approximately 74 to 75 million years ago, 6 to 7 million years before T. rex. It is not the oldest T. rex relative to have been discovered; however, it is the closest relative. Its size indicates that Tyrannosaurids, specifically T. rex’s group, the Tyrannosaurins, evolved their great size earlier than previously thought. The Scientific Reports study proposes that the evolution of giant Tyrannosaurs occurred as a response to the growing size of herbivorous prey dinosaurs.
The location of this early Tyrannosaurin may also help settle an ongoing debate on the geographic origin of the Tyrannosaurus genus. Some paleontologists believe that T. rex and its immediate relatives evolved in Asia and reached North America via land bridge, while others believe that they are indigenous to North America; the presence of T. mcraensis in New Mexico could point to the latter origin.
Doubt Over The Division
Some paleontologists, such as the University of Calgary’s Jared Voris, oppose the reclassification of the Hall Lake fossils, saying that the differences are too subtle to be considered a new species and can be accounted for by natural variation between specimens of T. rex. There is also some doubt about the fossils’ age, as all other Tyrannosaurus fossils are significantly younger.
If T. mcraensis does turn out to be merely a specimen of T. rex, it would not be the first time that such a misidentification has occurred. In the early 2000s, a half-complete skeleton of a young T. rex was identified as a smaller relative of its actual species and dubbed Nanotyrannus.
Image credits: Sergei Krasinski via Phys.org, Sebastian G. Dalman et al. via Scientific Reports
Edited by: Matsoarelo Makuke
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