Paleontologists have unearthed a huge new predatory dinosaur in Grand Staircase-Escalante National Monument, southern Utah.
Artist’s impression of Lythronax argestes. Image credit: Andrey Atuchin.
This giant creature is named Lythronax argestes. The genus name, Lythronax, translates as “king of gore.” The specific name, argestes, refers to its geographic location in the American Southwest.
A fossil skeleton of Lythronax was discovered in a geologic unit known as the Wahweap Formation, abundantly exposed in Grand Staircase-Escalante National Monument.
Lythronax was about 24 feet (8 m) long and weighed around 2.5 tons. It lived on Laramidia, a landmass formed on the western coast of a shallow sea that flooded the central region of North America, during the Late Cretaceous Period between 95-70 million years ago. Laramidia hosted a vast array of unique dinosaur species and served as the crucible of evolution for iconic dinosaur groups such as the horned and duck billed dinosaurs.
Lythronax belongs to a group of carnivorous dinosaurs called tyrannosaurids, the same group as the famous Tyrannosaurus rex. Among tyrannosaurs Lythronax possesses several unique features, a short narrow snout with a wide back of the skull with forward-oriented eyes. Previously, paleontologists thought this type of wide-skulled tyrannosaurid only appeared 70 million years ago, whereas Lythronax shows it had evolved at least 10 million years earlier.
Reconstructed head of Lythronax argestes. Image credit: Gary Staab.
“The width of the back of the skull of Lythronax allowed it to see with an overlapping field of view – giving it the binocular vision – very useful for a predator and a condition we associate with T. rex,” explained Dr Mark Loewen from both Natural History Museum of Utah and the University of Utah, who is the lead author of the article published in the open-access journal PLoS ONE.
The discovery of Lythronax also indicates that tyrannosaurid dinosaurs likely evolved in isolation on Laramidia.
Paleontologists have recently determined that the dinosaurs of southern Laramidia (Utah, New Mexico, Texas and Mexico), although belonging to the same major groups, differ at the species level from those on northern Laramidia (Montana, Wyoming, the Dakotas and Canada).
“Lythronax may demonstrate that tyrannosaurs followed a pattern similar to what we see in other dinosaurs from this age, with different species living in the north and south at the same time,” said co-author Dr Joseph Sertich, also from Natural History Museum of Utah and the University of Utah.
These patterns of dinosaur distribution across Laramidia lead the scientists to ask what might have caused the divisions between the north and south, given that an enterprising dinosaur could have walked from Alaska to Mexico if given enough time.
Co-author Dr Randall Irmis from Natural History Museum of Utah explained that by analyzing the evolutionary relationships, geologic age and geographic distribution of tyrannosaurid dinosaurs, the team determined that Lythronax and other tyrannosaurids diversified between 95 to 80 million years ago, during a time when North America’s interior sea was at its widest extent.
Skeleton of Lythronax argestes. Image credit: Mark Loewen.
“The incursion of the seaway onto large parts of low-lying Laramidia would have separated small areas of land from each other, allowing different species of dinosaurs to evolve in isolation on different parts of the landmass.”
“As the seaway gradually retreated after 80 million years ago, these differences in dinosaur species may have been reinforced by climate variations, differences in food sources (different prey and plants), and other factors. This hypothesis explains why the iconic Late Cretaceous dinosaurs of western North America are so different from those of the same age on other continents.”
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Bibliographic information: Loewen MA et al. 2013. Tyrant Dinosaur Evolution Tracks the Rise and Fall of Late Cretaceous Oceans. PLoS ONE 8 (11): e79420; doi: 10.1371/journal.pone.0079420
