'King of Gore' joins royal court of T. rex relatives

This new dinosaur’s name, if subservient to the true “tyrant king,” is formidable in its own right: “the king of gore.”

Paleontologists have announced the find of a new royal in T. rex’s dynastic lineage, a family tree that already includes “frightful,” “fierce” and “alarming” kings, as well as a “monstrous murderer.” The new dinosaur, called Lythronax argestes, or “the king of gore from the southwest,” ruled its ecosystem some 11 million years before T. rex’s reign.

The uncovered dinosaur, described in PLOS ONE, is at some 80 million years old the most ancient of T. rex’s closest relatives. But, in a perhaps counterintuitive point, Lythronax also appears to be T. rex’s closest known common ancestor, more related to the tyrant than is any other member of the group of T. rex relatives, called tyrannosaurids. The find suggests that the features that would go on to appear in the greatest tyrant of them all evolved much earlier than paleontologists had thought.

“We rewrote the family tree of T. rex,” says Mark Loewen, a paleontologist at the Natural History Museum of Utah and the lead author on the paper. “We’ve provided new evidence about where T. rex comes from.”

Lythronax, unearthed in 2009 in Grand Staircase-Escalante National Monument, in Utah, lived in what is known as Laramidia, the island that existed from about 95 to 70 million years ago, when elevated sea levels cut what is now North America into separate islands. The long slice of land, ranging from Alaska to Mexico, was a booming kingdom of dinosaurs – a kingdom in which Lythronax, for at least a couple million years, was the king of its own realm in the territory’s southern coastal region. 

And its reign was far from benevolent. “Lythronax, like all tyrannosaurids, was the apex predator in its world,” says Joe Sertich, a paleontologist at the Denver Museum of Nature & Science and a co-author on the paper, “hunting anything it wanted including duckbilled dinosaurs and horned dinosaurs like Diabloceratops.”

Like most T. rex relatives, Lythronax was big, weighing 2.5 tons and at 24 feet long, with a mouth full of treacherous teeth.

But Lythronax also had some unexpected features: a short, narrow snout but a wide skull, with forward-oriented eyes. These features gave it what the authors call “binocular vision,” a trait that this king shares with just two other dinosaurs in the T. rex's lineage of close relatives. Those are Tarbosaurus bataar, a 70-million-year-old T. rex cousin, and the 69-million-year-old T. rex itself.

Researchers had thought that such a face appeared in the T. rex lineage not before 70 million years ago, when Tarbosaurus, the “alarming” king, took control over its little swath of Asia. Lythronax, though, now pushes the evolution of those features back some 10 million years. This means that the tyrannosaurids had diversified into separate tyrannosaurid species as of 80 million years ago.

The researchers propose that high sea levels between about 85 and 72 million years ago might be responsible for this early diversification, splicing Laramidia into separate sectors where dinosaur species evolved in isolation from each other.

“Fluctuating sea levels during the Late Cretaceous period likely played a major role in the distribution and evolution of the group,” says Dr. Sertich, "eventually culminating in the appearance of the true tyrant king, T. rex at the end of the age of dinosaurs."

The early diversification of the tyrannosaurid group suggests that there are likely to be as of yet undiscovered other members of T. rex’s lineages, each of which once reigned over their small section of land in Laramidia, cruel fiefdoms of the prehistoric world, says Randall Irmi, a paleontologist at the Natural History Museum of Utah and a co-author on the paper.

“There's a slew of new tyrannosaurs waiting to be discovered out there,” he says. “We are just beginning to understand this 80-million-year-old ecosystem.”