Newly Discovered Tyrant Dinosaur Stalked Ancient New Mexico

In life, McDonald and colleagues hypothesize, Dynamoterror would have been about 30 feet long. Far larger than the earliest tyrannosaurs, though not quite as big as the celebrity T. rex, Dynamoterror is comparable in size to a few other tyrannosaurs of similar age—large enough to earn top predator status in its ancient realm.

Back in this tyrannosaur’s heyday, McDonald says, “the Menefee would have been much like the swamps and forest of the southeastern U.S.—hot, humid, and lush.” Shovel-beaked hadrosaurs, armored dinosaurs, and horned dinosaurs were some of the neighbors Dynamoterror rubbed shoulders with and likely preyed upon.

What makes Dynamoterror stand out, however, is that it’s another piece in an emerging picture of dinosaur evolution running riot between 80 and 75 million years ago. Back in the Late Cretaceous, North America was split in two by the Western Interior Seaway, a warm stretch of water that washed over the middle of the continent, with the western half known to experts as Laramidia. From the stony records of this subcontinent, paleontologists have been finding a slew of unexpected dinosaurs.

Historic finds in the northern parts of Laramidia, such as modernday Alberta and Montana, revealed rich communities of dinosaurs such as tyrannosaurs, horned dinosaurs, armored dinosaurs and more. Fossils found in southern rocks of the same age were often given the same names as the northern species. But in the past three decades, paleontologists have started to put together a very different picture. New discoveries and fossil revisions have shown that the dinosaurs found in Utah, New Mexico, Texas and Mexico were not the same as those found in the north. If you were to walk from Mexico to Alaska 80 million years ago, you’d find a gradient of different dinosaurs as you hiked along.

Dynamoterror is part of this story, and an important one as it’s from an area with few known fossils. No dinosaurs had been named from the Menefee Formation until earlier this year, when an armored dinosaur called Invictarx was identified. Dynamoterror is now the second, and the fact that it differs from other known tyrannosaurs of a similar age indicates that there were distinct evolutionary pockets along the length of the ancient subcontinent.

The new tyrannosaur also points to what may yet be found. Both Dynamoterror and Lythronax are from southern North America and are about 80 million years old. There seems to be a bias against the preservation of dinosaurs in rocks of this age, Williamson says, but the few and often scrappy fossils that have turned up have indicated that dinosaur diversity was just as rich as it was in the 75-million-year-old rocks where preservation is better.The search is taxing, but it means there are more dinosaurs to dig up.

Some of them will likely be tyrannosaurs. To the north, McDonald says, “roughly contemporaneous rocks have yet to produce diagnostic tyrannosaurid material.” It could very well be that there were other unusual tyrant lizards in northern Laramidia, now entombed in the rocks, waiting to be uncovered and help fill in the picture of how these tyrants came to rule North America.

*Editor’s Note, October 9, 2018: A previous version of this article incorrectly stated that Douglas Wolfe was the 2012 expedition volunteer who found the Dynamoterror bones, when, in fact Douglas Wolfe co-led the 2012 expedition, and volunteer Eric Gutierrez found the fossil. The story has been edited to correct that fact.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.