Agile and vicious Nanotyrannus was not just a teenage T. rex

By Will Dunham

WASHINGTON (Reuters) -At the twilight of the age of dinosaurs, an agile and vicious predator named Nanotyrannus prowled western North America, resembling a smaller version of Tyrannosaurus - about a tenth the body mass - but with several key anatomical differences.

That is the finding of new research concluding that Nanotyrannus - which for years has been at the center of a debate among paleontologists as to whether it was merely a teenage version of Tyrannosaurus - was, in fact, its own distinct dinosaur.

The researchers examined fossil specimens of Nanotyrannus unearthed in 1942, 2001 and 2006 in Montana dating to about 67 million years ago, determining that the individuals were mature and not juveniles based on traits in the bones including annual growth rings.

They also found that Nanotyrannus differed anatomically from its larger cousin by having more teeth, a crest in front of its eyes, an air sinus in a certain bone at the back of the skull and the presence of a vestigial third finger, unlike the two-fingered Tyrannosaurus.

Nanotyrannus and Tyrannosaurus both were members of a lineage of meat-eating dinosaurs called tyrannosaurs, but were not the same genus, the researchers said. A genus is a group of closely related species that share similar characteristics. For example, lions, tigers, leopards and jaguars are from the same genus, Panthera, but each represents a different species.

The genus name Tyrannosaurus means "tyrant lizard," with the species name "rex" meaning "king." The genus name Nanotyrannus means "dwarf tyrant."

"T. rex was a massive predator adapted to wielding incredible bite forces. Nanotyrannus was a slender, agile pursuit predator that could have run circles around the tyrant king," said paleontologist Lindsay Zanno of North Carolina State University and the North Carolina Museum of Natural Sciences, lead author of the study published on Thursday in the journal Nature.

The new analysis ends the debate around Nanotyrannus, Zanno said.

"It is not biologically possible to conclude that it is a juvenile T. rex," Zanno said.

Their size difference was stark. Nanotyrannus had about 10% the body mass - about 1,500 pounds (700 kg) versus 15,000 pounds (7,000 kg). And it had about half the linear dimensions - approximately 18 feet (5.5 meters) versus roughly 40 feet (12.2 meters) long, and 7 feet (2 meters) versus 13 feet tall.

"Nanotyrannus and T. rex are extremely different," said paleontologist and anatomist James Napoli of Stony Brook University in New York, co-author of the study.

Nanotyrannus was a predator built for speed and agility, with long legs, a long snout with blade-like teeth and strong arms for manipulating prey. Tyrannosaurus was a colossal predator that was built for strength, with stocky legs, an enormous head, thick, banana-shaped teeth and greatly reduced arms.

"I suspect that these two species would have occasionally come into conflict, as predators tend to do, but the long legs of Nanotyrannus, and its small size, suggest that it mostly hunted smaller, faster prey than Tyrannosaurus," Napoli said.

The researchers also determined that the Nanotyrannus specimens had sufficient anatomical differences to be divided into two separate species - Nanotyrannus lancensis and the newly named Nanotyrannus lethaeus. One of the Nanotyrannus fossils was part of the well-known "Dueling Dinosaurs" specimen, battling the horned dinosaur Triceratops at the time of death.

Some scientific research into how Tyrannosaurus matured was based on the notion that the Nanotyrannus fossils represented T. rex juveniles, including studies that hypothesized that this apex predator experienced an extreme growth rate.

"For decades, paleontologists have unknowingly used Nanotyrannus specimens as a model for teenage T. rex to understand the biology of Earth's most famous dinosaur. Those studies need a second look," Zanno said.

The finding that Nanotyrannus and Tyrannosaurus shared the same landscape, along with myriad plant-eating dinosaurs, offers the latest evidence that dinosaur diversity was rich prior to the asteroid impact 66 million years ago that ended the age of dinosaurs.

"This discovery shows us that dinosaurs continued to evolve, innovate and diversify right up until their reign was cut short. This fits into a growing body of evidence proving that dinosaurs were not declining for millions of years before they went extinct, contrary to what we once thought," Napoli said.

(Reporting by Will Dunham; Editing by Daniel Wallis)