By Will Dunham

WASHINGTON, Dec 4 (Reuters) - Paleontologists for decades debated whether meat-eating dinosaur Nanotyrannus was actually just a juvenile Tyrannosaurus. But within a span of five weeks, the matter seems to have been definitively resolved by two new studies showing that Nanotyrannus was clearly distinct from T. rex.

The latest study, published on ​Thursday, focused on a throat bone called the hyoid from the first Nanotyrannus fossil ever discovered, a skull unearthed in Montana in 1942. Researchers detected a record ‌of growth in the hyoid - akin to a tree's annual growth rings - showing that this individual was around 15-18 years old, so either fully grown or nearly so.

This finding follows a study published on October 30 by different ‌researchers that used other bones to establish a growth record and identified anatomical differences in fossils of Nanotyrannus and Tyrannosaurus, also contradicting the notion they were the same dinosaur.

The hyoid was part of a skull fossil that was the "holotype" specimen of Nanotyrannus, stored at the Cleveland Museum of Natural History.

"The key finding is that the holotype specimen of Nanotyrannus lancensis, the fossil that formally defines the species, represents a mature individual, and therefore cannot belong to an immature Tyrannosaurus rex, as had often been interpreted in the past. Instead, it shows that Nanotyrannus is a distinct species of carnivorous ⁠dinosaur that lived alongside T. rex," said Princeton University paleontologist Christopher ‌Griffin, lead author of the research published in the journal Science.

Paleontologists have been trying to figure out the identity of this dinosaur since the skull was discovered eight decades ago. The controversy really flared when it was given its own name in 1988 as separate from Tyrannosaurus.

"The fact that ‍these two studies came out so closely together really feels like a one-two punch for Nanotyrannus," Cleveland Museum of Natural History paleontologist and study senior author Caitlin Colleary said.

The studies show two big predators prowled western North America at the twilight of the age of dinosaurs during the Cretaceous period, which ended 66 million years ago when an asteroid struck Earth. Tyrannosaurus is the most massive meat-eating dinosaur known. ​Nanotyrannus, somewhere around a tenth T. rex's mass, was nonetheless a vicious and agile hunter.

Perhaps the largest Tyrannosaurus specimen is the one nicknamed Sue at the Field Museum in Chicago, ‌measuring 40-1/2-foot-long (12.3 meters). Nanotyrannus was around 5 meters (16.4 feet) in length.

"It probably weighed roughly 700 kg (1,543 pounds), so about the size of the largest predator on land today, the polar bear," Griffin said.

Tyrannosaurus and Nanotyrannus are closely related as members of the meat-eating dinosaur group called tyrannosaurs, but had differences beyond merely size.

Nanotyrannus was built for speed and agility, with long legs, a long snout with blade-like teeth, and strong arms for manipulating prey. Tyrannosaurus was built for strength, with stocky legs, an enormous head with a relatively shorter snout, thick, banana-shaped teeth, and puny arms.

The study published in October showed Nanotyrannus had 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 two-fingered Tyrannosaurus.

The new study was the first to use the hyoid ​to examine dinosaur growth. This is typically done with limb bones, but no limb bones were associated with the ​holotype specimen.

"The hyoid is a really cool little bone because it's the only bone in a vertebrate's body that is not articulated (connected) to another bone. It's in the throat and supports the tongue, and it's attached to muscles for swallowing and breathing," Colleary said.

In humans, the hyoid is horseshoe-shaped. In ‍Nanotyrannus, it is tubular.

The researchers also examined the ⁠hyoid from a "growth series" of Tyrannosaurus fossils at the Natural History Museum of Los Angeles County, including the large subadult nicknamed Thomas and a teenage specimen.

Confirmation that two formidable predators coexisted is forcing a reevaluation of this ecosystem.

"It had been suggested that immature T. rex filled the role of small- to medium-sized predators, which had been more ⁠common earlier in the age of dinosaurs. However, our findings suggest that juvenile Tyrannosaurus had to compete with fully grown animals like Nanotyrannus," Natural History Museum of Los Angeles County paleontologist and study co-author Zachary Morris said.

"Future ‌studies may help us better understand how such animals shared this ecosystem," Morris said. "Most crucially for smaller animals, there were multiple terrifying predators to avoid all ‌at once - truly a challenging time to be alive."

(Reporting by Will Dunham, Editing by Rosalba O'Brien)