SCIENCE LAB SPECIAL : NEW T. REX DISCOVERY.

The Rex Family Tree : ' Not the tyrant, but a fearsome forebear. ' : Researchers have announced the discovery of a new species of Tyrannosaurus from New Mexico, one that appeared in the fossil record five million years before the familiar tyrant lizard.

Their research, published in Scientific Reports, suggests a new chapter could be added to the origin story of Tyrannosaurus rex.

When staff members from the New Mexico Museum of Natural History and Science collected the partial skull of a large adult tyrannosaurus from the Elephant Butte Reservoir in the 1980s, they assumed the fossils belonged to T.rex.

But when the paleontologist Sebastian Dalman began working on the specimen in 2013, he noticed subtle differences.

The lower jaw of the specimen was more slender than the deep bone-crushing jaws of an adult T-rex. Its teeth were different, and it lacked the prominent ridge of bone found behind T.rex's eye, Mr. Dalman said.

The animal was about 39 feet long [ 12 meters ] about the same length as an adult T rex.

The team suggests the genus may have evolved to its giant size to prey on the enormous herbivores found in the region, like Alamosaurus, a sauropod up to 100 feet long, and Sierra Ceratops, an early relative of Triceratops.

T. rex fossils are believed to be 66 million to 68 million years old, the period recorded in the Hell Creek Formation of the Plains states, said Spencer Lucas, paleontologist curator of the museum and an author on the paper.

When the Elephant Butte fossil was discovered, researchers initially assumed the rock layers that produced it - the McRae Formation of New Mexico - belonged to the same period.

But the team's dating of the rocks now suggests that the McRae Formation was five million to seven million years older than Hell Creek and that the specimen was an earlier relative, which they named Tyrannosaurs mcraeensis after the formation where it was found. [ Asher Elbein ]