Study aims to bring tinier Tyrannosaur back from oblivion

NEW YORK: It is only 23 inches long, but one tyrannosaur skull has been a bone of serious contention among paleontologists for decades. In 1988, a team of researchers named it Nanotyrannus lancensis, suggesting that it represented a distinct animal that lived in the shadow of Tyrannosaurus rex. In 1999, another group argued that the skull and similar specimens were T. rex as a teenager, before the species underwent an extraordinary growth spurt that preceded adulthood.

For years, the teen T. rex hypothesis gained traction. “Most people bought into it, including me,” said Nick Longrich, a paleontologist with the University of Bath in England. But Dr. Longrich has changed his tune. In a study published Wednesday in the journal Fossil Studies, he and colleagues argue that enough evidence exists to resurrect Nanotyrannus as its own species among the larger Tyrannosaur family. Based on anatomical features, they argue, it isn’t even particularly closely related to T. rex. Other experts say the study is unlikely to end the debate.

“It’s sort of like Schrodinger’s dinosaur,” said Thomas Holtz, a paleontologist at the University of Maryland who was not involved with the paper. “This paper’s going to keep people talking about it, but it’s not going to really resolve it.” To make its case, Dr. Longrich’s team studied the original 23-inch skull and more recent finds named Jane and Petey, as well as a long-disputed tyrannosaur specimen, the “dueling dinosaurs.” All of these have been argued to represent adolescent T. rex, Dr. Longrich said. But his team said it found around 150 differences in their anatomy, including skull details; an extended, bladelike snout; and longer arms and claws than adult T. rex.

He also said the specimens had features consistent with mature animals, not adolescents. The growth rings inside the bone from three specimens — including Jane and Petey — likewise suggest slowing growth rates. The animals were on track to weigh over a ton, rather than the T. rex, which was four to five tons, the researchers estimated.

“We have three individuals, which basically rules out an individual variation or aberrant growth pattern,” Dr. Longrich said. “What we’re seeing is that the growth patterns are inconsistent with these animals being juveniles.”

Where, then, are the actual juvenile T. rex? Dr. Longrich believes he’s found a fragment of one — a piece of skull from the University of California, Berkeley, collections described in the paper. “In every single feature it was T.rex,” he said.

Other paleontologists are not ready to throw out the teenage T. rex hypothesis, and they raised strong objections to the paper. The specimens in question do show features in common with adult T. rex — among them the forehead, snout and braincase, said Thomas Carr, a paleontologist at Carthage College who first made the case that Nanotyrannus represented young T. rex. Moreover, he disagrees with the claim that they don’t fit the growth pattern in other tyrannosaur skulls.

“With T. rex and tyrannosaurs in general, differences between juveniles and adults are quite extreme and people are easily thrown,” Dr. Carr said.