Study aims to bring a smaller Tyrannosaurus back from obscurity
It’s only 2 feet long, but a tyrannosaurus skull has been a topic of serious debate among paleontologists for decades.
In 1988, a team of researchers named it Nanotyrannus lancensis, suggesting that it represented a separate 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 was gaining ground.
“Most people believed in it, including myself,” says Nick Longrich, a paleontologist at 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 his colleagues argue that sufficient evidence exists to resurrect Nanotyrannus as its own genus among the larger Tyrannosaurus family. Based on anatomical features, they argue that it is not even particularly closely related to T. rex.
Other experts say the investigation is unlikely to end the debate.
“It looks a bit like Schrödinger’s dinosaur,” says Thomas Holtz, a paleontologist at the University of Maryland who was not involved in the paper. “This article will keep people talking about it, but it won’t actually solve the problem.”
To substantiate this, Dr. Longrich the original 2-foot skull and more recent finds named Jane and Petey, as well a long-disputed tyrannosaurus specimen, the ‘dueling dinosaurs’. It is claimed that all of these represent the adolescent T. rex, said Dr. Longrich. But his team said it found about 150 differences in their anatomy, including skull details; an elongated, leaf-like snout; and longer arms and claws than adult T. rex.
He also said the specimens had characteristics consistent with adults, not adolescents. The growth rings in the bone of three specimens – including Jane and Petey – also indicate a slowing growth rate. The animals were on track to weigh more than a ton, ahead of the T. rex, which weighed four to five tons, the researchers estimated.
“We have three individuals, which basically rules out an individual variation or an aberrant growth pattern,” said Dr. Longrich. “What we’re seeing is that the growth patterns are not consistent with these animals being juveniles.”
So where are the actual juvenile T. rex? Dr. Longrich believes he has found a fragment of one specimen: a piece of skull from the University of California, Berkeley collections described in the paper. “In every feature film it was T.rex,” he said.
Other paleontologists are unwilling to dismiss the teenage T. rex hypothesis, and they have raised strong objections to the paper.
The specimens in question show features in common with adult T. rex — including the forehead, snout and braincase, said Thomas Carr, a paleontologist at Carthage College who first argued that Nanotyrannus represented the young T. rex. Furthermore, he disagrees with the claim that they do not fit the growth pattern of other tyrannosaurus skulls.
“In T. rex and tyrannosaurs in general, the differences between juveniles and adults are quite extreme and humans are easily discarded,” said Dr. Carr.
Holly Woodward, a paleontologist at Oklahoma State University who produced some of the growth data used by Dr. Longrich also disputed their conclusions. The distance between the inner growth rings in the bone tissue of a nearly mature adult T. rex indicates “lower growth rates at a younger age before the big growth spurt,” she said.
Dr. Woodward added that the team’s choice of mathematical models risks creating a distorted picture that shows younger animals having finished growing even when they haven’t.
“I’m just not convinced that the growth curve arguments support this hypothesis,” she said.
Dr. Longrich responded that teenage T. rex proponents haven’t proven their case either: “I’d throw it back in their camp and say, ‘Where’s the evidence for your hypothesis?'”
He explained that “if Nanotyrannus wants to turn into T. rex, it will require an extraordinary number of transformations.” No other dinosaur evolves like this, argues Dr. Longrich: Everything his team has studied fits neatly into the Nanotyrannus form or that of T. rex.
Credible paleontologists have historically argued both sides of the issue, Dr. Holtz said. Part of the problem is that most T. rex specimens are adults, with only a few subadults. Everyone recognizes that gap; they simply disagree about its meaning.
The discovery of an older adult Nanotyrannus or a young T. rex that differs from the Nanotyrannus form could clarify matters, said Dr. Holtz. That also applies to the upcoming data about Jane and the tyrannosaurus of the ‘dueling dinosaurs’. While the team’s article offers interesting suggestions, Dr. Holtz that it is not enough for him to reject the hypothesis “that these are young T.rex.”
The argument continues. For an animal that may or may not have existed, Nanotyrannus proves strangely difficult to kill.