For decades, paleontologists have debated whether Nanotyrannuslancensis, a "dwarf" species of tyrannosaur that lived contemporaneously with Tyrannosaurus rex, was merely an adolescent version of the T. rex - a "Teen" rex, if you will.
But a new study in the journal Science, which draws heavily on the extensive fossil collection at the Yale Peabody Museum, finds that the diminutive Nanotyrannus is a distinct species of its own. The finding agrees with another recent study, in the journal Nature, that also suggested Nanotyrannus is a separate species.
"Whether two individuals are different species or just different growth stages of the same species is a persistent problem in dinosaur paleontology," said first author Christopher Griffin, a curatorial affiliate at the Peabody Museum, former postdoctoral researcher at Yale, and assistant professor of geosciences at Princeton University.
"This finding gives us a new way to gauge maturity in individuals lacking limb bones. We think it can be broadly applicable to similar problems throughout the dinosaurian fossil record," Griffin said.
The maturity, or full growth, of a dinosaur can be determined by looking at growth rings in limb bones, similar to counting tree rings to ascertain the age of an oak tree. But in the case of Nanotyrannus, the holotype - the specimen that defines the species - was only an isolated skull with no limbs.
So, the researchers went for the throat.
For the study, Griffin and his team determined that throat bones - called hyoids - can also be used to determine maturity. They looked at the throat bones of modern relatives of dinosaurs (birds and crocodiles), as well as fossil specimens of dinosaurs such as Allosaurus, to prove the validity of the method.
Then they turned their attention to Nanotyrannus. The analysis of the Nanotyrannus hyoid had all of the indicators of being a fully mature individual - meaning it was a distinct species.
"This is the first direct determination of maturity for the holotype specimen, the specimen that holds the name Nanotyrannus lancensis and defines the species," Griffin said. "A lot of good work has been done on other individuals identified as Nanotyrannus based on comparison with the holotype, but this definitely shows that the holotype itself represents a mature individual."
Nearly all of the analyses for the study were conducted at Yale, Griffin said, including the sampling of the holotype hyoid. The Peabody collections provided essential samples of hyoids, limb bones, and ribs of crocodylians and birds, and an Allosaurus collected by famed Yale paleontologist Othniel Charles Marsh in the 1800s.
"The Peabody collections were a crucial piece in showing that this new method was viable," Griffin said.
The new finding reopens questions about the development of Tyrannosaurus, the researchers say, because much of that information had been based on Nanotyrannus. The finding also suggests a greater diversity of carnivores - and therefore more complex ecological interactions - among the last non-bird dinosaurs of North America.
Funding for the study came from a National Science Foundation Postdoctoral Research Fellowship.
