New study confirms Nanotyrannus holotype was distinct species from T. rex

New study confirms Nanotyrannus holotype was diminutive, distinct species from T. rex

New research from a team including the University of Nebraska State Museum’s Ashley Poust further moves a decades-long scientific debate toward a conclusion — the Nanotyrannus, a teacup variation of the T. rex, did exist.

Poust and the team, led by Christopher Griffin of Princeton University, used a novel approach to determine maturity of the holotype of Nanotyrannus, a fossil skull at the Cleveland Museum of Natural History. They examined the ceratobranchial, or hyoid bone, which was intact with the skull, and determined it was an adult when it died, rather than a juvenile Tyrannosaurus rex. Using bone histology, or the study of fossilized bone microstructure, the hyoid — a small bone in the throat — revealed a record of development. The findings were published Dec. 4 in Science.

“This small-bodied — in relation to the T. rex — meat-eater’s hyoid bone showed growth patterns that suggest maturity or approaching maturity,” said Poust, Voorhies Endowed Curator of Vertebrate Paleontology. “This lets us be confident in keeping the name Nanotyrannus, because this animal is clearly not on a growth path to becoming a Tyrannosaurus rex.”

It is estimated that the Nanotyrannus stood 18 feet tall, diminutive compared to a length of more than 40 feet for the T. rex.

Discovered in 1942, the holotype Nanotyrannus skull was originally classified as Gorgosaurus but was reidentified as Nanotyrannus lancensis in 1988 after further scientific work. The latest findings further validate its classification as its own species, and were somewhat surprising to the researchers.

“At the time, the prevailing consensus was that the Nanotyrannus holotype skull represented an immature Tyrannosaurus rex, and was not a separate species,” said Griffin, assistant professor of geosciences at Princeton. “Our expectations were simply following along with that consensus, but once we sampled the hyoid and saw features that strongly indicated maturity, we knew that we had to examine that idea more skeptically. 

To confirm their findings, Poust leaned on his expertise in bone histology to examine hyoids from additional species, including modern-day relatives of dinosaurs (ostriches, alligators and lizards) and additional dinosaur fossils. The methodology, previously untested on dinosaur fossils, may open new doors of inquiry, Poust said, especially for more limited specimens.

“It's expanding, in a small way, the ability to learn about animals’ past lives,” Poust said. “It was exciting to show that the growth signal is so conserved across the body. Maybe this is a tiny wedge to start investigating that in some different ways.”

Poust was surprised that the hyoid could be a reliable way to determine a fossil’s approximate age at death, adding that it seems to be as useful as a rib or femur bone, which are more traditionally used for fossil research.

The scientific debate regarding whether Nanotyrannus existed is likely concluded, as these findings quickly followed a recent paper in Nature that also examined a suspected Nanotyrannus fossil found in Montana, now kept at the North Carolina Museum of Natural Sciences.

“We investigated the holotype specimen because it is the one fossil that formally defines the species — any other specimens that are called Nanotyrannus lancensis are being referred to this one specimen, which holds the species name,” Griffin said. “Since this specimen is mature, this definitively shows that Nanotyrannus is distinct from Tyrannosaurus.”

The conclusion of one debate, though, raises new questions about predator diversity and ecosystem dynamics.

“You're left with at least two different sized meat eaters in the same environment, which has some big implications for ecology and the extinction of dinosaurs,” Poust said. “Knowing more about what existed gives us a sense of how big the fossil record is and how species change through time. And understanding the complexities of an ecosystem is important.”