New Study Strengthens Case That Nanotyrannus Was a Small Adult Species, Not a Juvenile T. rex

Nanotyrannus: Growing Evidence for a Distinct Small Tyrannosaur

A new independent study published in Science adds weight to recent findings that the controversial specimen known as Nanotyrannus lancensis was a small-bodied adult tyrannosaur, not a juvenile Tyrannosaurus rex. The team focused on a tiny throat bone — the ceratobranchial of the hyoid apparatus — from the Cleveland Museum of Natural History specimen that has long divided paleontologists.

Researchers examined the bone’s microscopic structure and compared it with equivalent hyoid elements from modern animals (ostriches and alligators) and fossil taxa (including T. rex and Allosaurus). The Cleveland specimen’s bone showed extensively remodeled, compact tissue and a stacked sequence of growth lines interpreted as an external fundamental system (EFS), a histological marker commonly associated with the cessation of skeletal growth.

“When we began, most of the chatter suggested the skull was a juvenile T. rex,” says Caitlin Colleary, paleontology curator at the Cleveland Museum of Natural History and co-author of the new paper. “Finding an EFS in the ceratobranchial pushed us to rethink that assumption — the specimen appears skeletally mature.”

The skull was discovered in 1942 in Montana’s Hell Creek Formation and was originally described as Gorgosaurus lancensis. In 1988 the specimen was redescribed as Nanotyrannus lancensis, a diminutive tyrannosaurid, sparking decades of debate over whether it represented a distinct genus or a young T. rex. This year two independent papers — one in Nature (October) and the new one in Science — have each provided evidence supporting the distinct-species interpretation.

Based on comparative anatomy and size estimates, researchers conclude that an adult T. rex would have been roughly three times longer and on the order of ten times heavier than Nanotyrannus lancensis, implying significant ecological and size differences between the two predators. The authors suggest the small tyrannosaur may have lived alongside T. rex in Late Cretaceous forests and floodplains, potentially occupying a different ecological niche and perhaps even competing with juvenile T. rex for prey.

Not all experts are fully convinced. Jared Voris (University of Calgary), who was not involved in the study, cautions that the feature identified as an EFS could alternatively be several closely spaced growth rings, which might indicate continued growth rather than full skeletal maturity. The paper’s authors and other researchers acknowledge this uncertainty and call for additional histological and comparative work on other Nanotyrannus specimens, including material discussed in the October study sometimes called the “dueling dinosaurs.”

Why this matters: If Nanotyrannus is valid, it increases documented tyrannosaur diversity and informs how small and large tyrannosaurs partitioned ecosystems near the end of the Cretaceous.

Christopher Griffin (Princeton), the study’s lead author, began by validating whether throat bones in living birds and crocodylians preserve histological signals that correlate with age. The Cleveland ceratobranchial — found in a box next to the skull when Colleary became curator — provided a direct fossil test of that approach.

Colleary emphasizes collaboration and specimen access: “More people working on something makes the science so much better. To have two studies come out so closely together feels like a one-two punch for Nanotyrannus.” Future analyses of additional specimens and independent histological sampling will be essential to settling the debate.