A recently published scientific paper lends weight to the theory that Nanotyrannus is a valid taxon. Writing in the journal “Science” a team of scientists, including researchers from the Cleveland Museum of Natural History used a novel approach to confirm that a skull does indeed represent skeletally mature adult. Their evidence confirms that the fossil specimen, known as the “Cleveland skull” does not represent a juvenile Tyrannosaurus rex. An often-overlooked bone helped make the breakthrough. The Nanotyrannus hyoid bone provided the vital information.
Whether Nanotyrannus lancensis represents a distinct taxon or an immature T. rex is a decades-long controversy. This new study comes shortly after a paper that concluded that Nanotyrannus was a valid taxon. Indeed, this study claimed that there were two species of Nanotyrannus in the known fossil record.
To read Everything Dinosaur’s blog post about this earlier study: Nanotyrannus – A New Chapter in Tyrannosaur Evolution.
The “Cleveland skull” – Nanotyrannus lancensis fossil material. Picture credit: Cleveland Museum of Natural History.
Picture credit: Cleveland Museum of Natural History
The Cleveland Skull
The Cleveland skull lies at the heart of this long-running controversy. Discovered in 1942, the specimen consists almost entirely of a skull, with no associated postcranial bones. As a result, palaeontologists found it difficult to determine whether these fossils represented a juvenile tyrannosaur or an adult animal. If the fossil skull represented an adult, then this would indicate a new tyrannosaur genus present in Hell Creek. Hence, the erection of a new tyrannosaur species (Gorgosaurus lancensis) in a paper published posthumously in 1946 by the American palaeontologist Charles Whitney Gilmore.
Dr Caitlin Colleary (Department of Earth Sciences, Cleveland Museum of Natural History), examining the Nanotyrannus skull material. Picture credit: Cleveland Museum of Natural History.
Picture credit: Cleveland Museum of Natural History
The Nanotyrannus Hyoid Bone
One small bone proved vital when trying to assess the skull material. The Nanotyrannus hyoid bone had detached from the skull and was stored separately in the museum collection. This slender throat bone had gone largely unnoticed until a visiting researcher recognised its potential importance. The hyoid supports the tongue and airway in living animals. Crucially, it also records growth in a measurable way. The researchers were able to demonstrate that the histology preserved in this bone was useful for assessing the ontogeny of the fossils. The research team therefore applied histological analysis to the hyoid, an approach never used on this bone before.
Before examining the Cleveland specimen, the scientists validated their method. They studied hyoids from birds and crocodilians, which represent living dinosaur relatives. They then compared the Nanotyrannus hyoid bone with those from theropod dinosaurs including Tyrannosaurus rex, Allosaurus, and Coelophysis.
Thin sections viewed under a microscope revealed clear growth patterns. Most importantly, the hyoid contained an external fundamental system. This structure forms when growth has ceased. The team demonstrated that hyoid bone histology is useful for ontogenetic assessment in extant and extinct archosaur.
Senior author of the paper, Dr Caitlin Colleary (Department of Earth Sciences, Cleveland Museum of Natural History), explained:
“By applying a new technique to a specimen that’s been in the collection for nearly 80 years, we unlocked information that’s been there all along.”
The Cleveland Skull Material Represents a Fully Grown Animal
The presence of this feature confirms that the Cleveland skull fossils belonged to a fully grown individual. It did not represent a juvenile Tyrannosaurus rex. Instead, it supports the interpretation of Nanotyrannus lancensis as a distinct taxon coeval with T. rex.
A Nanotyrannus (N. lancensis) left, competes for food with a juvenile Tyrannosaurus rex, whilst an adult T. rex watches in the background. Picture credit: Andrey Atuchin.
Picture credit: Andrey Atuchin
Nanotyrannus Hyoid Bone Study – The Implications
This study has significant implications for tyrannosaur research. The holotype defines the species. Demonstrating adult status strengthens the case for Nanotyrannus beyond reasonable doubt. The study also highlights the enduring value of museum collections. A single overlooked bone provided the decisive evidence. Sometimes, the most important discoveries wait quietly in storage.
For the moment, the debate has shifted decisively. The Nanotyrannus hyoid bone offers direct proof of skeletal maturity. As a result, this small tyrannosaur finally steps out from the shadow of Tyrannosaurus rex. This study and the earlier Nanotyrannus paper, indicate that the Hell Creek ecosystem was home to a wide assemblage of predatory dinosaurs.
Everything Dinosaur acknowledges the assistance of a media release from the Cleveland Museum of Natural History in the compilation of this article.
The scientific paper: “A diminutive tyrannosaur lived alongside Tyrannosaurus rex” by Christopher T. Griffin, Jeb Bugos, Ashley W. Poust, Zachary S. Morris, Riley S. Sombathy, Michael D. D’Emic, Patrick M. O’Connor, Holger Petermann, Matteo Fabbri and Caitlin Colleary published in the journal Science.
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