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The skull Gilmore described as "Gorgosaurus lancensis." From Gilmore, 1946.

The name “Nanotyrannus” is a polarizing one. Depending on who you ask, the remains attributed to the controversial dinosaur represent a small-bodied tyrannosaur distinct from Tyrannosaurus, the juvenile form of a previously unknown tyrannosaur genus, or the long-sought bones of young Tyrannosaurus. Even before the debate about dinosaur growth stages blew up last year with the suggestion that Torosaurus is a mature Triceratops, paleontologists were arguing over what, exactly, “Nanotyrannus” was.

I was reminded of the ongoing debate during the annual Society of Vertebrate Paleontology meeting last week when I happened upon a thin monograph tucked within a stack of old reprints. The 1946 paper was by the Smithsonian National Museum of Natural History paleontologist Charles Gilmore and was titled “A New Carnivorous Dinosaur From the Lance Formation of Montana.” I should have recognized the paper immediately, but I only realized what I was reading when I flipped to the illustrations at the back and saw the skull that would later carry the name “Nanotyrannus.”

Gilmore’s monograph didn’t mess around. After a quick note explaining that he doubted the validity of the dinosaur “Deinodon” because it was based on indistinguishable teeth, Gilmore jumped right into a description of a small tyrannosaur skull that had been found in the latest Cretaceous strata of Montana. The fossil was beaten up—a few bones were missing from the right side, and many of the teeth were damaged—but overall, the specimen was one of the most complete tyrannosaur skulls then yet found. He called the dinosaur Gorgosaurus lancensis, basing this decision on the large, rounded eye openings, the long and shallow jaws, and the small size of the specimen. The last plate in the monograph demonstrated how different the new dinosaur was. Compared to the skulls of an adult and juvenile Gorgosaurus, the new skull lacked the little ornamental flange of bone above the eye, and the profile of the snout had a slightly deeper and more rounded profile compared to the other small Gorgosaurus skull.

Gilmore also took the opportunity to clean house a little. As many as five different tyrannosaur species, called “deinodonts” at the time, had been named from the latest Cretaceous of North America. In addition to the species he just named, Gilmore found only one species, Tyrannosaurus rex, to be valid. Everything else had been named from teeth, skeletons without heads, or otherwise was difficult to diagnose. Gilmore concluded: “This brief review of the large Upper Cretaceous carnivorous Dinosauria focuses attention on the very unsatisfactory state of our knowledge concerning the nomenclatural status of many of the included forms.” Funny that Gilmore should say that—years later, his “Gorgosaurus lancensis” would play a role in the debate of just how many species of tyrannosaurs were stalking Late Cretaceous Montana.

Four decades after Gilmore’s initial description, the little tyrannosaur skull was re-cast as a different sort of predator. In 1988 paleontologists Robert Bakker, Phil Currie and Michael Williams hypothesized that the skull actually belonged to a unique genus of small tyrannosaur which shared the environment preserved in the Lance and Hell Creek formations with Tyrannosaurus. The primary line of evidence was the fusion of the skull bones. As animals age, the various bones that make up their skulls fuse along sutures, and the degree to which the bones have fused can sometimes be used to roughly determine age. Since all the skull bones in the Gilmore skull appeared to be fused, Bakker and colleagues stated, the tyrannosaur must have been a small adult and therefore distinct from the bigger, bulkier Tyrannosaurus rex. Appropriately, they called the hypothesized animal Nanotyrannus.

Here’s where things get tricky, though. The timing of when sutures between skull bones fuse in dinosaurs varies among individuals and may not be a good indicator of growth stage. And in a 1999 study of growth changes in tyrannosaurid skulls, paleontologist Thomas Carr found that none of the bone fusions claimed by Gilmore or Bakker and colleagues were actually visible. That, in addition to typical characteristics of immature animals such as large, round orbits and the texture of the bone, identified the skull as a juvenile tyrannosaurid, most likely a young Tyrannosaurus rex. This wasn’t the only time young tyrannosaurs have led researchers astray. In 2004, Carr and Thomas Williamson sunk three proposed tyrannosaurs—Aublysodon mirandus, Stygivenator molnari, Dinotyrannus megagracilis—as young Tyrannosaurus rex specimens, and more recently Denver Fowler and colleagues proposed that the “tiny tyrant” Raptorex was probably a juvenile Tarbosaurus bataar. Given that tyrannosaurids were so variable and underwent such dramatic changes from small, gracile juveniles into bulky, deep-skulled adults, it is little wonder that the over-splitting that gave Gilmore a headache remains with us.

Nevertheless, hints and rumors abound that “Nanotyrannus” may make a comeback. Aside from rumors of yet-unpublished specimens, last year Larry Witmer and Ryan Ridgely published a new analysis of the skull Gilmore had found, often called the “Cleveland skull” since it is now kept at the Cleveland Museum of Natural History. Their results were inconclusive—pending the study and publication of other tyrannosaur specimens that will provide a greater context by which to compare the Cleveland skull—but they noted that the skull might have some unique  features which could be used to argue that it was different from Tyrannosaurus rex.

The Cleveland skull and other supposed “Nanotyrannus” specimens will undoubtedly remain in contention for some time. The features already examined and cited by Carr indicate that the specimen was probably not fully mature, and the best-supported hypothesis so far is that this animal—much like the specimen known as “Jane“—was a young Tyrannosaurus rex. Still, there remains the possibility that someone is going to describe the skeleton of a larger, more mature tyrannosaurid from the latest Cretaceous that significantly diverges in anatomy from Tyrannosaurus rex. That seems like a long shot, but we will have to wait for the description of many mysterious specimens to find out.

References:

Carr, T. (1999). Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria) Journal of Vertebrate Paleontology, 19 (3), 497-520

CARR, T.; WILLIAMSON, T. (2004). Diversity of late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America Zoological Journal of the Linnean Society, 142 (4), 479-523 DOI: 10.1111/j.1096-3642.2004.00130.x

Gilmore, C. 1946. A new carnivorous dinosaur from the Lance Formation of Montana.” Smithsonian Miscellaneous Collections, 106: 1–19.

Witmer, L.; Ridgely, R. (2010). THE CLEVELAND TYRANNOSAUR SKULL (NANOTYRANNUS OR TYRANNOSAURUS): NEW FINDINGS BASED ON CT SCANNING, WITH SPECIAL REFERENCE TO THE BRAINCASE Kirtlandia, 57, 61-81