Fossilized skull of the Late Cretaceous tyrannosaur Nanotyrannus, possibly a juvenile Tyrannosaurus rather than a distinct genus
Nanotyrannus lancensis (Gilmore, 1946) theropod dinosaur skull (60 cm long) from the Cretaceous of Montana, USA (public display, CMNH 5741, Cleveland Museum of Natural History, Cleveland, Ohio, USA). A remarkable “pygmy” tyrannosaurid dinosaur was proposed in 1988 by Bob Bakker, Philip Currie, and Michael Williams on the basis of a well-preserved skull housed at the Cleveland Museum of Natural History (specimen CMNH 5741). The skull of Nanotyrannus lancensis (Gilmore, 1946) was first discovered in 1942 in Carter County, southeastern Montana, USA, by a team from the Cleveland Museum of Natural History. Nanotyrannus was found in the Hell Creek Formation of late Maastrichtian age (near-latest Late Cretaceous - 67 million years). The original published description placed this species in the theropod genus Gorgosaurus, which is now a junior synonym of Tyrannosaurus. Re-examination (including CT-scanning) of this skull by dinosaurologist Bob Bakker in 1987 and 1988 resulted in taxonomic reassignment to a new genus altogether - Nanotyrannus (“pygmy tyrant”). For a while, the new genus was being referred to as “Clevelanotyrannus”! The skull is small for a tyrannosaurid dinosaur (60 centimeters long), but paleontologists have noted the well developed ossification of the skull (the head bones are tightly sutured), which is widely perceived to be a feature diagnostic of an adult individual. Other perceived adult aspects of the skull convinced Bakker that this was not a juvenile specimen of an otherwise large theropod dinosaur, but an adult of a miniature theropod (extrapolated to be about 5 meters total body length). If correctly interpreted, Nanotyrannus represents a clade of tyrannosaurid theropods that underwent evolutionary dwarfism very late in the history of dinosaurs. This biologic validity of Nanotyrannus has not been universally accepted. Gregory Paul (1988) was inclined to assign the species to Albertosaurus. Thomas Carr (1999) perceived that Nanotyrannus has morphologic features consistent with juvenile Albertosaurus. Philip Currie (2003) suggested that Nanotyrannus possibly represents an early ontogenetic stage (juvenile) of Tarbosaurus or Tyrannosaurus. A re-examination of the skull by CT-scanning in the 2000s has shown that the skull morphology is not consistent with it being a juvenile individual (see Witmer & Ridgeley, 2010). Theropod were small to large, bipedal dinosaurs. Almost all known members of the group were carnivorous (predators and/or scavengers). They represent the ancestral group to the birds, and some theropods are known to have had feathers. Some of the most well known dinosaurs to the general public are theropods, such as Tyrannosaurus, Allosaurus, and Spinosaurus. Mostly synthesized from info. provided by: Bakker, R.T., M. Williams & P.J. Currie. 1988. Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana. Hunteria 1: 1-30. Bakker, R.T. 1992. Inside the head of a tiny T. rex. Discover 13: 58-69. Carr, T.D. 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria: Coelurosauria). Journal of Vertebrate Paleontology 19: 497-520. Cleveland Museum of Natural History Currie, P.J. 2003. Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North Amercia and Asia. Canadian Journal of Earth Sciences 40: 661-665. Paul, G.S. 1988. Predatory Dinosaurs of the World. New York. Simon & Schuster. 464 pp. Witmer, L.M. & R.C. Ridgely. 2010. The Cleveland tyrannosaur skull (Nanotyrannus or Tyrannosaurus): new findings based on CT scanning, with special reference to the braincase. Kirtlandia 57: 61-81.