The Cleveland Museum of Natural History

The Cleveland Museum of Natural History’s skull of a small tyrannosaurid theropod dinosaur (CMNH 7541) collected from the Hell Creek Formation has sparked controversy, with competing hypotheses suggesting that it represents a separate taxon of dwarf tyrannosaurid (Nanotyrannus lancensis), a juvenile specimen of Tyrannosaurus rex (the only other acknowledged Hell Creek tyrannosaurid), or a compromise position (a juvenile Nanotyrannus). Beyond this controversy, CMNH 7541 holds importance because of the anatomical information that such a well preserved skull can provide, and it is in this context that we have sought to probe the structure of the braincase region (e.g., pneumatic sinuses, cranial nerve foramina), as well as other regions of the skull. We subjected the skull to computed x-ray tomography (CT scanning), followed by computer analysis and 3D visualization. The braincase and a number of other bones (e.g., vomer, quadrate, quadratojugal, palatine, mandible) were digitally ‘‘extracted’’ from the CT datasets. Although the new findings strongly confirm the long-held view that CMNH 7541 pertains to a tyrannosaurid, the mosaic of characters it presents makes finer taxonomic assignment difficult. For example, some characters support affinities with T. rex, yet other characters argue for a much more basal position. The key question that awaits resolution is whether the differences observed can be attributed to juvenility, and such resolution will require information from new, as yet unpublished specimens. Nevertheless, some of the differences seen in CMNH 7541 (e.g., the pattern of pneumatic foramina in the basicranium) are highly divergent and are harder to attribute to ontogeny. Among other findings, we report here thin, laminar structures within the main nasal airway that are interpretable as being respiratory turbinates, which have potential implications for metabolic physiology.