natural variation within a species. Both Carr and O'Connor expressed concerns about the study's inability to determine which of the proposed species several well-preserved specimens belonged to. Another paleontologist, Philip J. Currie, originally co-authored the study but withdrew from it as he did not want to be involved in naming the new species. Paul rejected the objections raised by critics, insisting that they are unwilling to consider that Tyrannosaurus might represent more than one species. In a subsequent paper awaiting publication, Paul maintained the conclusion that Tyrannosaurus consists of three species. He pointed out that the criticism of the study naming T. imperator and T. regina only focused on two of the features used to distinguish the two new species (the number of small incisiform teeth and femur robustness), while the original study also compared the robustness of other bones as well (the maxilla, dentary, humerus, ilium and metatarsals). Furthermore, Paul argued that Tyrannosaurus can be separated into three different species based on the shape of knob-like bumps (‘postorbital bosses’) behind the eyes. Paul also argued that past research concluding that Tyrannosaurus only consists of one species (T. rex) has simply assumed that all Tyrannosaurus skeletons are a single species, and that many new dinosaur species have been named on the basis of fewer differences than he and his colleagues used when proposing T. imperator and T. regina. Nanotyrannus Other tyrannosaurid fossils found in the same formations as T. rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis, the latter being named Dinotyrannus megagracilis in 1995. These fossils are now universally considered to belong to juvenile T. rex. A small but nearly complete skull from Montana, long, might be an exception. This skull, CMNH 7541, was originally classified as a species of Gorgosaurus (G. lancensis) by Charles W. Gilmore in 1946. In 1988, the specimen was re-described by Robert T. Bakker, Phil Currie, and Michael Williams, then the curator of paleontology at the Cleveland Museum of Natural History, where the original specimen was housed and is now on display. Their initial research indicated that the skull bones were fused, and that it therefore represented an adult specimen. In light of this, Bakker and colleagues assigned the skull to a new genus named Nanotyrannus (meaning "dwarf tyrant", for its apparently small adult size). The specimen is estimated to have been around long when it died. However, In 1999,