found it probable that these specimens belonged to T. rex. The same year, Carr published a paper on T. rex's growth history, finding that CMNH 7541 fit within the expected ontogenetic variation of the taxon and displayed juvenile characteristics found in other specimens. It was classified as a juvenile, under 13 years old with a skull less than . No significant sexual or phylogenetic variation was discernible among any of the 44 specimens studied, with Carr stating that characters of potential phylogenetic importance decrease throughout age at the same rate as growth occurs. Discussing the paper's results, Carr described how all "Nanotyrannus" specimens formed a continual growth transition between the smallest juveniles and the subadults, unlike what would be expected if it were a distinct taxon where the specimens would group to the exclusion of Tyrannosaurus. Carr concluded that "the 'nanomorphs' are not all that similar to each other and instead form an important bridge in the growth series of T. rex that captures the beginnings of the profound change from the shallow skull of juveniles to the deep skull that is seen in fully-developed adults." Paleobiology Life history The identification of several specimens as juvenile T. rex has allowed scientists to document ontogenetic changes in the species, estimate the lifespan, and determine how quickly the animals would have grown. The smallest known individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only , while the largest, such as FMNH PR2081 (Sue) most likely weighed about . Histologic analysis of T. rex bones showed LACM 28471 had aged only 2 years when it died, while Sue was 28 years old, an age which may have been close to the maximum for the species. Histology has also allowed the age of other specimens to be determined. Growth curves can be developed when the ages of different specimens are plotted on a graph along with their mass. A T. rex growth curve is S-shaped, with juveniles remaining under until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young T. rex would gain an average of a year for the next four years. At 18 years of age, the curve plateaus again, indicating that growth slowed dramatically. For example, only separated the 28-year-old Sue from a 22-year-old Canadian specimen (RTMP 81.12.1). A 2004 histological study performed by different workers corroborates these