Abstract
BACKGROUND: Tyrannosaurus rex, one of the most iconic non-avialan dinosaurs, remains a central focus of paleobiological research. Growth modeling suggests T. rex exceeded 8,000 kg within two decades and had a lifespan approaching 30 years. However, this understanding of T. rex growth dynamics is dependent on single-point histological sampling of multiple skeletal elements and lacks specimens encompassing the earliest growth states. METHODS: We present the most comprehensive histological analysis of Tyrannosaurus ontogeny to date, based on transverse diaphyseal sections of femora and tibiae from 17 individuals ranging from small juveniles to large adults. Four alternative statistical models were tested, differing in the treatment of cortical growth marks, including annulus-like birefringent bands visible only in cross-polarized light. Due to high intraspecies morphological variability, the taxonomic status of many Tyrannosaurus specimens is debated, prompting use of the term "Tyrannosaurus rex species complex" to describe our dataset. RESULTS: The best-supported model incorporated all visible growth marks, produced the narrowest confidence bands, and indicated lower maximum growth rates and a delayed attainment of asymptotic size (~35-40 years) compared with earlier estimates. We also find that two immature specimens within the Tyrannosaurus rex species complex are not statistically compatible with the other growth series. Our approach is the first in dinosaur skeletochronology to simultaneously estimate the position of the earliest preserved growth mark across specimens, while fitting sigmoidal curves with simultaneous confidence bands. We find the inclusion of double growth marks and those visible only with cross polarized light provide better statistical model fits and this may have implications for modeling other taxa. Additionally, we find no strong link from extant vertebrates to support the idea that the growth inflection point is biologically significant and corresponds to sexual maturity. Our results suggest that the Tyrannosaurus rex species complex grew more gradually and over a longer lifespan than indicated by prior models, with a protracted period of subadult development.