Prolonged growth and extended subadult development in the <i>Tyrannosaurus rex</i> species complex revealed by expanded histological sampling and statistical modeling

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 <i>Tyrannosaurus rex</i> 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 <i>Tyrannosaurus rex</i> species complex grew more gradually and over a longer lifespan than indicated by prior models, with a protracted period of subadult development.