Ontogenetic histology ofApatosaurus(Dinosauria: Sauropoda): New insights on growth rates and longevity

ABSTRACT The bone microstructure of an ontogenetic series of Apatosaurus radii, ulnae, and scapulae suggests that Apatosaurus underwent three distinct osteogenic phases. Primary laminar to plexiform fibro-lamellar bone tissue, devoid of lines of arrested growth (LAG), occurs in individuals up to 91% adult size. LAGs and longitudinally vascularized lamellar tissue are deposited for the first time in the external cortices of sub-adult individuals. Slow growth and additional deposition of accretionary lamellar bone occurs in adulthood, and indicates attainment of maximum size in Apatosaurus. All scapulae examined show cyclicity in vascularity indicative of regular variation in speeds of osteogenesis. In contrast, Apatosaurus radii and ulnae show consistent bone depositional rates throughout ontogeny. Despite inter-element variability, all Apatosaurus bones sampled corroborate the hypothesis of sustained rapid growth rates for most of ontogeny, followed by gradual decline with attainment of maximum size. Estimation of ages of ~10 years for large sub-adults refutes the hypothesis that slow, indeterminate growth was required for Apatosaurus and other sauropods to achieve extremely large body sizes.