Abstract
The Upper Jurassic Morrison Formation sauropods Diplodocus (formerly "Seismosaurus") hallorum and Supersaurus vivianae are quantifiably the largest dinosaurian taxa from the formation, as well as being among the largest dinosaurs in the world. Their extreme body size (in particular body length, c. 50+ m) has fascinated the paleontological community since their discoveries and has sparked an ongoing discussion on the trends and limits of Morrison Formation sauropod body size. Although not an undeviating proxy, often the largest and skeletally most mature specimens are among the rarest (as exemplified in Triceratops). While their body size has no phylogenetic bearing, the extreme size and potential eco and biological significance of these two sauropod taxa are frequently discussed. Whether these rare and titanically proportioned sauropod specimens are large-bodied, senescent or both is an often-repeating rhetoric. To definitively make maturational inferences about these taxa, we osteohistologically sampled the holotype of D. hallorum (NMMNH P-25079) and the second known specimen of S. vivianae (WDC DMJ-021). Our age-determinant and maturational assessments indicate that both specimens were skeletally mature at their respective age of death. Retrocalculation methods for D. hallorum NMMNH P-25079 produce a maximum age-at-death estimation of 60 years, whereas S. vivianae WDC DMJ-021 lived well past skeletal maturity-so much so that reliable retrocalculated ages cannot be accurately determined at this time. Additionally, the rarity of such large sauropods within the Morrison Formation might be more parsimoniously explained as relating to their maturity as opposed to representing aberrant taxa on the Morrison landscape.