Abstract
A partial theropod skeleton from the Albian (ca. 105 ma) Cloverly Formation of Wyoming is shown to exhibit many features in common with members of Carcharodontosauria and is referred to Acrocanthosaurus atokensis on the basis of an autapomorphy and a unique combination of characters. The absence of neurocentral fusion in dorsal and caudal vertebrae and bone histology of the femur indicate that the specimen is a juvenile. The circumferences of lines of arrested growth were used to estimate mass over successive years of the animal's life. These mass estimates suggest that early in ontogeny, Acrocanthosaurus grew at rates on par with growth rates inferred in Allosaurus and most tyrannosaurid theropods, which are similar to rates expected for scaled-up precocial birds. Histological data from adult specimens suggest that Acrocanthosaurus reached adult body size in two to three decades. Gigantism in Acrocanthosaurus likely evolved via acceleration of growth rates relative to those of basal members of Allosauroidea, a transition also observed within tyrannosauroid theropods. Contrary to previous assessments, there is only evidence for one large-bodied theropod species in the Early Cretaceous of North America, though many fragmentary specimens are indeterminate to the genus level. Aptian–Albian and Maastrichtian-aged dinosaur communities were more similar to one another than to those of the intervening Campanian stage in that both seem to have featured a single, extremely large-bodied, fast growing, geographically widespread theropod dinosaur.