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A review of the type and referred specimens of Anchisaurus polyzelus (Hitchcock) and Ammosaurus major Marsh, from the Early Jurassic Portland Formation (Hartford Basin, Newark Supergroup), indicates that the latter is a junior synonym of the former. The material displays derived similarities with sauropod dinosaurs that are not present in their sister group, the prosauropods. Cladistic analysis strongly supports the hypothesis that Anchisaurus polyzelus is the most basal known member of Sauropoda. Thus A. polyzelus becomes the smallest known sauropod. Optimization of femur length using square change parsimony indicates that the lineage leading to Neosauropoda underwent gradual and sustained size increase for most of its history since its divergence from Theropoda. It also shows that A. polyzelus represents a reversal of this trend and has decreased in size relative to the sauropod common ancestor.