The Ichthyosauria is a group of reptiles with fish-shaped bodies from the Mesozoic (65-250 million years ago)(1,2). Their secondary adaptations to aquatic life have obscured their ancestral features(3,4), and basal ichthyosaurs, which would be expected to retain these ancestral features (plesiomorphies), are poorly represented in the fossil record(1). As a result, their relationships to other amniotes have been controversial for over 180 years(5,6). New specimens of Utatsusaurus hataii from the Lower Triassic (240 Myr ago) of Japan are the first basal ichthyosaurs to show detailed features for almost the entire skeleton, including previously unknown parts of the skull and pelvic girdle. Computer-assisted retrodeformation of fossil images : shows that Utatsusaurus retained features of terrestrial amniotes in both the skull and the postcranial skeleton, such as the connection between the vertebral column and the pelvic girdle. Phylogenetic analyses indicate that ichthyosaurs belong in the Diapsida, but that, unlike the sauropterygians(8,9), they are not included with the Sauria (the crown group containing lizards, crocodiles, birds and Sphenodon). Recent studies have reported that the addition of ichthyosaurs to the amniote data altered the relationships among basal saurians(10,11), but no major clades were affected by the inclusion of ichthyosaurs in our analyses.