Thalattosaurs (meaning "ocean lizards") are a group of prehistoric marine reptiles that lived during the mid-late Triassic Period. Some species of thalattosaur grew to over 4 metres (13 feet) in length, including a long, flattened tail used in underwater propulsion. While they bore a superficial resemblance to lizards, the exact relationships of thalattosaurs is unknown; they are widely accepted as diapsids, but experts have variously placed them on the reptile family tree near ichthyosaurs, archosaurs, or Lepidosauromorpha (lizards and their relatives), or as basal neodiapsids.