The latest common ancestor of the Plesiosauria was probably a rather small short-necked form. During the earliest Jurassic, the subgroup with the most species was the Rhomaleosauridae, a possibly very basal split-off of species which were also short-necked. Plesiosaurs in this period were at most five metres (sixteen feet) long. By the Toarcian, about 180 million years ago, other groups, among them the Plesiosauridae, became more numerous and some species developed longer necks, resulting in total body lengths of up to .