The typical plesiosaur had a broad, flat, body and a short tail. Plesiosaurs retained their ancestral two pairs of limbs, which had evolved into large flippers. Plesiosaurs were related to the earlier Nothosauridae, that had a more crocodile-like body. The flipper arrangement is unusual for aquatic animals in that probably all four limbs were used to propel the animal through the water by up-and-down movements. The tail was most likely only used for helping in directional control. This contrasts to the ichthyosaurs and the later mosasaurs, in which the tail provided the main propulsion.