Its name is based on Greek apatan, apatao, “illusory, deception”; a reference to the Y-shaped chevrons (or hemal arches) on the underside of the tail, that Marsh thought were deceptively like those found in some mosasaurs (Tylosaurus, Platcarpus, etc.).
Apatosaurus does not mean “headless lizard” as some seemed to have thought. Named by Othniel Charles Marsh (1831-1899) in 1877.
-sauria, -saurian + (page 3)