Mosasaurus depressus was named by Cope (1869) [exact location of type specimen not recorded beyond "from Bulington Co., N.J." (Cope, 1869-1870, p. 196); from the Navesink Formation of New Jersey]. It is a 3D body fossil. Its type locality is Pemberton Marl Company pit (Hornerstown), which is in a Maastrichtian marginal marine marl in the Hornerstown Formation of New Jersey.

It was recombined as Plioplatecarpus depressus by Russell (1967), Driscoll et al. (2019).