 A third species, "P. prosostealis", was named by Appleby, but it was removed from the genus in 2017 due to its similarity to "Ichthyosaurus".The genus "Protoichthyosaurus" was synonymized with "Ichthyosaurus" by Maisch and Hungerbuhler in 1997, and again by Maisch and Matzke in 2000.However, it was found to be distinct in 2017 by Dean Lomax and colleagues, who separated it from "Ichthyosaurus" on account of differences in the arrangement and shape of the carpal ossifications, as well as the absence of the fifth digit.