In 1872, Marsh argued that "Liodon proriger" and "Liodon dyspelor" are taxonomically distinct from the European genus and must be assigned a new one. For this, he erected the genus "Rhinosaurus", which means "nose lizard" and is a portmanteau derived from the Ancient Greek words (', meaning "nose") and (', meaning "lizard"). Marsh also described a third species based on a partial skeleton he collected near the southern portion of the Smoky Hill River that is now in the Yale Peabody Museum as YPM 1268, which Marsh named "Rhinosaurus micromus". Cope responded by arguing that "Rhinosaurus" was already a preoccupied synonym of "Liodon". He disagreed with Marsh's arguments but proposed that in case Marsh was indeed correct, the genus name "Rhamphosaurus" should be used. Marsh later discovered that the taxon "Rhamphosaurus" was preoccupied as a genus of lizard named in 1843. As a result, he suggested a move to a newly erected genus named "Tylosaurus". This name means "knob lizard" in another reference to the elongated rostrum characteristic of the genus. It is derived from the Latin "tylos" (knob) and Ancient Greek . Despite coining the new genus, Marsh never formally transferred the "Rhinosaurus" species to "Tylosaurus"; this was first done in 1873 by Joseph Leidy of the University of Pennsylvania by transferring "Rhinosaurus proriger" and "Rhinosaurus dyspelor" to "Tylosaurus". "Rhinosaurus micromus" was formally transferred to the same genus in 1894 by John Campbell Merriam, a paleontologist of the University of Munich at the time.