Analysis of Phosphorosaurus biology suggests that this mosasaur was a deep-water or nocturnal hunter, potentially preying on animals such as squid and bioluminescent fish similar to the modern lanternfish present in the same areas.

The large eyes of Phosphorosaurus had overlapping fields of vision, giving it depth perception, which would have given it an advantage when chasing such animals in poorly-lit conditions.

Studies also indicate that the animal was likely an ambush predator that would lie in wait for prey, as it was not as efficient a swimmer as larger mosasaurs, much like other halisaurines.

Belgian palaeontologist Louis Dollo defined the genus in 1889,[3] with the description of Phosphorosaurus ortliebi from the upper Maastrichtian Craie phosphateé of Ciply in Belgium.

They suggested that Phosphorosaurus ponpetelegans and Eonatator coellensis were more closely related to the genus Halisaurus than the type species of their respective genera.