With a length of around 3 m (10 ft), Phosphorosaurus was small compared to most other mosasaurs, but rather standard in size for a halisaurine.
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.

Konishi et al.
2015 listed the following unambiguous character states for the genus: "Dorsal exposure of prefrontal confined to preorbital segment of frontal; preorbital segment of frontal narrow, sloping anteroventrally; frontal preorbital borders subparallel with each other; interorbital emargination present; frontal lateral border forming a step-like junction between preorbital and interorbital segments; apex of posterodorsal triangular plateau on frontal reaching level of interorbital constriction; frontal articulation facet for postorbitofrontal squareshaped in outline; parietal postorbital process dorsally not separated by ridge from parietal table; and stapedial meatus parallel-sided".
With a length of around 3 m (10 ft), Phosphorosaurus was small compared to most other mosasaurs, but rather standard in size for a halisaurine. 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.
Konishi et al. 2015 listed the following unambiguous character states for the genus: "Dorsal exposure of prefrontal confined to preorbital segment of frontal; preorbital segment of frontal narrow, sloping anteroventrally; frontal preorbital borders subparallel with each other; interorbital emargination present; frontal lateral border forming a step-like junction between preorbital and interorbital segments; apex of posterodorsal triangular plateau on frontal reaching level of interorbital constriction; frontal articulation facet for postorbitofrontal squareshaped in outline; parietal postorbital process dorsally not separated by ridge from parietal table; and stapedial meatus parallel-sided".
Phosphorosaurus