Fully grown adult Protosphyraena averaged between six and nine feet (two and three meters) long, an unremarkable stature in oceans ruled by mosasaurs and plesiosaurs that grew more than 35 feet (11 meters) long.

Complete skeletons of Protosphyraena are rare, which may suggests that either scavengers devoured them or the corpses drifted as they decayed and fell apart. Like some Late Cretaceous sea monsters, Protosphyraenamost likely went extinct 65 million years ago.