When Tyrannosaurus rex prowled the land, the oceans of the Late Cretaceous weren’t particularly safe either. Cruising the waters at the time were large predatory reptiles known as mosasaurs, elongated and streamlined for hunting down their prey in the warm shallow seas. Some of these were true beasts reaching up to 18 meters (59 feet) in length, but others were a little more modest. Researchers have revealed the first mosasaur of its kind to be discovered in Japan, and discovered that it probably hunted at night using binocular vision.
The marine reptile in question is a species called Phosphorosaurus ponpetelegans, and came in at a relatively tiny – for mosasaur at least – 3 meters (10 feet) long. The remarkably well preserved skull is the only example known from Japan, and helps palaeontologists fill a geographic gap of the species from between the Middle East and the eastern Pacific. It also allows the researchers to determine that the animal quite possibly fed on bioluminescent fish and squid during the night, whilst their larger cousins dominated the sea during the daytime.
“The forward-facing eyes on Phosphorosaurus provide depth perception to vision, and it's common in birds of prey and other predatory mammals that dwell among us today,” explains Takuya Konishi in a statement. Konishi is a coauthor of the study, which is published in the Journal of Systematic Palaeontology. “But we knew already that most mosasaurs were pursuit predators based on what we know they preyed upon – swimming animals. Paradoxically, these small mosasaurs like Phosphorosaurus were not as adept swimmers as their larger contemporaries because their flippers and tailfins weren't as well developed.”
But when compared to their larger relatives, the vision of Phosphorosaurus is markedly different. On the bigger specimens, their eyes are located on either side of their head – not unlike a horse or deer today – which is thought to have helped streamline the reptile and allow it to swim faster to catch the turtles, sharks and other mosasaurs they fed on.
With the smaller species, however, the eyes are forward facing, which in nocturnal animals doubles the number of photoreceptors used to detect light. Because fossils of lantern fish and squid-like animals have been found in the same rock formations in Japan as Phosphorosaurus, the researchers suggest that the reptile might have been hunting at night. They even go on to postulate that perhaps this might have been a larger trend for other species of mosasaurs, with the larger animals hunting and chasing down prey during the day, while the smaller, more vulnerable ones only came out at night.
The excavation of the skull that enabled this discovery took a painstaking two years. It involved dipping the rock in which it was encased in acid overnight, and then washing it off every morning, gradually freeing the bones. These were then pieced together to re-create the original skull. The researchers now intend to look into how the species fits into the evolutionary tree of mosasaurs.
