Mini Mosasaur: Fossil discovered in Japan sheds new light on ancient 'binocular eyed' marine hunter

An international research team have announced the discovery of the first mosasaur to be found in Japan, unearthed in a creek in the northern town of Mukawa. The fossil, which belongs to the marine-lizard which swam the oceans 72-million years ago, is one of the most well preserved of its kind to be found.

Although some types of mosasaurs could grow up to 40 feet long, this species known as Phosphorosaurus ponpetelegans, only grew to about 10 feet. Previous mosasaur fossil discoveries have been made on the east and Pacific coasts of North America, Europe and North Africa.

The team led by Takuya Konishi, a University of Cincinnati assistant professor of biological sciences, said that this finding bridges "the gap between the Middle East and the Eastern Pacific." Konishi said this proves the species was able to colonise locations around the world.

Such was the preservation of the fossil that the team were able to determine that it had binocular vision due to the location of its eyes. Positioned at the front of its head, the mosasaur would have benefited from depth perception, different to its larger relatives who had eyes on the sides of their heads.

"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," said Konishi.

"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."

This has led Konishi and his team to the conclusion that this mosasaur was a night hunter, and fed on fish that illuminated in the night, according to the study published in the Journal of Systematic Palaeontology.

"If this new mosasaur was a sit-and-wait hunter in the darkness of the sea and able to detect the light of these other animals, that would have been the perfect niche to coexist with the more established mosasaurs," says Konishi.