Strange Mosasaur Fossils Reveal Smaller Species Had Binocular Vision and Hunted At Night

A strange mosasaur fossil that was recently unearthed in Japan is shedding new light on how this swimming lizard hunted along the way to colonizing the northern hemisphere. In fact, it used binocular vision powered by extra photoreceptors that detected light for night hunting glowing fish and squid.

An international team of researchers led by Takuya Konishi, a University of Cincinnati assistant professor of biological sciences, discovered the 72-million-year-old marine reptile fossil (Phosphorosaurus ponpetelegans) that fills in a biogeographical gap between the Middle East and the eastern Pacific, according to a news release.

"Previous discoveries of this particular rare mosasaur have occurred along the East Coast of North America, the Pacific Coast of North America, Europe and North Africa, but this is the first to fill the gap between the Middle East and the Eastern Pacific," Konishi said in the release.

Compared their larger cousins that could grow to lengths of 40 feet, this species was relatively small and could only grow 10 feet long. Furthermore, the rare mosasur fossils were so well-preserved that researchers were able to locate very large eye sockets on the front of it's face, suggesting it has binocular vision and depth perception like humans – an eye structure not observed in their larger cousins, whose eyes were positioned on each side of the head.

"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," Konishi added in the university's release. "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."

All of this proves that these smaller marine reptiles had very large binocular eyes to power extra photoreceptors needed to detect light at night when they hunted. In contrast, their larger relatives had big heads and bodies shaped to enhance streamlined swimming after prey during the day.

"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," Konishi concluded.

Their findings were recently published in the Journal of Systematic Palaeontology, a publication of the Natural History Museum in London.

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