Joel Kontinen
These exceptional fossils, which aaccordin to
evolution, are around 72 million years old, were discovered along the Aridagawa
River in Wakayama Prefecture on Honshu island. “They belong to a
never-before-seen species of mosasaur — a group of air-breathing
aquatic reptiles that were apex marine predators during the Cretaceous
period (145 million to 66 million years ago).”
In a new study
published Dec. 11 in the Journal of Systematic Palaeontology, researchers
named the new mosasaur Megapterygius wakayamaensis. The new genus Megapterygius translates
to "large-winged" after the creature's unusually large rear flippers
Mosasaurs share a
similar body plan and there is very little variation among species. But M.
wakayamaensis is something of an outlier, which has surprised
scientists.
“Like other
mosasaurs, M. wakayamaensis had a dolphin-like torso with four
paddle-like flippers, an alligator-shaped snout and a long tail. But it also
had a dorsal fin like a shark or dolphin, which is not seen in any other
mosasaur species. “
However, what
confused researchers the most was the size of the new mosasaur's rear flippers,
which were even longer than their front flippers. Not only is this a first
among mosasaurs but it is also extremely uncommon among all living and extinct
aquatic species.
Almost all
swimming animals have their largest flippers toward the front of their bodies,
which helps them steer through the water. Having larger flippers at the rear of
the body would be like driving a car by steering the rear wheels instead of the
front ones, which would make it much harder to turn quickly.
"We lack any
modern analog that has this kind of body morphology — from fish to penguins to
sea turtles," Konishi said. "None has four large flippers they use in
conjunction with a tail fin."
The researchers
suspect that instead of using the rear flippers to turn, M. wakayamaensis angled
them upward or downward to quickly dive down or ascend through the water
column, which may have helped make them adept hunters. The dorsal fin could
have made it easier for the creature to turn, which may have counteracted the
extra drag from the rear flippers, they added.
“M. wakayamaensis was about the same size as great white sharks (Carcharodon
carcharias), which grow to around 4.9 metersor or 16 feet ‘) long. But other
species could grow up to (17 m), which
is longer than a school bus.”
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