A new genus and species of mosasaurine mosasaur being named Megapterygius wakayamaensis has been identified from a largely complete skeleton found in Wakayama prefecture, south-western Japan.
Thank you for reading this post, don't forget to subscribe!
Megapterygius wakayamaensis was about the size of a great white shark. Image credit: Takumi.
Mosasaurs were a group of large predatory marine reptiles that inhabited all of the world’s oceans during the Late Cretaceous epoch, between 90 and 66 million years ago.
These creatures were contemporaries of Tyrannosaurus rex and other Cretaceous dinosaurs that ruled the Earth.
They were victims of the same mass extinction that killed off nearly all dinosaurs when an asteroid struck what is now the Gulf of Mexico.
Although their relationship to other reptiles is not completely certain, mosasaurs appear to be closely related to a group known as monitor lizards.
The newly-identified mosasaur species, Megapterygius wakayamaensis, lived approximately 72 million years ago.
Its extra-long rear flippers might have aided propulsion in concert with its long finned tail.
And unlike other mosasaurs, it had a dorsal fin like a shark’s that would have helped it turn quickly and with precision in the water.
The fossilized skeleton of Megapterygius wakayamaensis was found in 2006 in the Hasegawa Muddy Sandstone Member of the Toyajo Formation in Wakayama prefecture, south-western Japan.
“The specimen is the most complete skeleton of a mosasaur ever found in Japan or the northwestern Pacific,” said University of Cincinnati paleontologist Takuya Konishi.
“In this case, it was nearly the entire specimen, which was astounding.”
“The specimen has unique features that defy simple classification,” he added.
“Its rear flippers are longer than its front ones. These enormous flippers are even longer than its crocodile-like head, which is unique among mosasaurs.”
“I thought I knew them quite well by now. Immediately it was something I had never seen before.”
“Those big paddle-shaped flippers might have been used for locomotion.”
“But that type of swimming would be extraordinary not only among mosasaurs but among virtually all other animals.”
“We lack any modern analog that has this kind of body morphology — from fish to penguins to sea turtles. None has four large flippers they use in conjunction with a tail fin.”
The authors speculate that the large front fins might have helped with rapid maneuvering while its large rear fins might have provided pitch to dive or surface.
And presumably like other mosasaurs, its tail would have generated powerful and fast acceleration as it hunted fish.
“It opens a whole can of worms that challenges our understanding of how mosasaurs swim,” Dr. Konishi said.
The discovery is reported in a paper in the Journal of Systematic Palaeontology.
_____
Takuya Konishi et al. A new derived mosasaurine (Squamata: Mosasaurinae) from south-western Japan reveals unexpected postcranial diversity among hydropedal mosasaurs. Journal of Systematic Palaeontology, published online December 11, 2023; doi: 10.1080/14772019.2023.2277921
