Ancient Mosasaur found in Japan

Researchers recently unveiled the discovery of a Japanese mosasaur, a prehistoric marine reptile akin to a great white shark in size, that prowled the Pacific seas around 72 million years ago. This formidable creature, named Wakayama Soryu, meaning blue dragon, due to its origin in Wakayama Prefecture, boasted unique features setting it apart from other mosasaurs.

University of Cincinnati Associate Professor Takuya Konishi, along with international collaborators, detailed their findings in the Journal of Systematic Palaeontology. The mosasaur’s fossilized remains, found by Akihiro Misaki along the Aridagawa River in 2006, revealed an extraordinary specimen, nearly complete and unparalleled in its preservation.

Intriguingly, Wakayama Soryu possessed extra-long rear flippers, potentially aiding in propulsion alongside its elongated finned tail. Unlike its mosasaur counterparts, this ancient predator sported a dorsal fin reminiscent of a shark’s, enhancing its agility and precision in aquatic maneuvers.

Konishi, dedicated to the study of marine reptiles, emphasized the uniqueness of the Japanese find. The specimen’s distinctive characteristics, such as rear flippers longer than its front ones and an unprecedented combination of features, challenged conventional classification. Even for an expert in the field, the Wakayama Soryu presented a revelation, expanding our understanding of the diverse adaptations within this group of ancient marine creatures.

A mosasaur discovered in Japan was the most complete skeleton ever found in Japan or the northwestern Pacific. Credit: Takuya Konishi

Mosasaurs, apex predators of prehistoric oceans from around 100 to 66 million years ago, coexisted with iconic dinosaurs like Tyrannosaurus rex. The extinction event that wiped out most dinosaurs, triggered by an asteroid impact in the Gulf of Mexico, also marked the demise of these marine reptiles.

The recently discovered Wakayama Soryu, with features resembling both New Zealand and California mosasaurs, intrigued researchers. Classified within the Mosasaurinae subfamily, it was named Megapterygius wakayamaensis, reflecting its significant flippers. These “large-winged” appendages, an anomaly in the animal kingdom, posed questions about their function, challenging our understanding of mosasaur locomotion.

With nearly binocular vision, the Wakayama Soryu likely excelled as a lethal hunter. Its unique combination of features, including large front and rear flippers and a rudder-like tail, defies comparison to modern aquatic animals. The mystery surrounding the purpose of these hydrodynamic surfaces, whether for steering or propulsion, adds complexity to our grasp of mosasaur swimming behavior.

The Wakayama Soryu (blue dragon) was a mosasaur the size of a great white shark that had a dorsal fin and long flippers. Credit: Takumi

Notably, the Wakayama Soryu exhibited a dorsal fin, supported by the orientation of neural spines along its vertebrae. Drawing parallels with harbor porpoises, the hypothesis suggests a functional similarity, challenging traditional views of mosasaur anatomy.

The painstaking excavation process over five years involved removing sandstone matrix and creating a cast to document the mosasaur’s original skeletal alignment. These efforts provide valuable insights into the Wakayama Soryu’s anatomy and behavior, contributing to the evolving narrative of these enigmatic ancient marine creatures.

Source: University of Cincinnati

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