[Asia’s First] Globally Rare Selmasaurus Fossil Discovered on Koshiki Island, Kagoshima! 3 Reasons Why It Rewrites History

Satsumasendai City in Kagoshima Prefecture announced the discovery of what is believed to be a fossil of the extremely rare mosasaurid “Selmasaurus” from a Late Cretaceous (about 80 million years ago) stratum on Shimokoshiki Island, a remote island belonging to the city.

Mosasaurs are large marine reptiles known as the “kings of the sea” that dominated the oceans during the Late Cretaceous period.
Until now, Selmasaurus had only been discovered in the Americas, and if this fossil is confirmed, it will be an historic achievement as the fourth case in the world and the very first discovery in Asia (Japan).

In this article, we will explain in detail how this monumental discovery of the century came about, and the three major reasons why it has astonished the global paleontological community.

What is the King of the Sea, “Selmasaurus”? A Giant Lizard, Not a Dinosaur

What kind of creature was the “Selmasaurus” whose fossil was discovered this time?

Not a Dinosaur, but a Marine Reptile of the Order “Squamata”

As a major premise, although it has “saurus” in its name, it is not a dinosaur.
It is a large marine reptile belonging to the order “Squamata,” closely related to modern lizards and snakes, that lived in the seas during the Late Cretaceous period of the Mesozoic Era, the same era as the dinosaurs.
With sharp teeth lining its large mouth, it was a ferocious hunter reigning at the top of the marine ecosystem, but it disappeared along with the dinosaurs about 65 million years ago due to the mass extinction caused by a giant meteorite impact.

A Medium-Sized Species with an Estimated Length of 4 to 5 Meters

While typical mosasaurs can reach lengths of over 11 meters, the Selmasaurus individual found this time is estimated to have been about 4 to 5 meters long based on the size of its bone.

Why is it a “Discovery of the Century”? 3 Reasons It Rewrites History

The bone fragment discovered this time, measuring 9 cm long and about 4.5 cm wide, is highly regarded in the paleontological community.
The reasons are the following three points.

Reason 1: The First Discovery in Asia (Only the 4th Case in the World)

Until now, Selmasaurus-type fossils were considered “super rare species,” with only three cases found worldwide from inland areas of the North American continent (such as Kansas and Alabama).
Therefore, it was thought to be an endemic species around America, but by being discovered in Japan in Asia, across the Pacific Ocean, the conventional wisdom that it “only existed in the Americas” was brilliantly overturned.

Reason 2: Significantly Updating the Period of Existence (The Newest Fossil)

The past three fossil cases were found in strata from about 86 to 81 million years ago (or 83 million years ago).
In contrast, the fossil this time was unearthed from a stratum dating back “about 80 million years ago.”
This makes it highly likely to be “the fossil of a Selmasaurus that lived in the newest era so far,” a discovery that significantly extends their period of existence closer to modern times.

Reason 3: Proof of Diversity in Asia

The fact that a species thought to exist only in America was also swimming in the seas of Asia indicates that Late Cretaceous mosasaurs migrated over a much wider area than imagined and possessed rich diversity.
It represents a global contribution to understanding the evolutionary history of mosasaurs throughout Asia.

Behind the Scenes of the Discovery: The “Treasure Island of Fossils” Koshiki Island and the Drama of Expert Appraisal

This major discovery was brought about on Shimokoshiki Island (Himenoura Group) in Satsumasendai City, which is known as a “treasure island of fossils.”

The Discovery in July 2022

In the midst of an intensive excavation survey, Yuka Miyake, who was serving as a city curator at the time (currently a researcher at the Museum of Nature and Human Activities, Hyogo), dug up a strange bone fossil.

The Appraisal in 2023

During investigations at the Koshiki Museum, Dr. Takuya Konishi, an associate professor at the University of Cincinnati in the US and a world-renowned authority, visited the island.
From the shape of the protrusions and the curve of the bone, he recognized that it closely resembled the “quadrate bone” that makes up the jaw joint of mosasaurs. As a result of joint research, it was concluded that there is a very high possibility it is Selmasaurus.

Furthermore, while there had been three cases of mosasaur fossil discoveries in Kyushu until now, they were all “teeth.”
Finding a “bone other than a tooth (quadrate bone)” like this time is a first for Kyushu, making it an epoch-making event in that regard as well.

Voices of Joy from the Discoverer and Expert

Associate Professor Takuya Konishi

“Because it came out of southwestern Japan, far from North America, we can now say it wouldn’t be strange for it to come out of anywhere else. It’s a discovery that will help us better understand its distribution and evolutionary history.”

Ms. Yuka Miyake

“I am glad to have been able to make a global contribution. I look forward to future discoveries that can only be understood on Koshiki Island.”

Future Prospects: Special Exhibition at the Koshiki Museum

This historic quadrate bone fossil of Selmasaurus is on special exhibition at the “Koshiki Museum” in Satsumasendai City.

Director Shingo Omura expressed his hopes that it will serve as an opportunity to stimulate the curiosity of the next generation, stating, “I want many people, especially children, to know about it, and I hope they will visit Koshiki Island for their summer vacation independent research projects and experience dinosaurs. I hope they can feel the charm of dinosaurs (marine reptiles).”

Research to unravel the true forms of the kings that ruled the ancient seas will continue to be broadcast to the world from the “treasure island of fossils,” Koshiki Island.