[Tyrannosaurid] Japan’s First! Lower Jaw Fossil Discovered in Amakusa, Kumamoto. A Potential New 8-Meter Species?

In February 2024, groundbreaking news that rewrites the history of dinosaur research in Japan was announced.
In Reihoku Town, Amakusa District, Kumamoto Prefecture, a “lower jaw bone” of a large carnivorous dinosaur from the Tyrannosauridae family was discovered.

Previously, only “teeth” had been found in Japan, so this is the first time a “bone” has been confirmed.
Estimated to be 8 to 9 meters in length, this dinosaur is considered highly likely to be a new species that fills a gap in the evolutionary history of Asia.
In this article, we explain the details of this historic discovery and the surprising facts revealed by the fossil, such as the existence of “replacement teeth” identified via CT scans.

A Decade-Long Revelation! “Japan’s First Bone” Found in Black Rock

The setting of this discovery is Reihoku Town, Amakusa District, Kumamoto Prefecture, where geological strata from the Late Cretaceous period (approximately 74 million years ago) are exposed.

Discovery in 2014 and Cleaning

The fossil itself was found in 2014 during a joint survey by the Goshoura Cretaceous Museum and the Fukui Prefectural Dinosaur Museum.
Initially, it was encased in black rock and was indistinguishable, even thought to be “fossilized wood,” but subsequent careful cleaning raised the possibility that it was a “significant fossil.”

Identity Revealed by CT Scan

CT surveys beginning in 2022 revealed the internal structure clearly.
The results showed that the lower jaws (dentaries) of a carnivorous dinosaur were preserved overlapping left and right.
In February 2024, it was finally concluded to be “Japan’s first lower jaw bone of a Tyrannosaurid.”

Proof of a Robust Hunter: Fossil Characteristics and “Replacement Teeth”

The discovered fossil (Specimen Number: GCM-VP750) bore characteristics unique to the Tyrannosauridae family.

Robustness and Tooth Cross-Section

The jaw bone is very thick, with a height of about 8 cm, indicating it possessed powerful biting force.
Additionally, the cross-section of the teeth is a “bulging oval,” which matches the definitive characteristic of Tyrannosaurids, known for crushing bone along with meat.

Not Permanent Teeth? Discovery of “Replacement Teeth”

Even more surprisingly, the formation of “replacement teeth”—teeth waiting to grow in next—was confirmed inside the jaw bone.
The state of preservation is extremely good, making it a treasure trove of information for understanding the ecology of that time.

8 to 9 Meters Long! Possibility of a “New Species” Filling the Asian Void

Apex of Japan’s Food Chain

The total length estimated from the jaw size is approximately 8 to 9 meters.
Although smaller than the largest T. rex (approx. 12 m), it was undoubtedly the apex predator in the Japanese ecosystem of that time.

The “Missing Link” in Asian Evolutionary History

Definitive fossil records of Tyrannosaurids in Asia from about 74 million years ago are extremely rare.
While diverse species are known in North America, information has been lacking in Asia.
Therefore, this fossil is considered highly likely to be a new species different from known ones.

Why is “Bone” Important, Not Just “Teeth”?

The reason this discovery is being hailed as “Japan’s first” lies in the fact that it was “bone” rather than “teeth.”

Teeth

Because they fall out easily, finding one does not necessarily mean the animal died at that location.

Bone (Jaw)

Since it is a part of the body and does not detach easily, it is highly likely that the dinosaur died and was buried at that spot.

In other words, there is a possibility that other parts of the skull or a full skeleton lie nearby, raising high expectations for future additional discoveries.