A Fossil Treasure Trove: The Sasayama Group!

The Sasayama Group, spanning Tamba-Sasayama City and Tamba City, is a remarkable fossil site where numerous dinosaur and mammal fossils have been discovered. Among them are Sasayamamylos kawaii, one of Japan’s oldest known mammals, and Tamba Ryu (Tambatitanis amicitiae), one of the largest dinosaurs found in Japan.

The region also features many outcrops where visitors can observe the rock layers of the Sasayama Group, making it one of Japan’s premier open-air natural history museums.

Discovery of One of Japan’s Largest Dinosaurs: Tamba Ryu!

In August 2006, a local geology enthusiast discovered a dinosaur rib fossil along the Sasayamagawa River in Sannan Town, Tamba City. Subsequent excavations conducted in six phases from 2007 revealed that the fossil belonged to a herbivorous, titanosauriform dinosaur (a type of sauropod).

A remarkable number of fossils, including vertebrae, were unearthed in their natural alignment, a rarity in paleontology. Additionally, an atlas vertebra and braincase—both of which are rarely found worldwide—were also discovered, making this an exceptional paleontological find.

Primitive Mammal Fossils!

During excavations from 2007 to 2008, one of Japan’s oldest known mammal fossils was discovered in Miyata, Tamba-Sasayama City. Fossil finds of early Cretaceous mammals are extremely rare, and within the geological period of the Sasayama Group, only around a dozen examples have been recorded worldwide.

At the time, mammals were only about the size of a mouse, but this discovery provides a crucial missing link in the evolutionary history of mammals, helping to fill gaps in our understanding of their early development.

Other Fossil Resources!

In addition to the discoveries mentioned, fossils of troodontid and ceratopsian skeletons, as well as teeth from tyrannosaurids and therizinosaurs, have also been found.

Furthermore, complete skeletons of early Cretaceous frogs, along with fossils of lizards, clam shrimp, and other species, provide valuable clues to reconstructing the prehistoric ecosystem of the time.

The Sasayama Group is an exceptionally rare geological formation, home to a diverse range of fossils—including dinosaurs, mammals, and other prehistoric creatures—making it a site of global paleontological significance.