Small Iguanodont Dinosaur Discovered in Japan’s Cretaceous Marine Rocks

Fossilized bones discovered in northeastern Japan more than 20 years ago have been identified as belonging to one of the smallest iguanodont dinosaurs ever found in the country. The new study, led by researchers at the University of Tsukuba, provides fresh insights into dinosaur diversity in Japan’s Late Cretaceous coastal ecosystems.

Iguanodonts were a major group of ornithopod dinosaurs that lived from the Late Jurassic through the end of the Cretaceous Period. Their body lengths ranged from approximately 2 meters (6.6 feet) to more than 10 meters (33 feet).

These herbivorous dinosaurs achieved a global distribution, with fossils discovered on every continent. Asia played an important role in their dispersal across Laurasia, including migrations between Asia and North America as well as between Asia and Europe.

The Cretaceous deposits of East and Central Asia contain a particularly diverse record of iguanodont dinosaurs. In Japan, the number of iguanodont fossil discoveries has increased significantly over the past two decades. The country represents the easternmost part of the Cretaceous Laurasian continent, making its fossil record important for understanding dinosaur evolution and dispersal in Asia.

In the new study, Dr. Masami Kondo and his colleagues reexamined an isolated right sternum plate excavated in 1999 near Iwaki in Fukushima Prefecture, Japan.

The fossil was recovered from the Ashizawa Formation, part of the Futaba Group. This sequence of sandstone and mudstone is approximately 90 million years old and was deposited on the floor of an ancient marine environment.

The specimen was initially identified as part of a hadrosaurid, or duck-billed dinosaur, but it remained largely unstudied for more than 20 years.

After analyzing the shape and structure of the hatchet-like bone, the researchers concluded that it belonged to an indeterminate member of Styracosterna. This broad group within Iguanodontia includes hadrosauroids and their close relatives.

The bone’s characteristics most closely resemble those of more derived iguanodonts rather than primitive members of the group.

By comparing the fossilized sternum with those of related dinosaurs, the researchers estimated that the animal measured approximately 3 meters (10 feet) in length. This makes it unusually small compared with many other iguanodont dinosaurs.

“This is significantly smaller than other iguanodont fossils previously excavated from the same formation,” the researchers said.

Large iguanodonts, including hadrosaurs, are already known to have lived in Japan during the Late Cretaceous. The newly identified fossil suggests that iguanodont dinosaurs with a wide range of body sizes occupied coastal environments across East Asia.

“This discovery provides new insights that further support the diversity of ancient coastal ecosystems,” the researchers added.

The study was published on July 6 in the journal Paleontological Research.

_____

Masami Kondo et al. 2026. An iguanodont sternum plate excavated from the Upper Cretaceous Ashizawa Formation (Futaba Group) in Fukushima Prefecture. Paleontological Research 30: 122–134. doi: 10.2517/prpsj.250042