90-Million-Year-Old Fossil Reveals a Small Dinosaur on Japan's Ancient Coast

A fossil discovered on the grounds of the Iwaki City Ammonite Center in Japan's Fukushima Prefecture in 1999 has been identified, more than two decades later, as belonging to a small plant-eating dinosaur that lived along the coast of what is now Japan about 90 to 88 million years ago. The bone had originally been tentatively identified as the sternal plate, or breastbone, of a hadrosauroid dinosaur, but it went unstudied for over 20 years.

A new detailed morphological analysis has now identified the fossil as the right sternal plate of an indeterminate styracosternan — a derived group of iguanodontians, the plant-eating dinosaurs that lived from the Late Jurassic through the Late Cretaceous and spread widely across the Northern Hemisphere. Styracosternans include the hadrosauroids, a group that gave rise to duck-billed dinosaurs.

Comparisons with related species suggest the animal was about 3 meters long, making it substantially smaller than other iguanodontians previously found in the same rock formation, the Ashizawa Formation, which preserves sediments from a shallow-marine environment. Researchers have already described a nearly complete skeleton of a large iguanodontian, about 8 meters long, from Late Cretaceous Japan.

The presence of this much smaller dinosaur alongside larger ones suggests that iguanodontians spanning a wide range of body sizes occupied coastal environments in East Asia during the Late Cretaceous, adding new evidence for the diversity of these ancient ecosystems. Most of Japan's iguanodontian fossil record consists of isolated teeth and fragments, so complete specimens like this one are valuable for understanding how the group evolved as it moved between Asia, North America and Europe. The findings were published in the journal Paleontological Research.