A small dinosaur bone discovered in Fukushima Prefecture more than 25 years ago is giving scientists new clues about the variety of plant-eating dinosaurs that once lived along the ancient coast of Japan.
The fossil belonged to an iguanodontian, a large and diverse group of herbivorous dinosaurs that lived from the Late Jurassic through the Late Cretaceous periods.
Iguanodontians spread widely around the world, and some species grew to impressive sizes.
During their long history, these dinosaurs moved between Asia, North America and Europe. These movements helped produce the wide variety of iguanodontians that scientists know from the fossil record today.
Fossils from Japan are particularly valuable because they can help researchers understand how these dinosaurs lived and spread along the eastern edge of ancient Asia.
The newly studied fossil has an unusual history of its own.
It was discovered in 1999 on the grounds of the Iwaki City Ammonite Center in Fukushima Prefecture. The rocks in the area formed roughly 90 million to 88 million years ago during the Late Cretaceous period.
At that time, the region looked very different from modern Fukushima. Geological evidence indicates that the sediments were deposited in a shallow marine environment, meaning dinosaurs lived near a prehistoric coastline.
When the fossil was first discovered, researchers tentatively identified it as a sternal plate, or chest bone, belonging to a hadrosauroid dinosaur. But the isolated bone remained largely unexamined for more than two decades.
Scientists have now conducted a detailed analysis of the specimen. Reporting their findings in the journal Paleontological Research, they identified it as the right sternal plate of a dinosaur belonging to a group called styracosternans.
Styracosternans are an advanced branch of iguanodontians that also includes hadrosauroids, the broader group containing the famous duck-billed dinosaurs.
Although researchers could not identify the exact species, the size of the bone revealed something intriguing.
By comparing it with similar bones from related dinosaurs, the researchers estimated that the animal was only about 3 meters, or 10 feet, long.
That makes it considerably smaller than some other iguanodontians known from the same geological formation. Japan has produced evidence of much larger animals, including an iguanodontian skeleton representing a dinosaur around 8 meters, or 26 feet, long.
The difference in size suggests that Late Cretaceous coastal environments in this part of East Asia may have supported iguanodontians of very different body sizes.
It is not yet clear whether the smaller fossil represents a naturally small species or perhaps an individual that had not reached full adult size. Because the specimen consists of only one bone, many questions about the dinosaur remain unanswered.
Nevertheless, the discovery adds another piece to Japan’s growing dinosaur fossil record.
Most Japanese iguanodontian discoveries consist of isolated teeth or other fragmentary remains, making it difficult to reconstruct their diversity and ecology. Even a single forgotten bone can therefore provide valuable information.
The fossil suggests that around 90 million years ago, the coastline of what is now Fukushima may have been home to a surprisingly varied community of plant-eating dinosaurs, ranging from relatively small iguanodontians to animals more than twice their size.
