Small Dinosaur Fossil From Japan's Coast Reveals Surprising Range of Body Sizes

A newly described dinosaur fossil from the coast of Japan shows that plant-eating iguanodontian dinosaurs living there during the Late Cretaceous varied much more in body size than previously thought, according to a study published in the journal Paleontological Research.

Iguanodontians were a diverse group of plant-eating dinosaurs that lived from the Late Jurassic to the Late Cretaceous and spread across Asia, Europe and North America. A near-complete skeleton of a large, 8-meter-long iguanodontian has previously been found in Japan, but most Japanese fossils of the group are fragmentary, consisting of isolated teeth and other scattered remains, leaving their diversity and ecology poorly understood.

The new fossil, an isolated bone, was discovered in 1999 on the grounds of the Iwaki City Ammonite Center in Fukushima Prefecture, in rock formed roughly 90 to 88 million years ago as part of the Futaba Group, which represents a shallow marine environment. The bone remained unstudied for more than two decades until the new research identified it as the right sternal plate of a styracosternan, a derived group of iguanodontians that includes hadrosauroids, or duck-billed dinosaurs.

By comparing the bone with related species, the researchers estimated the animal was about 3 meters long — much smaller than other iguanodontians found in the same rock formation, indicating that coastal dinosaur communities in East Asia included animals of markedly different sizes.