Japanese Fossils Challenge Our Understanding Of Hadrosaur Evolution

Artist's impression of Yamatosaurus izanagii (center) along with two more common hadrosaurs. Given the distances between the sites where these fossils have been found, it is unlikely a scene like this ever occurred. Image Credit: Masato Hattori

Hadrosaurs were among the most successful dinosaur families of the late Cretaceous. Were it not for that pesky asteroid/comet they might be the world's most common large herbivores today. However, a new discovery reveals we've probably been wrong in some of our thinking about key aspects of their development.

Famous for their duck-like bills, hadrosaurids dominated North America 73-66 million years ago, being far more numerous than the predators that preyed on them and the sauropods that favored size over number. Last year we learned they had just made it to Africa before the world as they knew it came to a crashing halt. Who knows how common they might have become there, given more time?

The first hadrosaur fossils were discovered in North America, and it has subsequently been widely considered that this was where they evolved before conquering much of the world. However, the discovery of a new species, named Yamatosaurus izanagii in Scientific Reports, on the Japanese island of Awaji throws this into question.

Japan was still attached to Asia 71-72 million years ago when Yamatosaurus lived – its island status is just 15 million years old. Although dating to near the end of the dinosaurs' reign, Yamatosaurus' features indicate it was a relatively early break-away from more widespread hadrosaur lines. This makes its discoverers consider it more likely that the family initially evolved in Asia, and only later migrated to the continent on which they were most successful.

The flattened snouts that are hadrosaurs most recognizable feature housed hundreds of teeth, making them champion chewers. These teeth were endlessly replaceable, like those of sharks, with backups growing in when any tooth wore out from the task of breaking down so much tough vegetation.

Yamatosaurus was different from every other hadrosaur we have found, with no functional replacement available for many of its teeth. Combined with a difference in tooth shape from the hadrosaur norm, this led Professor Yoshitsugu Kobayashi of Hokkaido University Museum to conclude they lived on a different diet from their counterparts, although we don't know what that was.

Japan lacks the exposed fossil-rich outcrops that have provided most of our knowledge of the Cretaceous elsewhere, so Kobayashi and co-authors were excited when an amateur found the lower jaw, teeth, shoulder and neck and tail vertebrae of a Yamatosaurus in an Awaji Island quarry.

Despite this, hadrosaurid fossils have been found on all of Japan's four major islands, as well as Awaji, indicating how common they were. However, jawbones and teeth have proven to be a particularly precious find.

"These are the first dinosaurs discovered in Japan from the late Cretaceous period," Kobayashi said in a statement. "Until now, we had no idea what dinosaurs lived in Japan at the end of the dinosaur age."

The shoulder bone also proved a significant find. It appears to reveal a species reversing the process that created us, going from walking on two legs to getting around mostly on four, and suggesting this occurred near the start of the evolution of hadrosaurs.

The name combines an ancient name for Japan and a god credited with creating the Japanese islands, with Awaji being first.


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