Unraveling the mysteries of Nipponosaurus

June 9, 2017, Hokkaido University
The complete skeleton of Nipponosaurus sachalinensis is kept at the Hokkaido University Museum. Credit: Hokkaido University

Nipponosaurus sachalinensis—a controversial hadrosaurid dinosaur whose fossilized skeleton was unearthed in southern Sakhalin in 1934—is found to be a valid taxon and a juvenile that had not reached sexual maturity.

Nipponosaurus, a herbivorous dinosaur of the Late Cretaceous Period, was named so in 1936 by Professor Takumi Nagao of Hokkaido Imperial University (predecessor of Hokkaido University). The name refers to the Japanese word for Japan (Nippon), as Southern Sakhalin was Japan's territory before World War II. It was the first study of in Japan and Nagao's work is considered as the genesis of Japan's dinosaur research.

In 2004, a reanalysis of the dinosaur by a graduate student of Hokkaido University revealed that Nipponosaurus was a juvenile and closely related to the North American hadrosaurid Hypacrosaurus. Since then, several conflicting hypotheses were proposed, including some denying an independent taxonomic status of Nipponosaurus. This theory stemmed from the fact that the fossilized bones came from an immature dinosaur, so its bones would have changed as it grew older.

Ryuji Takasaki, a of Hokkaido University, associate Professor Yoshitsugu Kobayashi at the Hokkaido University Museum and their collaborators in Canada and the U.S. investigated the developmental stage of the Nipponosaurus by dissecting three (a femur, rib and chevron). They found the orientation of vascular canals in the thighbone change from reticular in the inner cortex to laminar in the outer cortex. They also figured out that the number of lines indicating arrested growth, similar to growth rings in a tree, is limited to two in all bones. Both of these features are showing that Nipponosaurus was a juvenile yet to reach .

Cross section of a thighbone of Nipponosaurus (A) with an enlarged photo of the rectangular area (B). The latter shows the changing directions of the vascular canals. Credit: Takasaki R., et al., Historical Biology, May 5, 2017

The researchers also investigated the bones of hadrosaurids in each up to adulthood to examine how the bones transformed as they grew. Some bones did not change their form through these stages.

By comparing the bones of Nipponosaurus and other hadrosaurids, the team discovered unique characteristics within the bones of Nipponosaurus that should not change as it develops. This finding led to the conclusion that Nipponosaurus is indeed a valid taxon. The unique characteristics are a wide shelf-like structure on the lower jaw, coronoid process stretching vertically from the shelf-like structure, and extremely short front legs.

The researchers concluded Nipponosaurus is a more primitive hadrosaurid than previously thought and closely related to Europe's hadrosaurid Blasisaurus and Arenysaurus, indicating Nipponosaurus is one of the dinosaur species that migrated from Europe, not North America, to the Far East.

"Our study clarified the phylogenetic status of Nipponosaurus, and we are now interested in the relationship between Nipponosaurus and other Japanese dinosaurs, whose fossils have been unearthed one after another in recent years. We aim to discover how diverse dinosaurs inhabited East Asian coastal areas," says Ryuji Takasaki.

Explore further: Japan's largest complete dinosaur skeleton discovered

More information: Ryuji Takasaki et al, Reanalysis of the phylogenetic status of(Ornithopoda: Dinosauria) from the Late Cretaceous of Southern Sakhalin, Historical Biology (2017). DOI: 10.1080/08912963.2017.1317766

Related Stories

Japan's largest complete dinosaur skeleton discovered

June 6, 2017

The complete skeleton of an 8-meter-long dinosaur has been unearthed from marine deposits dating back 72 million years at Japan's northern island of Hokkaido, making it the largest dinosaur skeleton ever found in Japan, according ...

Researchers discover Moabosaurus in Utah

April 13, 2017

Move over, honeybee and seagull: it's time to meet Moabosaurus utahensis, Utah's newly discovered dinosaur, whose past reveals even more about the state's long-term history.

Dinosaur species innocent of cannibalism

September 26, 2006

A U.S. paleontology student is being credited with discrediting a theory that the dinosaur species Coelophysis bauri practiced cannibalism.

Dinosaurs: Juvenile, adult or senior?

February 7, 2017

How old were the oldest dinosaurs? This question remains largely unanswered. The natural life span of these long-extinct giants is of interest to scientists, in combination with questions regarding how fast they could grow ...

80-million-year-old fossil found in Japan

October 15, 2007

The skull of a platypus-like dinosaur estimated to be more than 80 million years old has been discovered on a Japanese mountaintop near the town of Mifune.

Recommended for you

Study sheds new light on ancient human-turkey relationship

January 17, 2018

For the first time, research has uncovered the origins of the earliest domestic turkeys in ancient Mexico. The study also suggests turkeys weren't only prized for their meat—with demand for the birds soaring with the Mayans ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.