90 million year old tooth unearthed in Japan may provide clues on mammal evolution in Asia

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A fossilized tooth of an extinct mammal thought to be a close relative of marsupials has been unearthed in central Kyushu, a find that dates back about 90 million years and may provide clues to how mammals evolved in Asia, the Mifune Dinosaur Museum said Thursday.

90 million year old tooth unearthed in Japan may provide clues on mammal evolution in Asia
Life reconstruction of a Deltatheridium, an ancient relative of marsupials which include kangaroos and 
koalas. The Mifune Dinosaur Museum in Kumamoto Prefecture said a fossil tooth of a marsupial relative 
has been unearthed in the town of Mifune [Credit: Akio Ito & Yasuko Okamoto via Kyodo]

The tooth, the first of its kind excavated in Japan, was uncovered from sandstone in the Tashiro district in Mifune, Kumamoto Prefecture. The stone was found in March 2014 from a geological layer formed in the late Cretaceous period, according to the museum.

The uncovered tooth measures about 2 mm in length and 3 mm in width. Based on the shape and size, it is assumed to be a back tooth from the upper left jaw of a carnivore that was about 10 to 15 cm tall.

The fossil is believed to be of an animal similar to a Deltatheridium, an ancient relative of marsupials that lived in Mongolia in the Cretaceous period about 145 million to 66 million years ago.

90 million year old tooth unearthed in Japan may provide clues on mammal evolution in Asia
The fossilized tooth of a marsupial relative unearthed in Mifune, 
Kumamoto Prefecture [Credit: Kyodo]

Marsupials — a subclass of mammals that today include kangaroos and koalas — and Deltatheridiums are classified in the metatheria group. In Asia, some metatheria fossils have been discovered in a geological layer of the late Cretaceous period in inland areas of Mongolia and China.

Kazunori Miyata, an associate professor at the Dinosaur Research Institute of Fukui Prefectural University, said the fossil’s discovery is “significant.”

“It suggests the possibility that primitive marsupial ancestors were thriving on the east bank of Asia including Japan in the late Cretaceous period when dinosaurs were still dominant.”

Source: The Japan Times [August 25, 2017]

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