BEIJING -- Scientists from China and the United States have found frogs around 100 million years old preserved in amber, the earliest frogs ever found in amber.

The specimens were excavated in Myanmar between 2015 and 2016 and are well preserved.

"One of the frogs is around 2.2 cm long and its skull, part of its backbone and most of its limbs are preserved," said Xing Lida of the China University of Geosciences in Beijing, co-author of a paper recently published by Scientific Reports.

Co-author David C. Blackburn of Florida Museum of Natural History, said, "The frogs are tiny. They lived in wet tropical forests in northern Myanmar. Unluckily, they were trapped by tree resin which formed into amber."

Xing said it was the first time that frog traces had been found in rainforests from the Cretaceous period.

"The diversity of frog habitat during the period is beyond our expectations," he said.

Frogs in amber are quite rare. Previous examples found in the Dominican Republic and Mexico date back only 20 million - 30 million years.