Scientists have discovered the earliest examples of salamanders - some specimens up to 165 million years old - in fossil beds in Mongolia and China.
By Jonathan Amos
BBC News Online science staff
Scientists say they have found literally thousands of the animals preserved in volcanic ash.
The researchers describe one juvenile in particular that reveals the amphibian's eye, folds in its tail and a stomach bulging with clam shrimps.
The discoveries are part of an ongoing excavation programme being conducted by staff from the University of Chicago, US, and Peking University in Beijing, China.
Before these extraordinary finds, the oldest known salamander fossils dated back only to the Tertiary Sub-Era, which began 65 million years ago.
"What excites us is that we're not only seeing the earliest known salamanders in the fossil record, but we've thousands of them," Professor Neil Shubin told BBC News Online.
"There are whole bodies, impressions of soft tissue preserved, and stomach contents. It's really unusual that you have such a view of the early evolution of a group of animals like this."
In the journal Nature this week, the scientists describe in detail an amphibian they call Chunerpeton tianyiensis.
The digging team has pulled out thousands of specimens
It is said to resemble the North American hellbender, a common salamander currently found in Asia, as well as in the Allegheny Mountains near Pittsburgh in the US state of Pennsylvania.
The bones in the front of its skull, its fingers, toes and ribs are all somewhat different, however.
Other complete fossils, including some with those rare soft tissue impressions, offer a wealth of new information on the salamander's origin and life cycle.
"You have tiny guys who might be just a millimetre long right up to adults that might be 20 centimetres long. It's remarkable," said Shubin, who is professor and chairman of organismal biology and anatomy at the University of Chicago.
"Some of the adults look like big larvae; some of them have retained their larval features as adults. And we have some that have clearly metamorphosed.
"The exquisite condition of these fossils offers clues to evolutionary strategies - larval details such as gills in adult animals, for example."
PENCHANT FOR CLAMS
In one small larval specimen, the contents of its last meal are still in the stomach
The finds confirm, in Shubin's and colleague Ke-Qin Gao's view, that salamanders originated in Asia.
"About 200 million years ago, the world had one supercontinent, [Pangea]. Then the continents began to split apart - the big split being between a northern landmass called Laurasia and a southern landmass called Gondwana.
"Gondwana has Africa, South America, Australia, Antarctica. Laurasia has North America, Europe and northern Asia. It seems salamanders evolved around this split so that today they are almost entirely Laurasian in distribution.
"The creatures we are finding in China are relatives of the salamanders in Asia and North America today."