A small fish crawling out of a drying desert pond underlines a theory that ties up the fishes with the amphibians.
"Such a plucky hypothetical ancestor of ours probably could not have survived the overwhelming odds of perishing in a trek to another shrinking pond," said Gregory J Retallack, professor of geological sciences at University of Oregon, who led the study.
This scenario comes from the late Devonian period, roughly 390 million to 360 million years ago, which late Harvard palaeontologist Alfred Romer propounded, the Journal of Geology reported.
Challenging Romer's theory on fish survival, Retallack said that the transitional fossils were not associated with drying ponds or deserts, but were found consistently with humid woodland soils, according to a university statement.
"Judging from where their fossils were found, transitional forms between fish and amphibians lived in wooded floodplains. Our distant ancestors were not so much foolhardy or opportunistic to take advantage of floodplains and lakes choked with roots and logs for the first time in geological history," said Retallack.
"Limbs proved handy for negotiating woody obstacles, and flexible necks allowed for feeding in shallow water," he said.
"By this new woodland hypothesis, the limbs and necks, which distinguish salamanders from fish, did not arise from reckless adventure in deserts, but rather were nurtured by a newly evolved habitat of humid, wooded floodplains," Retallack said.
"Ancient soils and sediments at sites for transitional fossils around the world are critical for understanding when and under what conditions fish first walked," he added.