This undated image courtesy of The Field Museum shows an Embolomere fossil. Scientists have long posited the earliest water animals to transition to land had amphibious tadpole features, going through a metamorphosis akin to today's frogs. But new research out June 18 in Science challenges that assumption, presenting analysis of rare fossils scientists say fills knowledge gaps on the early development of the creatures that gave rise to the first land-dwelling vertebrates. The research centers on specimens excavated from the Mazon Creek fossil beds in northern Illinois. The world-renowned site features iron carbonate concretions that formed some 309 million years ago, fossilizing within them ancient creatures that had once thrived in the area's lush swamps, shallow seas and river deltas. (Photo by Arjan MANN / Field Museum / AFP via Getty Images) / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO/ FIELD MUSEUM / Arjan MANN / HANDOUT- NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS - NO ARCHIVE