Analysis of exceptionally rare 309-million-year-old fossils indicates that early tetrapods—the first vertebrates to transition from water to land—may not have undergone a metamorphic phase akin to modern frogs. Paleontologists examining the remains found no evidence of features associated with a tadpole stage, such as gills or a lateral line system.
This finding upends a long-standing hypothesis that the earliest terrestrial vertebrates began life with amphibious, tadpole-like characteristics before developing limbs. The fossil specimens, preserved in what is now a coal swamp environment, suggest a more direct developmental pathway.
The study's authors emphasize the fossils are remarkably complete, offering a rare glimpse into the anatomy of juvenile tetrapods. Without traces of aquatic larval traits, the researchers argue these animals likely emerged from eggs as miniature versions of adults.
If confirmed, this would push back the evolutionary origins of direct development—where young resemble adults—by tens of millions of years. The discovery raises new questions about how and when metamorphosis evolved in the vertebrate lineage.
Other scientists caution that the sample size remains limited, and similar fossils from other locations could tell a different story. Further excavations and comparative analyses are needed to solidify the claim.