Newly examined fossils indicate that the earliest four-limbed vertebrates developed directly into adults without a larval phase featuring external gills.
A study of 300-million-year-old specimens from the Mazon Creek site in Illinois reveals that embolomeres, large aquatic predators of the Carboniferous Period, hatched with adult-like features.
Researchers Jason Pardo and Arjan Mann at the Field Museum analyzed two well-preserved 2-centimetre baby embolomeres. These fossils showed an external yolk sac and no external gills, differing from modern amphibians.
“The absence of external gills across early development in these animals is the smoking gun,” Pardo said. The findings challenge the long-held view that a metamorphosis bridged water-to-land transition.
The study, published in Science, also examined other early tetrapods from the same period and location, finding no evidence of a tadpole-like stage. John Long of Flinders University noted that this suggests early land invaders did not require such a phase.