Overview
- A study published in Science on Thursday reports that exceptionally preserved hatchlings from the Mazon Creek beds lack external gills and retain yolk sacs, traits consistent with direct development rather than an amphibian-style larval stage.
- Researchers reexamined museum and collector specimens using scanning electron microscopy to reveal soft tissues, skull and skeletal features that match adult anatomy even in the smallest juveniles.
- The result comes from multiple lineages — including embolomeres, a megalichthyid and an aïstopod — which strengthens the case that the absence of a tadpole-like phase is not a single-specimen anomaly.
- Authors say the findings overturn a long-standing textbook assumption that metamorphosis was ancestral to the first land-dwelling vertebrates and instead imply that amphibian-style life cycles arose later in some lineages.
- Because the fossils were preserved in iron carbonate concretions and come from late-surviving members of deep lineages, the study clarifies early life histories while leaving open questions about how often and where different developmental strategies evolved.