300-Million-Year-Old Fossils Show Early Tetrapod Juveniles Lacked External Gills

Researchers at the Field Museum examined 300-million-year-old embolomere fossils preserving two 2-centimetre juveniles that had external yolk sacs but lacked external gills. The animals lived 307 million to 309 million years ago at the Mazon Creek site south-west of Chicago, where the specimens were collected between the 1960s and 1990s.

The juveniles lacked external gills and already possessed the skull and skeletal elements found in adults that reached about 2 metres in length.

Jason Pardo of the Field Museum said the fossils demonstrate that embolomeres remained morphologically similar from hatching onward and did not undergo rapid tadpole-like change. Pardo and colleague Arjan Mann also reviewed remains of two other early tetrapod species from the same Carboniferous Period deposits.

None showed evidence of a larval stage with external gills, a pattern also seen in early lungfishes and coelacanths, Pardo stated.

Embolomeres were primarily aquatic predators that possessed small legs. The study concludes that a tadpole phase was not required for the earliest tetrapod-like vertebrates to occupy terrestrial environments. The findings were published in the journal Science under DOI 10.1126/science.aeb7635.

John Long of Flinders University noted that the work fills a gap in understanding developmental patterns among lobe-finned fish ancestors.