Baby Embolomere Fossils Show No External Gills and Adult-Like Skull Features

Palaeontologists examined two 2-centimetre-long baby embolomere fossils from the Mazon Creek site in Illinois that preserve soft tissues and external egg yolk sacs. The fossils date to the Carboniferous Period between 307 million and 309 million years ago. Adult embolomeres reached about 2 metres in length and ranked among the largest tetrapods and top predators of that time.

The young specimens lacked external gills and already possessed the skull and skeletal elements found in adults. Jason Pardo of the Field Museum of Natural History said the absence of external gills is consistent with direct development. He added that the animals remained unchanged from hatching until adulthood, aside from growth and shifts in proportion.

Pardo and colleague Arjan Mann also examined fossils of two other early tetrapod species from the same site and period. None of the specimens showed evidence of a tadpole-like stage. The team concluded that embolomeres did not undergo metamorphosis between juvenile and adult forms.

The fossils were collected between the 1960s and 1990s. The study appears in the journal Science with DOI 10.1126/science.aeb7635. John Long of Flinders University said the findings indicate early tetrapod-like fishes did not require a tadpole phase to move onto land.