Fossils of Three Carboniferous Tetrapods Show No Tadpole-Like Larval Stage

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 specimens date to the Carboniferous Period between 307 million and 309 million years ago. The young embolomeres possessed an external yolk sac similar to that seen in some lungfish and lacked external gills.

Their skulls and skeletons already contained the major elements found in adults, which reached approximately 2 metres in length. Jason Pardo of the Field Museum of Natural History in Chicago said the fossils show embolomeres did not undergo metamorphosis.

The animals spent most of their time in water yet possessed small legs that could have allowed movement onto land, where they ranked among the largest tetrapods and top predators of the period.

Pardo and colleague Arjan Mann, also at the Field Museum, studied the embolomere remains along with fossils of two other early tetrapod species from the same location and time. None of the specimens displayed evidence of a tadpole-like stage. John Long of Flinders University in Adelaide, Australia, said the data indicate early tetrapod-like fishes living about 308 million years ago did not require a tadpole phase to move onto land.

The fossils were originally collected between the 1960s and 1990s. The study appears in the journal Science.