Fossils of Two 2-Centimeter Baby Embolomeres Show No Tadpole-Like Stage

Fossils of two 2-centimetre-long baby embolomeres from the Mazon Creek site show that these early tetrapods hatched with fully formed skulls and skeletons and lacked external gills, New Scientist reported. The specimens, collected between the 1960s and 1990s south-west of Chicago, Illinois, also retain visible egg yolk outside the body, a feature seen in some modern lungfish.

Jason Pardo of the Field Museum of Natural History in Chicago said the absence of external gills across early development is the smoking gun.

The young embolomeres measured about 2 centimetres long, while adults reached roughly 2 metres, yet the fossils contain all major skeletal elements already present at hatching. Pardo and colleague Arjan Mann examined the embolomere material along with fossils of two other early tetrapod species that lived at the same location 307 million to 309 million years ago during the Carboniferous Period.

None of the specimens showed evidence of a tadpole-like larval stage, Pardo stated.

Embolomeres were the largest tetrapods of their time and among the top predators, spending most of their lives in water while possessing small legs that could have allowed brief movement onto land. The new fossils indicate these animals remained more or less the same in body plan from hatching until adulthood, Pardo said.

John Long of Flinders University in Adelaide, Australia, said the findings show early tetrapod-like fishes living about 308 million years ago did not need a tadpole phase to invade land, contrary to some earlier assumptions.

Aeb7635. Today's reptiles, birds, mammals and amphibians all belong to the tetrapod group, which evolved from lobe-finned fish around 390 million years ago. The Mazon Creek specimens provide direct evidence that at least some of the earliest members of this lineage developed without the metamorphic transition long assumed to have bridged aquatic and terrestrial life.