300-Million-Year-Old Baby Embolomere Fossils Show External Yolk Sac and No External Gills

Two 2-centimetre-long baby embolomere fossils from the Mazon Creek site in Illinois show an external yolk sac and lack external gills, indicating that these early tetrapods hatched with adult body plans rather than passing through a tadpole-like stage.

The fossils were collected between the 1960s and 1990s at the Mazon Creek fossil site south-west of Chicago. The preserved animals lived 307 million to 309 million years ago during the Carboniferous Period.

Adult embolomeres reached about 2 metres in length and ranked among the largest tetrapods and top predators of that time. Jason Pardo at the Field Museum of Natural History in Chicago examined the specimens with colleague Arjan Mann. The skull and skeleton of the juveniles contain all the major elements seen in adults.

“The skull and skeleton have all the important parts seen in an adult embolomere,” Pardo said. Embolomeres spent most of their lives in water but possessed small legs that allowed them to move onto land. The young fossils retained an external yolk sac similar to that of some modern lungfish and showed no trace of the external gills characteristic of amphibian larvae.

“Pardo said the lack of external gills in the juveniles is key evidence for direct development in these specimens. The fossils indicate that embolomeres grew larger and changed proportions but did not undergo rapid metamorphic change between hatching and adulthood. Pardo and Mann also examined fossils of two other early tetrapod species from the same time and place.

None displayed evidence of a tadpole-like stage. Early lungfishes and coelacanths likewise lack such a stage. Tetrapods evolved from lobe-finned fish around 390 million years ago. Today’s reptiles, birds, mammals and amphibians all descend from that group.