300-Million-Year-Old Baby Embolomere Fossils Show Direct Development Without External Gills

Palaeontologists at the Field Museum of Natural History examined two 2-centimetre-long baby embolomere fossils from the Mazon Creek site south-west of Chicago that preserved external egg yolk sacs and lacked external gills. The specimens, dated to 307 million to 309 million years ago in the Carboniferous Period, contained complete skulls and skeletons matching adult structures.

Jason Pardo stated that the absence of external gills across early development constitutes the key evidence.

Adult embolomeres reached approximately 2 metres and ranked among the largest tetrapods and top predators of the period, spending most time in water while possessing small legs. The fossils were collected between the 1960s and 1990s. Pardo and colleague Arjan Mann also reviewed remains of two additional early tetrapod species from the same location and time, finding no signs of a tadpole-like larval stage in any of the material.

Pardo noted that the young embolomeres retained an external yolk sac similar to certain lungfish, while their bodies remained unchanged in structure from hatching onward. aeb7635, concludes that this direct developmental pattern was typical for early terrestrial ancestors.

Tetrapods evolved from lobe-finned fish around 390 million years ago.

The examined fossils indicate that a metamorphic larval phase was not required for the water-to-land transition in these lineages.