Remarkable fossils rewrite the story of how animals conquered the land

Remarkably preserved fossils that date back 300 million years have given fresh insights into early four-limbed vertebrates, suggesting they did not undergo significant metamorphosis from juveniles to adults. This finding challenges long-held views on the evolution of terrestrial life.

“For a long time, we have assumed these creatures were primarily amphibian-like, with life cycles bridging water and land existence,” explains Jason Pardo from the Field Museum of Natural History in Chicago.

The group known as tetrapods, which includes modern reptiles, birds, mammals, and amphibians, evolved from lobe-finned fish approximately 390 million years ago. However, the early development of these ancestral fish remains largely unknown, notes John Long from Flinders University in Adelaide, Australia.

Pardo, along with Arjan Mann from the Field Museum, studied fossils discovered between the 1960s and 1990s at the Mazon Creek site near Chicago. These fossils are from animals that lived between 307 and 309 million years ago, during the Carboniferous Period.

Embolomeres, which could grow up to 2 meters as adults, were the largest tetrapods of their time and among the top predators. Primarily aquatic, they used their small legs to move onto land.

The collection includes two baby embolomeres, each measuring 2 centimeters, so well preserved that soft tissues and even egg yolk are visible.

While tadpoles retain their yolk sac internally for a few days post-hatching, the young embolomeres had their yolk sacs externally, akin to some fish like lungfish.

Unlike amphibian larvae, which possess external gills for underwater breathing, the young embolomeres lacked these features. “The absence of external gills during early development is key,” states Pardo.

The skull and skeleton of these fossils show all the essential features of an adult embolomere, according to Pardo. This indicates that embolomeres did not significantly change from hatching to adulthood.

Pardo compares this to human development, noting that while human bodies grow and change proportions, they do not undergo the rapid transformations seen in frogs or salamanders. “Our fossils indicate this kind of life cycle was typical for our earliest land ancestors as well,” he adds.

Despite being aquatic, Pardo suggests that early terrestrial ancestors did not experience a tadpole-like stage. The team examined fossilized remains of two additional early tetrapod species from the same period and location as the embolomeres.

“None of these show any signs of a tadpole-like stage,” says Pardo. “Nor do other fish-like tetrapod relatives, such as early lungfishes and coelacanths. While it’s possible a tadpole stage emerged and was lost, the data suggests it’s highly unlikely.”

This research addresses a significant gap in knowledge, according to Long. “It reveals that early tetrapod-like fishes from around 308 million years ago didn’t require a tadpole phase to transition to land, contrary to some scientific beliefs.”

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