Fossilized babies of ancient crocodile

In an unexpected turn of events, scientists have made a groundbreaking discovery about the origins of life on Earth. Contrary to long-held assumptions, the first four-legged vertebrates, or tetrapods, did not undergo a typical amphibian metamorphosis from fish-like creatures.

Instead, research has revealed that these early land-dwelling animals were less like modern amphibians than previously thought. This new finding challenges scientists' understanding of how animals conquered the land and upends their previously held premise.

According to Jason Pardo, a research associate at the Field Museum and co-lead author of the study, "This is the first time we've had these early, early hatchling animals." The discovery comes from fossilized baby tetrapods found in the Mazon Creek site, which has become one of the best locations for soft tissues and delicate fossils.

The study uses dozens of Mazon Creek fossils to represent the evolutionary transition between fish and four-legged animals. Two "centerpiece" fossils are babies of an animal called an embolomere, a type of early tetrapod that was among the top predators in rivers, lakes, and swamps from 350 million to 280 million years ago.

"I first saw the baby embolomere fossil about 10 years ago," says Arjan Mann, the Field Museum's assistant curator of early tetrapods. "It's one of the many fascinating fossils I've worked with over the years."