All terrestrial animals with four limbs and a backbone – whether a frog, a bird, a lizard, or even a mammal such as ourselves – can trace their evolution back to a fish-like ancestor that made its way from the ocean onto land.

It's assumed that the life cycle of this ancient aquatic relative echoed that of modern amphibians, complete with a tadpole phase.

A new study throws that picture into doubt.

“The original thought was that these animals had a life cycle like modern amphibians. This comes from the old Great Chain of Being ideas; fish evolved into amphibians, which evolved into reptiles, and so on,” Jason Pardo, postdoctoral fellow at Vilnius University in Lithuania, a research associate at the Field Museum, and coauthor of the study, tells Refractor.

That idea held up for one reason.

“Metamorphosis means you can evolve terrestrial adaptations in adults, and can put off reproductive adaptations until later,” Pardo says.

But nobody had fossil evidence to back up the hypothesis.

So Pardo and his colleague Arjan Mann went looking. They examined fossils from Mazon Creek, near Chicago, a site known for preserving soft tissue in extraordinary detail.

"It's one of the best fossil sites in the world, especially for soft tissues and delicate little fossils like these baby tetrapods,” says Mann, study co-author and the Field Museum’s Assistant Curator of Early Tetrapods. "Mazon Creek fossils are time capsules that capture the impossible."

They looked at embolomeres, megalichthyid fish, and snake-like creatures called aïstopods. None of them showed external gills.

Instead, the fossils, even the baby embolomeres, already had adult anatomy. Fingers. Toes. Lungs. Just smaller. They hatched essentially complete, then grew bigger; the same way human babies grow into the parts they're born with, instead of transforming through a larval stage.

The centerpiece fossil is a baby embolomere, catalogued as FMNH PR 1082. Embolomeres are an extinct group of crocodile-like predators that dominated rivers, lakes and swamps through the Carboniferous and Permian periods. In adulthood, they can be as long as three meters (9.8 feet). FMNH PR 1082 is around 308 million years old, and just about two centimeters (0.8 inches) in length.

"FMNH PR 1082 was the fossil that really brought all this together,” Pardo says. “What's particularly neat about it is that we know it hadn't started eating yet; it has yolk still in its belly, and doesn't have any food in its intestines."

All these imply that though this specimen was unambiguously young, it was built like an adult, with no external gills. It had "skipped" the presumed larval stage.

Pardo put the takeaway plainly: "Our study shows that this basic underlying premise, that the first four-legged vertebrates grew up like amphibians, is wrong."

Their finding doesn't rule out an aquatic origin for tetrapods, or even gills. Pardo notes that the absence of external gills doesn't mean the animals had none.

"It just means they were probably on the inside, like modern fish," he says. "I suspect that a lot of these early animals with fingers and toes were much more fishlike than we sometimes imagine, and that includes a lifecycle more tightly tied to water than we want to believe."

He raised another possibility, too.

"Maybe some of these animals did lay soft eggs on land in moist places," the scientist considers. "And maybe they were living on land their entire lives."

Ultimately, the textbook version – of fish, to tadpole, to terrestrial animal – doesn't hold up anymore. What replaces it is still an open question; internal gills, soft eggs laid in moist soil, or something else entirely?

For now, Pardo and Mann's work is an indication that some of paleontology's biggest answers may already be sitting in museum drawers, waiting for someone to look closely enough.

This research was published in the journal Science.

Source: Vilnius University via EurekAlert!

Fact-checked by Mike McRae