Fossil hatchlings challenge the metamorphosis story

New fossil evidence reported in Science on June 18 suggests early tetrapods did not go through an amphibian-like metamorphosis. Researchers studying hatchlings roughly 308 million years old from Mazon Creek in Illinois found they appeared to emerge from eggs a

For decades. the story of how vertebrates first conquered land has carried a familiar beat: hatch. metamorphose. and only then truly become an adult that can live on land. The fossils released in a June 18 report in Science put that rhythm into question—because they preserve something too early to hide behind speculation.

“They came out of the egg looking like the adult,” paleontologist Jason Pardo of the Field Museum in Chicago said.

The transition to land helped drive the evolution of tetrapods. the group of four-limbed animals that includes all reptiles. amphibians. birds and mammals. Scientists have historically thought that the first vertebrates to venture onto land underwent metamorphosis from a larval form during development. In amphibians today. that rapid transition involves losing features such as external gills and tail fins and gaining others such as expanding lungs and new limbs as animals move from an aquatic life to a partly terrestrial one.

But the new evidence looks different.

Researchers led by Pardo and Arjan Mann. both paleontologists at the Field Museum. investigated fossils of newborn early tetrapods—roughly 308 million years old—found at Mazon Creek in Illinois. They relied on scanning electron microscope images to extract fine anatomical details from exceptionally preserved specimens. including soft tissues such as skin and cartilage.

The babies, the researchers say, died shortly after hatching. And the fossils of those hatchlings lack features tied to an amphibian-like larval stage, such as external gills and specific undeveloped bones.

“We’ve got a pattern of none of these animals having anything that looks like a larval stage, let alone metamorphosis,” Pardo said.

That conclusion isn’t based on a single creature. Pardo and Mann examined hatchlings from three different groups: an embolomere, a megalichthyid, and an aïstopod.

In total, the fossils include evidence from hatchings from two early tetrapods and a close relative. The skeletal evidence suggests the animals were not born as larvae that metamorphosed into adult forms.

The late-stage timing matters, too. The earliest known tetrapod relatives that could crawl on land lived about 375 million years ago. and fossilized tracks provide evidence of even earlier land expeditions. The animals in this study lived millions of years later. but they appear to be late-surviving examples of older lineages—meaning their forebears likely grew without a rapid change via metamorphosis.

Evolutionary biologist Laura Porro of University College London, who was not part of the study, framed the contradiction bluntly. “We kind of assume that this metamorphosis is ancestral to all terrestrial vertebrates,” Porro said. “And this pretty conclusively shows that it’s not.”

Pardo and Mann also didn’t just stop at what the hatchlings lacked; they considered what the fossils suggested these newborns already had.

For the embolomeres. one animal described in the paper as a predator that hunted in lagoonlike environments during the Carboniferous Period—about 360 million to 300 million years ago—short legs allowed them to crawl onto land even though they spent most of their time in the water. Pardo described them as “kind of like a cross between a crocodile and an eel.”.

In the new study, the researchers analyzed two embolomere fossils, each only a centimeter or two long. One of them still had an internal yolk sac, a pocket of nutrients that the newborn survived on before it started eating food.

Porro said, “The fact they still have a yolk sac suggests that these are very, very young animals.”

Skeletal evidence suggests adult embolomeres probably had lungs for gulping air, and possibly bony structures to support internal gills as well. What isn’t clear is whether hatchlings breathed air or relied on internal gills in the water until they grew bigger and needed more oxygen.

The other two groups help broaden the picture. One was a megalichthyid, a fishlike creature with some skeletal features of later tetrapods. The third was an aïstopod—described as looking like a snake, but actually a tetrapod that lost its limbs. Both probably had lungs, while megalichthyids possessed internal gills as well. Scientists have speculated that aïstopods may have been able to breathe through their skin, like amphibians.

Together, these three animals represent early tetrapod development, and Porro said, “I think what makes the case so strong is it’s got those three different groups.”

There are still many unanswered questions about how vertebrates set out on land. the extraordinary transition that altered terrestrial ecosystems across the planet. Scientists don’t know how many times animals may have independently made this leap, for example. But with evidence suggesting they didn’t need metamorphosis to make the move. a clearer picture of the first steps in the sand is emerging.

“I think that’s going to be written into future textbooks,” Porro said.

tetrapods fossils Mazon Creek metamorphosis amphibian-like transition early land vertebrates Science June 18 embryonic development embolomere aïstopod megalichthyid