Transition from water to land did not require amphibian-like metamorphosis, fossils reveal
New fossil evidence overturns a long-held hypothesis about how vertebrates first moved from water to land. The newborns of three different animals linked to the first visitors to the earth show that the animals did not undergo a metamorphosis similar to that of amphibians, researchers report June 18 in Science. “They came out of the egg looking like adults,” says paleontologist Jason Pardo of the Field Museum in Chicago.
The transition to land resulted in the evolution of tetrapods, the four-membered group of animals that includes all reptiles, amphibians, birds, and mammals. Scientists have always thought that first vertebrates to venture onto land underwent metamorphosis from a larval form during their development. In today’s amphibians, this rapid transition from hatching to adult involves the loss of features such as external gills and tail fins and the acquisition of other features such as the expansion of lungs and new limbs as the animals transition from an aquatic to a partially terrestrial life.
“We sort of assume that this metamorphosis is ancestral to all terrestrial vertebrates,” says evolutionary biologist Laura Porro of University College London, who was not involved in the study. “And it shows quite conclusively that that’s not the case.”
Pardo and Arjan Mann, a paleontologist also at the Field Museum, studied newborn tetrapod fossils, approximately 308 million years old, from Mazon Creek in Illinois. Researchers used scanning electron microscope images to uncover details of the exceptionally preserved fossils, which include soft tissues such as skin and cartilage. The fossils of the babies, which died shortly after hatching, do not show the characteristics associated with an amphibian-like larval stage, such as external gills and specific undeveloped bones.
“We have the impression that none of these animals have anything resembling a larval stage, much less a metamorphosis,” says Pardo.
The first known tetrapods capable of crawling on land lived around 375 million years ago, and fossilized tracks provide evidence of even earlier terrestrial expeditions. Although the animals studied in the new study lived millions of years later, they are examples of older lineages that survived late. This suggests that their ancestors also grew without rapid change through metamorphosis.
Fossil evidence
Fossils from the hatchings of two early tetrapods and a close relative show no signs of external gills and some bones are already developing. Evidence suggests that the animals were not born as larvae metamorphosed into adult forms. This contradicts the hypothesis that the first vertebrates that moved from water to land had to undergo an amphibian-like metamorphosis to make this change. Click through the slideshow to see the fossils of the three animals.
One animal, called an embolomere, evolved into a large predator that hunted in lagoonal environments during the Carboniferous period, approximately 360 to 300 million years ago. These animals spent most of their time in water, but their short legs allowed them to crawl on land. “It’s a bit like a cross between a crocodile and an eel,” says Pardo.
The researchers analyzed two embolomere fossils, each measuring just one or two centimeters long. One of them still had an internal yolk sac, a sac of nutrients by which the newborn survived before starting to eat.
“The fact that they still have a yolk sac suggests that these are very, very young animals,” Porro says.
Skeletal evidence suggests that adult embolomeres likely had lungs for swallowing air and possibly bony structures to also support internal gills. It’s unclear whether the hatchlings breathed air or relied on internal gills to breathe in water until they grew and needed more oxygen.
One of the other animals was a megalichthyid, a fish-like creature that exhibited some of the skeletal characteristics of later tetrapods. And the third was an aistopod, which looked like a snake but was actually a tetrapod that had lost its limbs. Both probably had lungs, while megalichthyids also had internal gills. Scientists hypothesize that aistopods might have breathed through their skin, like amphibians.
Together, these three animals represent the early development of tetrapods. “I think what makes our argument so strong is that it brings together these three different groups,” Porro says.
There are still many unanswered questions about how vertebrates arrived on earth, an extraordinary transition that has altered terrestrial ecosystems across the planet. Scientists don’t know, for example, how many times animals were able to make this jump independently. But with new evidence showing they didn’t need shapeshifting to achieve this, a clearer picture has emerged. the first steps in the sand.
“I think this will be written in future textbooks,” Porro says.