Life on land likely evolved when one of our ancient fish ancestors developed legs and came ashore. These early creatures, known as tetrapods — creatures with four limbs — would eventually evolve into the birds, reptiles, amphibians, and mammals of today.
Previous studies suggest that these early tetrapods may have been similar to modern-day amphibians because they likely laid eggs and, when the young hatched, underwent a tadpole-like phase before morphing into their full-grown forms.
New findings of fossilized baby tetrapods may, however, be shaking up our understanding of how these creatures grew into adulthood. Published in the journal Science, these early fossils suggest that these creatures skipped the tadpole phase of their metamorphosis and were not as similar to modern-day amphibians as previously thought.
“When a lot of us were in high school, we were taught this simplified story of evolution: that some fish evolved into amphibians, and some of those amphibians evolved into reptiles, and some of those reptiles evolved into mammals. And our study shows that this basic underlying premise, that the first four-legged vertebrates grew up like amphibians, is wrong,” Jason Pardo, a research associate at the Field Museum in Chicago and the study’s co-lead author, said in a press release.
Read More: 275-Million-Year-Old Fossil With a Twisted Jaw Reveals an Unexpected Tetrapod
An Ancient Crocodile-Like Ancestor
The fossils the research team analyzed were from Mazon Creek in Illinois, a fossil bed about 60 miles southwest of Chicago. The babies appear to be from a now-extinct species known as embolomeres. These tetrapods were crocodile-like and likely among the top predators in the lakes, rivers, and swamps they inhabited from 350 million to 280 million years ago.
Fossil baby embolomere.
(Image Credit: Arjan Mann)
“This is the first time we’ve had these early, early hatchling animals. This discovery is really a testament to the power of Mazon Creek, the site where these fossils came from,” Arjan Mann, the Field Museum’s assistant curator of Early Tetrapods and the study’s other co-lead author, said in a press release. “It’s an hour’s drive southwest of Chicago, and it’s one of the best fossil sites in the world, especially for soft tissues and delicate little fossils like these baby tetrapods. Mazon Creek fossils are time capsules that capture the impossible.”
According to the study, the fossilized baby embolomeres were only a few centimeters long, but could reach lengths of over 10 feet in adulthood.
“I first saw the baby embolomere fossil about ten years ago, when I was working on my Ph.D.,” said Mann. “It’s in the collections at the Field Museum, and the curator of tetrapods at the time, John Bolt, pulled it out of a drawer and showed it to me when I was visiting. At the time, it hadn’t yet been identified as an embolomere, but I was really drawn to it, and John loaned me the fossil to study.”
A Fossil That Rewrites What We Know About Early Evolution
During their time as Ph.D. students in Canada, Mann and Pardo worked together to learn more about the fossil specimen.
“We had so many conversations over the past decade about what the heck this thing was,” Mann said. “Every night, we’d go back and forth saying, what’s this feature? What could this thing be?”
The pair eventually used a scanning electron microscope at the Canadian Museum of Nature to analyze the fossil. This scan helped confirm that the specimen was a baby embolomere. But what was puzzling about the fossil was that it didn’t show signs of tadpole-like features, like external gills.
The research team noted a similar finding in another baby embolomere fossil they analyzed, as well as in other fossilized baby tetrapods related to embolomeres.
“We looked at a number of different species that represent different lineages in the transition from fish to tetrapods, and what we found is that none of them have anything that looks remotely like a tadpole. And if you don't have a tadpole, then you don't have a metamorphosis,” Pardo said. “These early tetrapods’ life cycles are more like ours, or like those of fish, than they are like amphibians.”
These results are changing what previous studies have found and rewriting the previously held hypothesis that mammals and reptiles likely evolved from an amphibian-like animal.
“The story was that metamorphosis is the tool by which animals made the transition from fossil to land. That story doesn’t work anymore; it’s dust in the wind,” Pardo said.
Read More: A Massive Pliocene Crocodile May Have Hunted Lucy and Other Early Hominins, 3 Million Years Ago
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in Science: Direct development of stem tetrapods across the fin-to-limb transition
