By Kyrmyzy Turebayeva
For decades, scientists believed that the earliest four-limbed vertebrates developed much like modern amphibians: They hatched from eggs, passed through a larval tadpole stage and then underwent metamorphosis into adults.
However, a new study published in Science challenges that long-held view.
Researchers from the Field Museum in Chicago have concluded that early tetrapods — the ancient vertebrates from which modern amphibians, reptiles, birds and mammals ultimately evolved — developed without a tadpole stage or amphibian-style metamorphosis.
The researchers reached that conclusion after studying fossilized juvenile embolomeres, crocodile-like early tetrapods that lived about 350 to 280 million years ago.
Their findings suggest that the first vertebrates to colonize land were far less similar to modern amphibians than previously believed.
“The main takeaway from our study is that we can’t look directly to modern amphibians to understand what the earliest tetrapods were like,” said Jason Pardo, a research associate at the Field Museum and one of the study’s co-lead authors. “This discovery changes how scientists understand the transition of vertebrates from water to land.”
The discovery was made possible by fossils from the Mazon Creek fossil site in Illinois.
Located about an hour southwest of Chicago, Mazon Creek is one of the world’s most important fossil localities, particularly for preserving soft tissues and tiny, delicate fossils such as juvenile tetrapods.
Researchers describe Mazon Creek fossils as “time capsules” because they preserve rare details of ancient life.
It was there that scientists discovered rare fossils of juvenile early tetrapods, including embolomeres.
According to the researchers, adult embolomeres could grow to more than 10 feet (3 meters) long and were among the top predators in ancient rivers, lakes and swamps.
The juvenile specimens recovered from Mazon Creek, however, measured only a few centimeters in length.
According to Arjan Mann, assistant curator of early tetrapods at the Field Museum and one of the study’s co-lead authors, these fossils provided an extraordinary opportunity to examine the earliest stages of tetrapod development.
“This is the first time we’ve had these very early hatchlings. This discovery is really a testament to the power of Mazon Creek, the site where these fossils were found. It’s one of the best fossil localities in the world, especially for preserving soft tissues and delicate little fossils like these baby tetrapods,” Mann said.
The study was the result of nearly a decade of research.
Mann first encountered the juvenile embolomere fossil about 10 years ago while working on his Ph.D. At the time, the specimen was housed in the museum’s collection but had not yet been identified.
Mann and Pardo, who were graduate students in Canada at the time, spent years trying to determine exactly what this unusual fossil represented.
Eventually, scanning electron microscopy conducted at the Canadian Museum of Nature confirmed that the specimen was indeed an embolomere.
But that discovery raised even more questions.
According to Mann, the researchers initially wondered whether the absence of a tadpole stage was unique to this single specimen or characteristic of early tetrapods more broadly.
To answer that question, they expanded their analysis to include species representing different branches of the evolutionary transition from fish to tetrapods.
The team examined thousands of fossils from the Field Museum, the Lauer Foundation for Paleontology, Science and Education and other collections associated with the Mazon Creek site.
They used scanning electron microscopy and other imaging techniques to examine the fossils and their preserved soft tissues.
Their analyses showed that early tetrapods lacked a tadpole-like developmental stage – the juvenile animals already possessed limbs and many features characteristic of adults.
They also had teeth, indicating they were active feeders, and lacked physical structures typical of amphibian tadpoles, such as external gills.
“We looked at several species representing different branches of the evolutionary transition from fish to tetrapods, and we didn’t find anything that even remotely resembled a tadpole. And if you don’t have a tadpole, then you don’t have metamorphosis,” Pardo said via email.
The findings prompt scientists to reconsider long-standing ideas about the evolution of early land vertebrates.
If animals such as embolomeres lacked a tadpole stage and true amphibian-style metamorphosis, then the assumption that the earliest tetrapods developed like modern amphibians no longer holds.
The study also changes how scientists view modern amphibians.
Rather than representing a simple intermediate stage between fish and other land vertebrates, amphibians appear to represent their own distinct and highly successful evolutionary strategy.
The researchers emphasize that this discovery was possible only because of the collaboration of scientists, fossil collectors and citizen scientists.
“Every specimen used in this study was the result of a joint effort involving the Earth Science Club of Northern Illinois, the Lauer Foundation for Paleontology, Science and Education, and the Field Museum,” Mann said.
Several citizen scientists and fossil collectors, including Paul Demkovich, Ben Riegler, Rich Rock and Tom Testa, provided specimens they discovered for scientific study.
“Those fossils have changed our understanding of tetrapod evolution. This is a monumental discovery, and it would not have been possible without citizen science,” Mann said.