Scientists have long hypothesized that the earliest aquatic animals that moved to land had the characteristics of amphibious tadpoles, undergoing a metamorphosis similar to what occurs in today’s frogs.
But new research published Thursday in the journal Science challenges this traditional assumption. It provides an analysis of rare fossils that scientists say fills knowledge gaps about the evolution of creatures that led to the emergence of the first vertebrates to live on Earth.
The research focuses on specimens excavated from the Mason Creek fossil beds in northern Illinois, southwest of Chicago.
The world-famous site includes concrete blocks of iron carbonate formed about 309 million years ago, fossilized within them are ancient organisms that once flourished in the region’s fertile swamps, shallow seas and river deltas.
It is known for its exceptionally well-preserved specimens including soft tissues.
The new study analyzes dozens of fossils to study evolution between fish and tetrapods, or four-legged animals.
At the center was a specimen that was most likely determined to be the baby of a crocodile-like creature known as a sigil, which lives mostly in water but has developed small legs.
In the juvenile stage, folk thought would have expected it to exhibit tadpole-like features such as external gills, explained Jason Pardo, a research associate at the Field Museum of Chicago and co-lead author of the study.
But it didn’t happen, he said.
Instead, the baby’s body — which the researchers said was the size of a short, narrow spaghetti — showed evidence of direct development, meaning it was put together more or less the way it would be in adulthood.
This is not what we would expect to see in amphibians, whose transformation from tadpoles to adults involves a more dramatic rearrangement and development of organs and limbs.
“We actually now have some direct evidence from the fossil record that this transition, the amphibian-like life cycle that we assumed for 150 years was part of our history, turns out to have been not part of that at all,” Pardo told AFP.
John Long, an Australian paleontologist who has also conducted extensive research in this area, described the study as “absolutely fascinating.”
“Not much was known about their early life stages,” he told AFP about the animals that gave rise to the first tetrapods.
“This detailed work on a group of fascinating fossils proves that they moved directly to the juvenile stage, so they did not need to go through the tadpole stage.”
Jason Anderson of the University of Calgary said the “impressive” research highlights “the power of fossils to address questions we thought impossible given that they occur over short timescales, and in tissues that are not typically preserved over hundreds of millions of years.”
He and Pardo also noted that the study confirms that amphibians are evolutionary impressive creatures in their own right.
“Our amphibians, rather than being remnants of earlier stages in the evolutionary history of tetrapods, are themselves highly advanced creatures,” Anderson told AFP.
The fossil that served as the focal point of the study had been in the Field Museum’s collections for a long time when the then-director showed it to the paper’s co-author Arjan Mann, who became intrigued.
While they were both doctoral students in Canada, Mann and Pardo had been puzzling over this for years.
Final analysis using a scanning electron microscope at the Canadian Museum of Nature allowed researchers to confirm that it was likely emboli.
During their research, the duo analyzed the juvenile features of that fossil along with other smaller tetrapods and other small fossil tetrapod relatives.
Mann — the Field Museum’s assistant curator of early tetrapods — noted that their research was made possible by the remarkable discoveries at the Mazon Creek site and the amateur scientists who combed it for decades, a hobby that over the years led to the specimens analyzed in the paper.
“This paper, in a way, is a love letter to them, showing the power of what we can do by working with this community to put together really high-impact new research,” Mann told AFP.
