Never-before-seen fossils of newborn crocodile-like creatures are shedding new light on how our aquatic ancestors conquered land.
The early four-limbed vertebrates (tetrapods), which would eventually give rise to humans, first stepped onto land in the Devonian period, about 419 million to 359 million years ago, one of the most important periods in the evolutionary history of animals.
Now, a new study was published Thursday (June 18) in the journal Science have revealed that these early tetrapods were less like amphibians and more like us. Rather than having a tadpole stage in their development, many amphibian As they do today, new evidence suggests that they were direct developers – like their ancestors, humans and many other animals, moving from smaller to larger versions of themselves.
“This study is an important contribution to our understanding of the reproductive developmental biology of early tetrapods,” he said. tim smithsonA visiting academic at the University of Cambridge who specializes in early tetrapods but was not involved in the study.
This suggests that “the earliest tetrapods that took their first steps on land were able to rely on the successful reproductive and developmental strategies of their ancestors,” Smithson told Live Science in an email. “Direct development made life easier – one less thing to worry about!”
The new research was based partly on fossils of an early land-dwelling hunter called Ambolomer. These animals looked like a mix of crocodiles and eels and ruled river, lake and swamp habitats between 350 million and 280 million years ago during the Carboniferous and Permian periods. While these creatures can grow to more than 10 feet (3 meters) long as adults, the study revealed rare fossils from Mazon Creek near Chicago that preserve Embolomeres as hatchlings that ranged from a few days to a few weeks old.
“These are intimate details of the first moments of life for these animals, and we have never seen anything like this before in this entire part of the evolutionary tree,” said the study’s co-author. jason pardoa postdoctoral fellow in evolutionary biology at Vilnius University in Lithuania and a research associate at the Field Museum in Chicago, told Live Science.
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Researchers studied exceptionally well-preserved fossils from Mazon Creek, Illinois.
(Image credit: Arjan Maan)
These fossils did not show evidence of external gills and other tadpole-like features that researchers would have expected from an early land dweller. Researchers then examined other fossils before and during the “fin-to-limb transition”, but found no evidence of an amphibian-like life cycle.
“For as long as we’ve understood evolution, we’ve taken for granted the story of how we transitioned from water to land,” Pardo said. “Actually we have a completely different story.”
Science turned upside down?
A statement released by the Field Museum claimed that the study overturns scientists’ understanding of how animals conquered the land. However, experts Live Science spoke to disagreed with this claim.
“The Mazon Creek material is amazing, the study is interesting and the interpretation of the fossils is good, but I don’t think the results are very surprising,” per ahlberga professor of evolutionary biology at Uppsala University in Sweden told Live Science in an email.
Ahlberg, who was not involved in the new study, specializes in the early evolution of tetrapods. He said that scientists knew that some early tetrapods had a larval stage similar to that of modern salamanders – that is, they belonged to the Temnospondyli group, which he described as the ancestral stock of modern amphibians. However, he argues that this does not mean that scientists have assumed that every early tetrapod was alike.
“Nobody in recent years has been arguing that all early tetrapods had such a larval stage or that it was necessary to enable the transition to land,” Ahlberg said. “I mean, I’ve been working at the core of this research area for 40 years and I’ve never thought about it that way.”
In response, Pardo agreed that experts in the field recognized that the data did not support that amphibian-like evolution occurred in early tetrapods. However, he argued that even among experts, there were still misconceptions about metamorphosis – A major developmental change, such as a tadpole turning into a frog – and an amphibian-like body.
The young embolomeres illustrated here suggest that early tetrapods did not undergo amphibian-like metamorphosis.
(Image credit: Berit Godering)
“On something bigger”
Study co-authors earning valueassistant curator of early tetrapods at the Field Museum, first saw the study’s first infant embolomere fossil during a visit to the Field Museum in 2016 while working on her doctorate. At the time, the fossil was a mystery.
According to the museum’s statement, Mann and Pardo pondered the fossil’s identity for years before a high-resolution scan with scanning electron microscopy at the Canadian Museum of Nature confirmed that the ancient creature was an embolomer.
“I think Jason and I both knew we were onto something big, because fossils of animals like this and from this stage, and this developmental state in early tetrapod evolution, have never been found or studied before,” Mann told Live Science in an email.
Along with embolomeres, the researchers looked at megalichthyid fish before the land change and at limbless, snake-like creatures known as aestopods during the land change. All showed signs of visible growth, he said.
“I think the message of this study is that we should always challenge conventional wisdom in science, especially when these old ideas don’t have enough support,” Mann said.