One of the greatest transitions in evolutionary history was the emergence of tetrapods, or four-legged vertebrates, onto land. By about 340 million years ago, fins had become fingers and limbs, shoulder and hip joints had changed to bear weight on land, and an entire array of amphibious creatures had begun to live along the water’s edge. But an analysis of some early tetrapods now suggests that not long after they made a home on land, some species became adapted to life in the water all over again.
Aja Mia Carter at the University of Pennsylvania and colleagues focused on a group of early amphibians called temnospondyls, roughly salamander-like tetrapods that spun off a great diversity of species between 330 and 295 million years ago.
Rather than looking at the limbs of these animals, though, Carter’s team analysed the backbone anatomy of over a dozen temnospondyl species. They also used a previously published evolutionary tree to understand how these species were inter-related, and searched the scientific literature for information on the likely lifestyles of each species – in particular whether that was either more aquatic or terrestrial.
Temnospondyls, Carter and colleagues found, most likely evolved from a land-dwelling ancestor. Surprisingly, from there, some species changed course and became adapted to life in the water all over again in an evolutionary reversal.
The analysis also revealed that relatively stiff backbones were not an adaptation to life on land. Researchers have typically assumed that early land animals evolved a stiffer spine to help support their bodies, but it was actually the water-dwelling temnospondyls that had a more rigid spine.
“I was stunned to see that between individual vertebrae, aquatic species were stiffer than terrestrial species,” Carter says. In other words, a stiffened spine was not essential for these early amphibians to walk on land.
“This research is resetting how we think about locomotion in [early] amphibians,” says Julia McHugh at the Museums of Western Colorado.
The study suggests temnospondyl backbone anatomy is a good predictor of the complicated evolutionary history of these early four-legged animals. It can help establish whether a species lived on land but had an aquatic ancestor, or lived in water but had a land-based ancestor.
“This study is the best of modern science,” McHugh says, putting longstanding ideas to the test.
Journal reference: PLoS One, DOI: 10.1371/journal.pone.0251983
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