For decades, biologists thought that early tetrapods, ancient vertebrates that started conquering the land over 300 million years ago, developed like modern amphibians—beginning their lives as purely aquatic tadpoles and then metamorphosing into terrestrial adults. “A lot of that comes from this old ‘scala naturae’ idea that you had fish that evolved into the next stage up, which were amphibians, and then amphibians evolved into the next stage up, which were reptiles that evolved into birds and mammals,” said Jason Pardo, a research associate at the Field Museum.
We’ve never had evidence that early tetrapods had an amphibian lifestyle; we have assumed it because it made intuitive sense. “It’s easier to make the transition from water to land if you’re already making that transition as part of your life cycle,” Pardo said. But now, a new Science study that Pardo co-authored with Arjan Mann (the Field Museum's assistant curator of early tetrapods) shows our most basic assumptions about the first tetrapods that started living on land might be wrong.
Baby monsters
The researchers' study focused mainly on embolomers, an extinct group of large predators that lived roughly 300 million years ago. Embolomers looked like a cross between a crocodile and an eel, with large skulls full of sharp teeth, followed by long, eel-like bodies. It had short, stocky limbs adapted mainly for paddling in water, but also capable of powering brief, clumsy excursions on land. They are thought to be one of the first vertebrates that made a partial transition from an aquatic to a terrestrial lifestyle. These animals could reach over three meters in length, but to understand the very beginning of their life cycle, scientists focused on examining some of their centimeter-scale babies.