Just when you thought it was safe to go back in the swamp...

We tend to close the curtain on evolution after some of its most impressive acts. Small, fuzzy dinosaurs evolving their feathers and their flaps during the Jurassic embody one of our favorite transitions - even as we tend to forget that birds were then just one type of dinosaur among many until that ill-timed asteroid. We love a whale of a tale, too, but disproportionately focus on when snaggletoothed beasts were splashing around near shore over all the changes cetaceans have been through since they’ve committed to the life aquatic. And the same is true for one of our most cherished narratives, how our fishy ancestors started dragging their bellies across the mudflats and brought vertebrate life to the land.

So far as our self-interest goes, the consequences of fish with fingers clambering through ancient swamps seem relatively straightforward. Such a harsh environmental adjustment certainly required adaptive tweaks to the skeleton, lungs, eyes, and other body parts that culminated into something reptile-like, which is not too far from a fuzzy mammal. The story acknowledges the legacy of those fishy parts we see in our own skeletons. But of course tetrapods did not simply flee the water en masse once the first species dipped their toes out of the pool. Not only did ancestors and relatives of today’s amphibians find themselves quite at home among the lakes and swamps, but evolution spun off some early forms that hung on for tens of millions of years longer than anyone expected.

At a glance, the creature’s rose-colored bones look like those of a giant salamander. The creature may have lived very much like one, perpetually soaked as it swished through the swamps of prehistoric Namibia in search of smaller creatures to gobble up. But Gaiasia jennyae was not an amphibian as we know them today. The 280 million-year-old animal was something much more ancient, a form of tetrapod that was supposed to have gone extinct much earlier.

Estimated to grow to lengths exceeding eight feet, Gaiasia is among the largest early tetrapods yet found. Credit: Gabriel Lio

So far as paleontologist Claudia Marslcano and colleagues were able to determine in their description of the animal, Gaiasia was a tetrapod. It shares features with other ancient relatives of ours that shade across water and shore, such as Tiktaalik and Acanthostega. It’s closest relatives were elongated species with tiny limbs, suggesting that these tetrapods primarily lived in the water and were not especially adept even if they did venture onto land.

If Gaiasia were found in rocks about 307 million years old or so, its form wouldn’t be all that surprising. During that time, around the prehistoric equator, Earth’s waterways were brimming with early tetrapod species that were diverging into different forms. Some were becoming more terrestrial, but others found limbs just as useful for swimming as crawling. But Gaiasia is far younger than these relatives, a fish out of time.

Based on previous fossil finds, it seemed as if the earliest, fishier tetrapods were vanishing around 307 million years ago and were being replaced by forerunners of modern amphibians. And yet Gaiasia has turned up in rocks tens of millions of years younger than the supposed shift, around 280 million years old, living in a Permian habitat far distant from the Equator.

The nature of the fossil record sets us up for such revisions. Earth’s crust isn’t like a jawbreaker with nested, even layers recording all prehistory. Some ancient habitats were preserved while others were not, and to differing extents. We know quite a bit about what was going on around the ancient Equator 307 million years ago, for example, while we have relatively patchy knowledge of ancient life that lived in other places at the same time. Gaiasia is proof of the point, that even the best-represented areas are just a fraction of what once existed. A new fossil from a distant locality can always complicate what we expect.

Aside from being an impressive creature in its own right - an eight-foot, triangle-toothed fish that slid through ancient swamps - Gaiasia hints that the epilogue to the great tetrapod transition isn’t as straightforward as we thought. Early forms persisted for a very long time, spreading far from the warm waters of the Equator. What paleontologists previously viewed as a significant extinction that set up early amphibians and amniotes for success might instead represent a broader global reshuffle, not so much a replacement but a remix involving tetrapods all over Pangea. At the very least, there is now a fossil gap of more than 25 million years between Gaiasia and when such tetrapods were thought to have vanished, a vast pool of time to go searching for fish worth bragging about.

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