This Bigheaded Fossil Turned Up in a Place No One Expected to Find It

Some 280 million years ago, a large predator glided through the chilly waters of a supercontinent in the Southern Hemisphere. The eight-foot-long hunter had tiny limbs, an eel-like body and a flat head full of jutting fangs. And according to existing ideas about vertebrate evolution, it shouldn’t have existed.

“It was displaced in time, displaced regionally and also far too big,” said Claudia Marsicano, a paleontologist at the University of Buenos Aires and an author of a paper describing the animal in the journal Nature on Wednesday. “There were a lot of things that made it unique.”

The giant salamander-like creature, which Dr. Marsicano and her colleagues named Gaiasia jennyae, could only have been a relic of a family thought to have been extinct for millions of years. Their finding might suggest that additional research on the emergence of tetrapods — vertebrates with four limbs and feet instead of fins — is in order.

Spencer Lucas, a paleontologist at the New Mexico Museum of Natural History and Science who was not involved in the study, said that the discovery “is not enough to force a rethink of most of what we think about late Paleozoic tetrapod evolution. But it is a nudge in that direction.”

Dr. Marsicano and her colleagues discovered Gaiasia fossils in 2014 and 2015 during successive expeditions to the brutal deserts of the Huab Basin of northwest Namibia in Southern Africa. They recovered fossils from four animals, including a pair of skulls and a nearly complete skeleton.

Piecing together the specimens, the team says that Gaiasia belonged to a family of bigheaded, swamp-dwelling vertebrates called colosteids. This family had split off from other land animals long before the ancestors of more modern lineages like amphibians, reptiles and mammals evolved.

Roughly 400 million years ago, colosteids and other early tetrapods evolved from fish amid equatorial jungles. Paleontologists’ understanding of the Paleozoic Era comes almost exclusively from the study of deposits found in North America and Europe, said Jason Pardo, a paleontologist at the Field Museum in Chicago and an author on the paper.

So when archaic tetrapods disappeared from those jungles 307 million years ago, researchers assumed they went extinct everywhere.

But Gaiasia turned up in rocks 20 million years later than expected, Dr. Marsicano said, early in the Permian Period. And while previously known members of the creatures’ family had skulls that could fit in a person’s hand, Dr. Marsicano said, Gaiasia’s skull reached two feet long or more, making it the largest animal of its kind ever found.

The environment that Gaiasia lived in was particularly intriguing. During the early Permian, what is today Namibia lay along the southern supercontinent of Gondwana, a landscape thick with walls of glaciers and cold, temperate forests like those found in Canada or Norway.

Gaiasia’s appearance in such a landscape would be like finding a crocodile living happily in a lake in Manitoba, Dr. Pardo said, “but this animal was certainly able to make a living and get quite large in that environment.”

In the jungles around the Equator, competition with big fish — and later, the larger relatives of modern amphibians — might have kept early tetrapods small, the team concluded. But these animals seem to have had better luck in the cool waterways in the shadow of Gondwana’s glaciers even as the Equator’s tropical forests splintered into dryer ecosystems full of finbacked predators like Dimetrodon.

Gaiasia and its habitat add an intriguing wrinkle to the story of tetrapod evolution, Dr. Marsicano said. Researchers have tended to assume that most tetrapods evolved in the tropics and lingered there, with more specialized groups spreading into cooler climates only around 280 million years ago. Gaiasia’s presence, on the other hand, suggests that its ancestors reached high latitudes before that time.

Early tetrapods “were more adapted to different types of environments from the beginning,” Dr. Marsicano said.

While the global story of tetrapod evolution is based largely on data from fossils found in North America and Europe, Dr. Pardo said, the discovery of Gaiasia highlights the importance of studying sites in South America and Africa, which have historically been marginalized in paleontology.

“Getting these faunas from the Southern Hemisphere is really critical to understanding the story of our own lineages,” Dr. Pardo said. “Especially the question of whether those are just local stories, as opposed to something happening on a global scale.”