Artist’s concept of the Trans-Saharan Seaway.
Illustration: Carl Buell (AMNH)

The Sahara might seem like one of Earth’s most lifeless regions today, but its fossils show it was once a vast seaway filled with giant fish and some of the largest sea snakes the planet has ever seen.

From 100 million to 50 million years ago, a large seaway up to 160 feet deep covered much of West Africa, leaving behind lots of marine fossils, including vertebrates, invertebrates, plants, and microbes. Many of them were surprisingly large. It’s difficult to study this region due to harsh geopolitical and physical climates, so a group of scientists decided to compile and synthesize lots of existing research on the area. They paint a surprising picture of the ancient world during a time period that straddled the event 66 million years ago that put an end to large dinosaurs.

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“The K/Pg extinction is one of the largest mass extinction events,” study author Maureen O’Leary from Stony Brook University explained to Gizmodo. “We don’t have a section in the rock record to tie in the fossils at the species level and see these intricate changes on every continent.” Data from the Trans-Saharan Seaway could fill in some blanks about how the extinction played out in an understudied part of the world.

That the western Sahara was once a waterway has long been known to those who live there; the Tuareg people have found remnants of sea life in the desert, O’Leary told Gizmodo, and the idea has been part of the paleontological discourse for over a century. During warm periods between 50 million and 100 million years ago, rising sea levels caused a waterway to develop, probably connecting the Gulf of Guinea to the Mediterranean Sea. Scientists from the United States, Mali, and Australia have analyzed fossils and sediment from three expeditions to the Sahara in Mali, and have published a bulletin detailing everything they learned so far about the region, hoping to illustrate what the ecosystem looked like.

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The shallow bodies of water that covered the Sahara were likely similar to other large seas sitting on top of continents today, like the Caspian Sea, or to the shallow warm waters around some Caribbean islands. The seaway’s shores likely featured forests of mangroves, trees found today around the world on tropical and subtropical shores. The fossils they found were weird and often very large, including a 5-foot-long catfish, 40-foot-long sea snakes, and other large fish, as well as relatives of today’s crocodiles, sharks, and elephants, according to the synthesis of their findings in the Bulletin of the American Museum of Natural History

But why were the fish so big? There’s a theory called island gigantism in which species can grow extra large on islands because they’re protected from predators and have less competition for resources. Perhaps there’s a similar version in aquatic environments—as the seaway dried up, it might have left “islands” of water where similar evolutionary forces could have generated large snakes and fish.

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The region’s fossil history offers another place to study the K/Pg extinction event. The effects of the event weren’t quite as dramatic as O’Leary expected, she explained, with some of the species persisting both before and after the time of the extinction. But this paper’s purpose was more to lay the groundwork for future researchers to build upon when trying to see how the mass extinction unfolded around the world.

Much of the previous research on this area was published in French, so many paleontologists may not be familiar with this region and its sediments, O’Leary explained. But Mali is the site of ongoing conflicts, making it unsafe for scientists to perform field work.

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The research demonstrates just how complex an enterprise paleontology can be. It includes discoveries on the smallest scale—one of the researchers found a fossil invertebrate that had burrowed into previously fossilized poop, for example—as well as bigger-picture thinking about the history of the planet, beyond just dirt and bones.

It’s a lot to think about.

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