Some of the biggest catfish and sea snakes to ever exist lived in what is today the Sahara desert, according to a new paper that contains the first reconstructions of extinct aquatic species from the ancient Trans-Saharan Seaway.

The sea was 50 metres deep and once covered 3,000sq km of what is now the world’s biggest sand desert. The marine sediment it left behind is filled with fossils, which allowed thescientists who published the study to build up a picture of a region that teemed with life.

Between 100m and 50m years ago, today’s arid, boulder-strewn northern Mali “looked more like modern Puerto Rico”; the sun shone on some of the earliest mangroves, and molluscs lined the shallow seabed, according to Maureen O’Leary, the palaeontologist who led the study.

The study also formally named the geological units, literally putting the area on the geological map for the first time, showing how the sea ebbed and flowed over its 50m years of existence, and building up information about the K-Pg boundary, the geophysical marker of one of Earth’s five major extinction events, in which the non-avian dinosaurs became extinct.

With 1.6m catfish, 12.3m sea snakes and 1.2m pycnodonts – a type of bony fish – O’Leary and the other scientists developed the idea that in the late Cretaceous and early Paleogene period, the animals were experiencing gigantism.

Evolutionary biologists have long talked about the phenomenon of island gigantism, where species that live on small islands can sometimes develop very large bodies, possibly because they have more resources or there are few predators, or both.

“We put out the idea that maybe this island gigantism can pertain to islands of water,” O’Leary said. “If you get a sea that’s moving in and going out, maybe it’s leaving behind pockets of water in which new variables are taking place that control the body size.”

Facebook Twitter Pinterest Reconstruction of sharks feeding on a dyrosaurid crocodilyform. Photograph: American Museum of Natural History 2019

Though considerably less watery and verdant today, there is no shortage of human life in the Sahara – people for whom the presence of ancient sea creatures is not news, as the team found during their expeditions in 1999, 2003 and 2009.

“The Sahara is full of people. Sometimes we would be working in what seemed like the most remote desert, and someone would just drive up on a moped. It’s a very alive environment,” said O’Leary, who is from Stony Brook University in the US. “The local people … knew that the sea had passed, and they would talk about the shells that they found and know that they’re marine shells.”

People in northern Mali have been living through a “security nightmare”, particularly since Tuareg rebels and then jihadis occupied it in 2012. While this has prevented O’Leary and the 10 other scientists – from Mali, the US and Australia – getting on the ground since 2009, it also forced them to compile their findings and come up with a comprehensive picture.

Facebook Twitter Pinterest Reconstruction of an extinct lungfish from the study of the Trans-Saharan Seaway. Photograph: American Museum of Natural History 2019

When setting off on their fourth expedition in 2009, the team got as far as Gao when they received calls urging them to abandon their trip. They did so, but it was only years later, reading the Canadian diplomat Robert Fowler’s book on being kidnapped with an aide by al-Qaida in the Islamic Maghreb, that O’Leary realised what the trigger for those calls had been.

She said significant paleontological riches were buried under the sands of northern Mali, and there was also extensive archaeological work to be done, but the security situation that had left so many Malians dead or displaced meant this work would be on hold indefinitely.

Facebook Twitter Pinterest Fossil teeth of chondrichthyes and reconstructions of the animals from which they came: shark, sawfish shark and electric ray. Photograph: American Museum of Natural History 2019

“This is such a terrible thing to happen to such an interesting country with such a rich legacy. I hope that by telling our scientific story, it gets the word out that there’s many important and interesting things to be known about Mali that are probably not that well understood. There’s a tremendous amount of scientific work to be done there and it’s impossible.”

The study builds on the work of British expeditions to Mali in the 1980s which, among other things, had discovered but not written about a large turtle shell and an important collection of fossils.

The collected fossils remain Mali’s property, but are on loan to the American Museum of Natural History for scientific study.

O’Leary said the fact that the Sahara was once under water showed there was a precedent for climate change and sea level rises that should give climate deniers pause for thought.

“Hopefully, by understanding these historical examples, in the current dialogue people can be more accepting that what scientists are telling them is true, and that not only is it true, but there are historical examples of much greater magnitude where the planet has changed,” she said.

“Perhaps talking about human-induced climate change seems like ‘Oh, is this the first time’. It’s not the first time. There’s been a lot of change in Earth’s history. And understanding those examples may make it more palatable to people, set their expectations differently.”