This giant tortoise was chomped by an ancient mystery croc

The Aldabra giant tortoise has a lot in common with elephants. On the islands they call home, these reptiles are the largest land animals around – up to 250kg (550lbs) – and serve as the ecosystem's top herbivores, maintaining the unique local vegetation. Also like elephants, the tortoises are so large and tough that few predators can threaten them at full size. New fossils from the Aldabra Atoll, however, reveal that it wasn't always this way. The tortoises of the past had an ancient predator to watch out for: crocodiles.

Image: David Stanley/Flickr

The Aldabra Atoll consists of several coral islands in the Seychelles and is currently home to around 100,000 giant tortoises. After a long history of overhunting and human-induced environmental change, the big reptiles are classified as Vulnerable by the IUCN (another thing they share with elephants, unfortunately), but the atoll has long been a UNESCO World Heritage Site, aiming to protect the tortoises and their habitats.

A recent fossil-hunting expedition to the area, led by Dennis Hansen of the University of Zurich, produced nearly 200 new fossils from at least 90,000 years ago, including many bits of bone and shell from giant tortoises. Intriguingly, several of these remains had conspicuous markings on them – pits, gouges and scratches – which Hansen and other scientists were able to identify as the characteristic signatures left behind by the unique teeth and feeding style of a hungry crocodilian.

The shells were covered by pits, gouges and scratches – evidence of chomping by an ancient mystery croc. Image: Royal Society Open Science (2018)

"The absence of healing on the tortoise shells would indicate that the animal did not survive the encounter with the crocodilian," the palaeontologists wrote in their study. So, either this was a case of scavenging on a carcass, or some croc successfully attacked and killed this big tortoise.

Although there are no crocodiles living alongside the tortoises today, 90,000 years ago the atoll was home to a now-extinct croc called Aldabrachampsus. Bones of this species were found alongside the tortoise fossils Hansen's team collected, but from what we know so far, these were small crocs, and the scientists aren't sure "whether a crocodilian of 2-2.5m (6.5-8ft) length would attack a fully grown giant tortoise with straight carapace lengths approaching or exceeding 1m (3ft)."

Hansen and company might have found evidence of the true culprit, however. Among the croc fossils they examined, a handful of jaw and skull bones were unusually thick and large. Based on the size of these few fossils, the scientists suspect there was another – larger – croc present at that time, as big as 3.5 metres (11.5 feet) in length.

Unfortunately, there aren't enough remains from this bigger croc to identify it yet. Maybe it was simply a big honkin' Aldabrachampsus. Or it could have been a different species – saltwater crocodiles were known to live in the atoll until as recently as the early 1800s, and there's always the possibility that there were other (now extinct) crocs around back then.

For now, the perpetrator and circumstances of this prehistoric murder mystery remain unknown, but these fossils reveal a predator-prey dynamic in the Aldabra Atoll that doesn't exist there today, one of many ways these islands have changed over the millennia. Other islands have had similar ecological shifts – there's evidence, for example, that Cuban crocs lived alongside giant tortoises in the Bahamas until only 1,000 years ago.

Crocodilians chomping on turtles is nothing new. In fact, it's a bit of a long-standing tradition. The most ancient examples go all the way back to the Cretaceous Period, more than 70 million years ago, where bite marks on sea turtle shells have been attributed to the super-giant gator cousin Deinosuchus (which also chomped on dinosaurs).