Fossilised faeces of a crocodile-like predator living 200 million years ago reveal the animal was infected with multiple parasite species. Evidence of ancient parasites is notoriously hard to find in the fossil record, so this discovery can help give us a picture of how they spread from species to species.

Parasites infect animals’ soft tissues, which rarely preserve well over time. So at Mahasarakham University in Thailand and his colleagues analysed a sample of fossilised dung, also called a coprolite, that was first unearthed in 2010 from the Huai Hin Lat Formation in north-eastern Thailand.

“I wanted to know what’s inside the coprolite, so I decided to cut it open and examine its internal structure,” says Nonsrirach.

The shape and contents of the faeces helped the researchers narrow down which creature it came from. They first photographed and measured the coprolite before hardening it with an epoxy resin. They then cut the log – 7 centimetres long and 2 centimetres thick – into thin, salami-like slices.

When the team examined the slides under the microscope, they found parasite eggs in a range of sizes and shapes trapped in the droppings. The eggs were mostly round and oval, and around the thickness of a human hair. The team suspects as many as six parasite species – including intestinal worms called nematodes from the order Ascaridida – are represented in the ancient faeces.

The researchers concluded that the excrement was probably left by an armoured, semi-aquatic reptile that looked like and lived similarly to a modern crocodile. “Considering that crocodiles appeared around 100 million years ago during the Late Cretaceous, it is likely that the coprolite came from a crocodile-like animal or one that co-evolved with crocodiles, such as phytosaurs,” says Nonsrirach.

Based on the remains of ancient plants and animals also found in the area, the researchers estimate the specimen is from the early Late Triassic Epoch, around 237 million to 208 million years ago. “This discovery is crucial for understanding the variety of parasites and how they interacted in ancient ecosystems,” says Nonsrirach. He suspects the animal ingested the parasites by feeding on infected fishes, amphibians or other reptiles.