Jurassic fossil of a parasitic worm fills evolutionary gap

A 165-million-year-old fossil discovered in China is shedding light on the origin of a bizarre group of parasites.

Scientists believe the fossil to be an early species of thorny-headed worm. These are endoparasitic organisms, which means they live inside a host animal. Their common name comes from the rows of backwards-facing hooks on a retractable organ called a proboscis, which they use to anchor their body to the gut wall of their hosts.

This newly described species, which has been named Juracanthocephalus daohugouensis, lived during the Jurassic Period. This makes it the oldest known specimen of thorny-headed worm ever discovered.

The important find is now helping scientists understand how these worms are related to other invertebrates and how they could have evolved from a non-parasitic ancestor into their current living forms.

Professor Edmund Jarzembowski, an associate scientist at the Natural History Museum and co-author of the study, says, “It was a totally unexpected find and amazingly fitted a gap in the evolutionary tree.”

“The thorny-headed worms are difficult to pin down biologically because they are so specialised and have lots of peculiar features which are so unlike other worms.”

Details of the discovery have been published in the journal Nature.

Why is this fossil important?

Because they live inside other animals, endoparasitic worms are rarely found in the fossil record. This means that little is known about their evolutionary history and relationships with other species.

The remains of Juracanthocephalus were discovered in the Daohugou Lagerstaette, a fossil deposit in Inner Mongolia in northeastern China. This site is known for its large number of remarkably well-preserved fossils of arthropods, vertebrates and plants that lived during the Jurassic Period.

The organisms found at Daohugou were buried rapidly by volcanic activity 165 million years ago, resulting in the exceptional preservation of many species.

“These thorny-headed worms are soft-bodied, so normally they easily decay, leaving no record,” says Edmund. “But our fossil was unusually preserved in the bottom of a Jurassic volcanic lake.”

“The worms are usually well-hidden inside their hosts, so finding one fossilised loose in ancient sediment is a very rare event.”

Thorny-headed worms are a large group of parasites that infect a wide range of hosts, including amphibians, invertebrates, fish, birds and mammals. They can sometimes be found in humans, so understanding more about them is also of medical interest.

More than 1,400 species of thorny-headed worms have so far been described, but much of our understanding of how these parasites evolved has remained a mystery due to a lack of fossilised specimens.

Scientists have now been able to study this early fossil in fine detail using instruments such as scanning electron microscopes and X-rays machines.

What has this fossil taught us about thorny-headed worms?

Based on some common features, thorny-headed worms have previously been considered close relatives to several different groups, including flatworms and penis worms.

Molecular analysis has also placed them as members of the rotifers, which are a group of microscopic aquatic animals. But as rotifers and thorny-headed worms have very different body plans, including the fact that the worms lack a mouth, their relationship was uncertain. Their lifestyles are also very different, with most rotifers being free-living organisms compared to the parasitic thorny-headed worms that rely on a host animal.

Analysis of Juracanthocephalus is now helping to fill in this evolutionary gap.

The fossil show distinct features characteristic of modern thorny-headed worms, such as their hooked proboscis and a bursa, which is a bell-shaped reproductive organ at the tail end. But unusually for these parasites, Juracanthocephalus also had solid jaws and signs of a digestive tract.

As living thorny-headed worms get all their food from their host, they have no mouth. This fossil therefore suggests that the worms may have evolved from a free-living ancestor.

As a jaw and digestive tract are common traits in the rotifers, this new discovery helps bridge the gap between the free-living rotifers and parasitic thorny-headed worms. The scientists finally have fossil evidence that the two groups may have shared a common ancestor.

But many questions about these animals still remain.

“Although this discovery was important, there is still a lot we don’t know about this animal,” says Edmund. “For instance, as it had jaws, did it still have a functional gut, unlike modern thorny-headed worms?”

“The specimen we found was male, so what was the female like? It’s a relatively big worm, so what were its hosts in Jurassic times? Perhaps they were dinosaurs? Not only do we need to look for more adults, but we also need to search for the tiny eggs in fossilised animal droppings. These discoveries could tell us so much more about these mysterious animals.”