For a long time, scientists believed that parasitic relationships between marine species developed much later in Earth’s history. But new fossil evidence from Morocco suggests otherwise. A recent study published in iScience found signs of parasitic worms living inside the shells of an ancient sea creature about 480 million years ago, during the Early Ordovician period. The fossils show that these worms drilled into the shells of a small mollusc and lived there, much like some worms do today. This discovery pushes back the timeline for when parasitism first evolved and gives scientists a clearer picture of how early marine ecosystems worked.
Where and how was the fossil found
The fossils were found in the Fezouata Shale formation in Morocco, one of the world’s most remarkable fossil sites. This site, which dates to the early part of the Ordovician period, is famous for its detailed preservation of marine life. It provides scientists with a rare look at the animals that lived soon after the Cambrian explosion, a major period of evolutionary change when many animal groups first appeared.In the Fezouata Shale, researchers studied 22 fossils of Babinka, a small clam-like bivalve that lived buried in seafloor mud. Bivalves are not common in this fossil deposit, which made the discovery even more interesting. Seven of these Babinka fossils showed strange, question mark-shaped marks on their shells. These markings matched those made by spionid worms, a type of small marine worm that still bores into shells today. The shape and pattern of the borings suggested they were not random scratches, but evidence of ancient parasitism.
Signs of ancient worms inside the shells
The researchers used high-resolution imaging, including micro-computed tomography scans, to study the fossils in detail. These scans revealed that the borings were filled with iron oxide, which had replaced the original shell material over time. This process, known as pyritisation, happens when bacteria feed on decaying organic matter and form iron sulphides, which later turn into iron oxides. As a result, the worms’ burrows were preserved even though the original shells dissolved long ago.The shape and position of the borings were very similar to those made by modern spionid worms. Today, these worms make small tunnels inside shells where they live and feed on organic matter. They often create mud-filled chambers as part of their activity. The fossils from Morocco show the same looping, curved burrows, suggesting that ancient worms behaved much like their modern relatives.This level of preservation also tells scientists about the environment in which these animals lived. The presence of sulphur-reducing bacteria and low oxygen levels indicates that the seafloor conditions helped preserve both the shells and the worm traces. Such environments were ideal for fossilisation, explaining why the Fezouata Shale holds so many well-preserved species.
Parasitism evolved earlier than scientists thought
Before this discovery, scientists believed that shell-boring parasites first appeared during the Devonian period, around 60 million years after the time these fossils formed. The new findings show that parasitic behaviour existed much earlier, in the Early Ordovician. This makes the Fezouata Shale fossils the earliest known example of parasitism in molluscs.Parasitism, when one organism benefits while the other is harmed, is one of the most complex forms of ecological interaction. The fossils suggest that even during the early stages of marine evolution, animals were already forming such specialised relationships. The researchers think these ancient worms might have been kleptoparasites, feeding on food particles or nutrients that their bivalve hosts collected, rather than harming the hosts directly.This discovery also changes how scientists view the evolution of marine ecosystems. It suggests that by 480 million years ago, ocean communities were already complex, with predators, prey, scavengers, and parasites all interacting in a network of life much like today’s seas.
How the fossils formed and survived
The Babinka fossils were preserved as internal moulds, the inner impressions of their shells, rather than as complete shells. Over time, the original shell material dissolved, leaving behind the shapes of the interiors. The worm tunnels were preserved because minerals filled them before the shells disappeared. This is why the borings are now visible as raised traces made of iron oxide.Some fossils showed simpler or incomplete borings, which could mean that the worms had only just started their burrowing, or that multiple worm species were present. Such variations are also seen in modern shellfish infected by different kinds of spionid worms. The researchers say this makes the Fezouata fossils especially valuable for understanding how parasitic behaviour developed and was preserved over millions of years.The team also noted that the chemistry of the Fezouata Shale played a key role in preserving such delicate details. The mineral-rich mud and the lack of oxygen slowed decay and allowed iron minerals to form around the burrows. This process captured a record of soft-bodied behaviour that would normally be lost to time.
What the fossils tell us about early marine life
Interestingly, while Babinka fossils with worm traces are rare, around one-third of the specimens examined showed signs of infestation. Other fossilised shell-bearing animals from the same rock layers, such as brachiopods, did not show any such marks. This suggests that the worms targeted Babinka specifically, showing a clear host–parasite relationship rather than a random occurrence.This pattern means that even in the early Ordovician seas, marine animals were forming selective relationships, much like in modern ecosystems. The worms may have lived attached to or inside their host shells, feeding safely while the bivalve continued its normal life.The discovery extends the known timeline for spionid-like worms and shows that the ancestors of modern marine parasites were already evolving sophisticated behaviours at a very early stage in Earth’s history. It also helps fill a major gap in our understanding of annelid evolution, the group of segmented worms that includes earthworms and leeches.By revealing the oldest known evidence of parasitism, the Fezouata Shale fossils offer a rare glimpse into how ancient ecosystems functioned. They show that even hundreds of millions of years ago, life had already developed the complex web of relationships, including parasitism, that still shapes the natural world today.Also Read | Why scientists are rethinking how gold was made: Magnetars may be the universe’s first cosmic goldsmiths