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One of the earliest examples of a symbiotic relationship between invertebrates has been found in 520-million-year-old fossils from China.
The fossils, featured in a study published in Nature Ecology & Evolution, show two species of marine worms, each with one to 15 smaller worm-like animals attached to them.
The specimens come from the Cambrian Period (541-485 million years ago). This is when Earth saw a rapid burst of evolution - life forms diversified and adapted to new ecological niches and began to interact with each other in more complex ways.
Symbiotic relationships are when two species interact closely over a prolonged time. They are common in nature.
However, there are few examples of symbiotic relationships between prehistoric soft-bodied invertebrates because the bodies are often lost during fossilisation.
'This beautiful example shows how these associations began to develop as ecosystems became more complex in the Cambrian Period,' says Museum scientist Dr Greg Edgecombe, who worked on the study.
The two species of marine worm, Cricocosmia jinningensis and Mafangscolex sinensi, were typically a few centimetres long. They lived at the bottom of shallow seas and were able to bury into the muddy sediment of the seafloor.
Although other fossils of these species have been found, these are the first reported examples to show other animals attached to them.
The smaller, worm-like guests have been given the Latin name Inquicus fellatus. They measure up to three millimetres long, shaped like a bowling pin with an elongated body that tapers to a slightly bulbous head.
The guests are attached at their bottom ends to the hosts' stiff skin, their feeding ends pointing outwards.
Although I. fellatus are attached to their host worms, the authors of the paper think it unlikely that the relationship was directly parasitic.
'The attachment disc of Inquicus fellatus does not seem to penetrate the host's cuticle [skin], its mouth faces away from the host and the creature appears to have been stiff, with no evidence that it could bend backwards to feed off its host,' says Dr Edgecome.
The guests may have simply found a good place to cling on to while browsing for food - or perhaps they just needed a ride.
However, some of the marine worms seem to have been infested, with one example of C. jinningensis carrying 15 I. fellatus organisms. This may have been detrimental to the worms' lifestyle.
Symbiotic relationships occur on a spectrum from parasitism (one species benefits at the expense of the other, such as a tick that sucks its host's blood) to mutualism (both organisms benefit, like when certain ants farm aphids for their honeydew while in return protecting them from predators).
In between these two extremes is commensalism, where one of the organisms benefits, while the other is affected, but only a little. For example, wingless flower mites hitch a ride on foraging bees or butterflies to travel to new flowers. The scientists who studied the fossils believe C. jinningensis and I. fellatus may have had a commensal relationship.
'But beyond their scientific importance, what I find especially exciting about these fossils is that they give sure a pure snapshot of life and death hundreds of millions of years ago,' adds Dr Edgecombe.
'It’s a moment of animals interacting, frozen in the rock.'