One of the earliest examples of two invertebrate species living together in a symbiotic relationship has been found in 520-million-year-old fossils from China.

The fossils, discovered by a team including researchers from the University of Leicester, show two species of marine worms with other, smaller worm-like animals attached to the outer surface of their body.

The study, published in Nature Ecology and Evolution, was produced by a group of scientists including The Natural History Museum, London, the University of Leicester and Yunnan University in China.

Symbiotic relationships, which involve two different kinds of organism interacting with close physical contact, are common in nature. However, few prehistoric examples involve soft-bodied animals because they are normally not fossilised.

Although fossils of the two species of marine worm, Cricocosmia jinnigensis and Mafangscolex sinensi, have been found before, these are the first reported examples to show other animals attached to them.

The smaller worm-like guests, a new species named Inquicus fellatus, are up to 3mm long and attached at their bottom ends to the stiff skin of their hosts, with their feeding ends pointing away.

Despite the fact that Inquicus fellatus are attached to their host worms, there is little indication they were feeding by penetrating the skin of their hosts, causing the authors to conclude it was unlikely the relationship was directly parasitic.

Sarah Gabbott, Professor of Palaeobiology from the University of Leicester's School of Geography, Geology and the Environment, said: "When we first saw the large worm curled around, almost hugging, lots of tiny worms, we suspected that we had uncovered an adult with offspring. But, careful inspection with a high-powered microscope, revealed that the large and small worms were different species -- so that theory was completely blown away, and we realized that a symbiotic relationship was most likely.

"We then asked ourselves -- was it parasitic or not -- were the small worms feeding on the large one? We could tell that it was always the posterior of the small worms, and not the mouth, that was attached so this was altogether a more 'neighborly' relationship."

Dr Greg Edgecombe from The Natural History Museum in London, a co-author on the study, says: "Evidence of symbiotic relationships are rare in the invertebrate fossil record, and this beautiful example shows how these associations began to develop as ecosystems became more complex in the Cambrian Period. But even beyond their scientific importance, what I find especially exciting about these fossils is that they give a pure snapshot of life and death hundreds of millions of years ago. It's a moment of animals interacting, frozen in the rock."

The specimens in the study come from the Cambrian Period, a time when Earth saw a rapid burst of evolution that led to the first appearance of most major groups of marine animals. The fossils show the earliest evidence of two aspects of symbiosis, firstly in the specific choice of host and secondly in the ability to shift its choice of host and colonise a new one.

Xiaoya Ma from Yunnan University and The Natural History Museum, co-author of the paper, said: "The symbiotic fossils' ability to demonstrate both host specification and host shift is particularly fascinating. Despite many other species of marine worms on the fossil bed, only Cricocosmia jinningensis and Mafangscolex sinensi were found to be acting as hosts. These two host worms were closely related and shared similar morphology and ecological niche, which might allow Inquicus fellatus to infest one of them initially and then also colonise the other. This compelling case of symbiosis is certain to inspire further discussions on the complexity of Cambrian ecosystems."