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An ancient worm that lived 518 million years ago has been identified as the first animal to evolve the loss of a body part no longer needed.
Living in muddy sediment, Facivermis secondarily lost its legs so that it could spend its life in tubes.
The small invertebrate known as Facivermis yunnanicus was an unusual creature.
With five pairs of spiny arms, it would likely have collected food particles drifting in the current. But the arms of these little animals were followed by an elongated, legless body that ended in a swollen, pear-shaped structure with three rows of hooks surrounding its anus.
The mystery surrounding these ancient worm-like creatures concerned what group they belonged to.
The existence of its five pairs of limbs and a long smooth stalk had led many to believe it was an intermediary between two other groups of invertebrates - the fully limbed lobopodians and the entirely legless cycloneuralians.
New beautifully preserved fossils of Facivermis collected in China have now revealed that it would once have lived in tubes on the sea floor, swaying gently in the current - not unlike what we see with tube worms today.
These fossils have confirmed that the worms were not actually an intermediary species, but in fact a full member of the lobopodians, and that Facivermis had lost a number of its hind limbs to better suit its tube-living existence.
These new fossils have been analysed by Richard Howard from the University of Exeter, along with colleagues from Yunnan University and the Museum. The results are published in Current Biology.
'A key piece of evidence was a fossil in which the lower portion of a Facivermis was surrounded by a tube,' explains Richard. 'We don't know the nature of the tube itself, but it shows the lower portion of the worm was anchored inside by a swollen rear end.
'This is the earliest known example of secondary loss - seen today in cases such as the loss of legs in snakes.'
The Cambrian Period (541-485.4 million years ago) was a time of incredible innovation in animal diversity.
It was during this period that the Cambrian explosion occurred. This was when most of the major groups of animals first started appearing in the fossil record as the different forms of life rapidly expanded. Lasting for just 20 million years, this event is considered one of the most crucial in the evolution of animals.
This new evidence from Facivermis fossils shows that it was also during this period that animals first started experimenting with losing features that they had already evolved.
'We generally view organisms evolving from simple to more complex body plans, but occasionally we see the opposite occurring,' says senior author Dr Xiaoya Ma. 'What excited us in this study is that even at this early stage of animal evolution, secondary loss modifications - and in this case, reverting back to lose some of its legs - had already occurred.'
What we know about the lifestyle of these animals can further be expanded by the exquisite preservation of the fossils coming from the Chengjiang Biota in Yunnan Province, south-west China.
Out of 30 fossils of Facivermis, two show evidence that these animals lived in tubes on the sea floor. This further supports their morphology, helping to explain the worms' curious appearance of a long, slim body that ended in a bulbous sac covered in hooks.
'Living like this in tubes, the animals' lower limbs would not have been useful and over time the species ceased to have them,' says Richard. 'Most of its relatives had three to nine sets of lower legs for walking, but our findings suggest Facivermis remained in place and used its upper limbs to filter food from the water.'
Dr Greg Edgecombe is a Merit Researcher at the Museum who focuses on the evolutionary history of invertebrates and the diversification of animals during the Cambrian explosion, and was a co-author on this latest paper.
'For several years we and others have been finding lobopodians from the Cambrian period with pairs of appendages along the length of the body - long, grasping ones in the front, and shorter, clawed ones in the back,' says Greg. 'But Facivermis takes this to the extreme, by completely reducing the posterior batch.'
This new data shows that some of these lobopodians were in fact highly derived, adapted for a lifestyle previously unknown from this group, in which the animals were neither walking nor crawling.