A group of British and Canadian palaeontologists have found fossils that show the earliest evidence of animal locomotion.
The team from the University of Oxford and Memorial University of Newfoundland, found fossilised trails left by Ediacarans, an enigmatic assemblage of soft-bodied creatures that lived 30 million years before modern animals evolved.
The find, in 565 million-year-old rocks at Mistaken Point in Newfoundland, Canada, appears in the current issue of the journal Geology.
The discovery of 70 fossilised trails, each about 5 to 17 centimetres long, is comparable to the kinds of marks left in the sea floor by modern animals like sea anemones, the researchers say.
Although they can't pin the trails to a specific creature, the discovery shows at least some of the Ediacarans were mobile, and hence must have had muscles.
Similarities in the trails to the modern-day anemone Urticina suggest the organisms that left the fossil traces may have had a muscular 'foot', the researchers say.
"This is exciting because it is the first evidence that creatures from this early period of Earth's history had muscles to allow them to move around, enabling them to hunt for food or escape adverse local conditions and, importantly, indicating that they were probably animals," says University of Oxford PhD student Alex Liu.
The Ediacarans are the earliest complex organisms before the Cambrian 'explosion of life' which marked the development of modern complex life.
But debate continues over just exactly what the Ediacarans looked like, or even what they were.
Stranger than fiction
"Some of the later species - particularly in Australia and the White Sea - do in my view seem to be early animals," says Lui. "But the morphological characteristics of earlier forms, [such as] those in Newfoundland, leave their biological affinities difficult to resolve at present - though we are working on it."
Professor Pat Vickers-Rich of Monash University in Melbourne, has recently been studying Ediacaran fossils in Namibia. She says palaeontologists originally thought the Ediacarans were jellyfish, worms and soft corals.
"Now we know they are so different to anything we know today," she says.
"Some of them were absorbers, absorbing their nutrients directly through the chemical environment with no mouth parts at all. Others, like Rangea had a kind of proboscis that grazed microbial mats."
Vickers-Rich says the Namibian fossils, from a locality called the Nama group in southern Namibia, represents the "last gasp of the Ediacaran fauna".
No one knows how the Ediacarans became extinct, but Rich hypothesises it may have been due to a build up of oxygen and changing oceanic chemistry, which may have favoured the new Cambrian animals.