Furrows preserved in 565-million-year-old rocks are now the first evidence that some of Earth's earliest and mysterious living things had muscles to move themselves -- and so were truly animals.
That means muscles may have evolved earlier and been part of a long evolutionary fuse that led to the so-called "Cambrian Explosion," 30 million years later, of the many lineages of marine animals still found in the oceans today.
"If you saw (these furrows) in rocks the same age as dinosaurs, you'd say it was something crawling along the sea floor," said Duncan McIlroy of Memorial University in Newfoundland, Canada. "But when it's this old you have to be very cautious about it."
The so-called Ediacarans are the earliest complex organisms before the Cambrian Explosion, and debate continues over what the Ediacarans looked like and even, what they were.
When McIlroy's doctoral student Alex Liu sent him pictures of the furrows he'd found in Precambrian, deep-sea rocks near Mistaken Point, Newfoundland, McIlroy was pretty certain they were from an animal. But because they were from rocks dated to a time before true animals are thought to have evolved, it required a bit more work to make the case, he told Discovery News.
So McIlroy, Liu and Martin Brasier carefully reviewed all the things known to paleontologists which could possibly create track marks -- both living and non-living. In the end, they found themselves back where they started. The researchers' careful analysis appears in the February issue of the journal Geology.
"It needs to have had muscles to push and create the ridges in the tracks," McIlroy explained, referring to the fine, crescent-shaped structures in the furrows. So it had to be some kind of animal.
To further test this idea, the researchers went so far as to grind up rocks from the same location and recreate the mud of the 565-million-year-old seabed. Then they dropped a modern sea anemone onto it. The result was a similar trail, McIlroy said, made by the anemone moving about, trying to find a firmer, rockier place to settle down.
"The other question that remains to be answered is who is making these," said trace fossil expert Whitey Hagadorn of Amherst College. At the moment there are no candidates among the strange and mysterious soft-bodied creatures that inhabited the seas of the Ediacaran period, just before the Cambrian.
The great irony of the discovery, said Hagadorn, is that it was made at a location and in rocks that have been studied by literally thousands of geologists for more than a century.
"This stuff happens all the time in geology," Hagadorn told Discovery News. "Until you know what to look for, you won't see it."
Now that the discovery has been made, it would not be surprising if other people start recognizing similar features in other Ediacaran rocks elsewhere.
"That will be the proof of the pudding," agreed McIlroy, "if others start finding other trails."