Researchers have revealed remarkably well preserved fossils of soft-bodied marine creatures that are between 470 and 480 million years old.
Prior to this find, scientists were unsure whether such creatures died out in an extinction event during an earlier period known as the Cambrian.
The fossils were preserved in rocks formed by layers of ancient marine mud in south-eastern Morocco.
They are described in the latest issue of the journal Nature.
The research team that studied the fossils described them as marine animals that lived during the early part of a period that followed the Cambrian, known as the Ordovician.
Professor Derek Briggs from Yale University in New Haven, US, who was an author of the study, told BBC News that the discovery provided "a much more complete record of early marine life than we’ve every had before".
The creatures, he explained, closely matched those found in the Burgess Shale of British Columbia, a locality in Yoho National Park, which is famous for yielding rare fossils of soft-bodied marine creatures from the Middle Cambrian period.
"There was an anomaly in the fossil record," said Dr Peter Van Roy, the lead researcher on the study, who is also based at Yale University. "Most of these animals just seemed to disappear at the end of the Middle Cambrian."
The transition between the Cambrian and the Ordovician periods is crucial in evolutionary history.
The "Cambrian explosion" saw the sudden appearance of all the major animal groups. It was followed by the "great Ordovician biodiversification event" when the number of marine animal groups increased exponentially over a period of 25 million years.
Professor Briggs explained: "[These specimens have] shown that some of the organisms that we thought were exclusive to Cambrian actually persisted until the Ordovician."
Dr Jean-Bernard Caron, a palaeontologist from the Royal Ontario Museum in Canada who was not involved in this study, told BBC News that the discovery was "very exciting".
He said that the fossil record had never before demonstrated that certain lineages of Cambrian animals survived until this later period.
Preserving life’s record
The specimens show that poor fossil preservation, rather than mass extinction, was probably responsible for this gap in the fossil record.
Because hard shells fossilise, and are therefore more readily preserved than soft tissue, scientists had an incomplete and biased view of the marine life that existed during the Ordovician.
But the conditions at this Moroccan site, Professor Briggs explained, were special.
"Very thick" marine muds, he said, were laid down in the deep ocean, trapping the creatures’ bodies below the influence of storms.
These mud layers also excluded oxygen, creating conditions conducive to forming some of the minerals in which fossils are preserved.
Dr Van Roy, who has been working at this site in for around a decade, discovered this particular group of fossils just one year ago.
But he expects to find even more and he and his team have planned further expeditions to Morocco. "We’re only scratching the surface," he said.
"I’m certain there will be more spectacular fossils coming out of this site in the near future."
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