Soft-bodied giants roamed oceans longer than thought

By Martin Leggett - 25 May 2011 17:0:1 GMT
Soft-bodied giants roamed oceans longer than thought

(Image Caption: Anomalocaridids had long, spiny head limbs presumably used to snag prey, and a series of blade-like filaments in segments across the animal's back, which scientists think might have functioned as gills.)

Image Credit: Esben Horn

Amazingly well-preserved fossils of monstrous soft-bodied animals - discovered in south-east Morocco last year - are recasting palaeontologists ideas about life in the oceans 480 million years ago.

The work, out in today's Nature, reveals that these predatory sea-creatures - known as anomalocaridids - survived 30 million years longer than scientists had previously thought, well into the Ordovician period. It serves as a reminder that the fossil record gives us a biased history of life's long journey, because soft-bodied organisms are only rarely preserved as fossils.

Anomalocaridids were bizarre sea-creatures, that could grow up to 6 feet (2m) in length, with a striking pair of spiky feelers, and a disc-like mouth. Because the jointed feelers - and the harder circular mouth - were often found separately from the rest of the body, until the 1980's their remains were classified as 3 different species.

It is thought that these large early predators rippled over the muddy sea-bottom, seeking out prey such as worms and trilobites, and perhaps scavenging too.

They first appear in the fossil record way back in the Cambrian period - a time stretching from 540 to 490 million years ago. That was when a massive evolutionary explosion populated the oceans with a diverse array of new complex animals. And the anomalocaridids may have been 'top dogs' of life's early scene.

But scientists believed they became extinct at the end of the Cambrian, which coincided with the rise of hard-bodied organisms, like the cephalopods. It now seems, though, that their absence in the Ordovician was more down to the lack of good conditions, for the preserving their soft tissues.

It is thought that the site in Morocco was witness to great sediment clouds, 480 million years ago, which quickly, but gently, buried the anomalocaridids - as well as other soft-bodied animals living on the sea-floor.

''The anomalocaridids are one of the most iconic groups of Cambrian animals,'' said Derek Briggs, from the Yale Peabody Museum of Natural History, and one of the authors of the study.

He went on ''These giant invertebrate predators and scavengers have come to symbolize the unfamiliar morphologies displayed by organisms that branched off early from lineages leading to modern marine animals, and then went extinct. Now we know that they died out much more recently than we thought.''