A lobster-like animal that was the biggest predator of its kind survived much longer and gota lot bigger than anyone thought.

Based on a set of fossils found in Morocco, paleontologists from Yale University found that anomalocaridids, survived 30 million years past the date when they were originally thought to have died out.

Anomalocaridids are arthropods, related to crabs, insects and spiders. The creatures appeared between 540 and 488 million years ago, in what is called the Cambrian explosion. The period is so named because at that time there was the sudden appearance of many major animal groups that exist today (and a few that no longer do).

Previously, scientists thought that the anomalocaridids hadn't survived past the end of the Cambrian, though during that 62 million-year span they diversified into many different types. The fossils that were found seemed to show that the biggest they ever got was about two feet long, but the new discovery is a creature that was about 3 feet. The fossil the Yale team found dates to the Ordovican, a period that followed the Cambrian which was also marked by a lot of diversification.

Anomalocaridids had two spiny, tentacle-like front limbs, with the front half of their bodies covered in a shell and the rear half segmented. They also had a series of blade-like filaments on their backs, which might have functioned as gills. They are rather like shrimp or lobsters, though they appear long before either one. Another major difference is modern crustaceans have gills that are on their undersides, rather than on their backs.

Most paleontologists think the two limbs would snag prey which was then drawn into a circular mouth that was lined with plates. It is something like a combination of a parrot's beak and a nutcracker, said Derek Briggs, director of the Yale Peabody Museum of Natural History and one of the lead researchers. It was probably one of the single largest predators in the world at the time - there were not yet any fish big enough to tackle it.

Anomalocaridids aren't ancestors to anything living, as they are a branch of the arthropod family that died out, much like the trilobites (who they shared the seas with).

The specimens are just part of a new trove of fossils that includes thousands of examples of soft-bodied marine fauna dating back to the early Ordovician period, 488 to 472 million years ago. Briggs said such finds are rare. Only about 40 percent of species have shells that fossilize, he said. Anomalocaridids - like modern lobsters and horseshoe crabs - had a shell made of chitin, which decays unless it gets buried in silt very quickly. The rest of the animals don't have anything like that. That, Briggs said, means a lot of the time you only get a very incomplete picture of what life was like millions of years ago. The animals found in Morocco inhabited a muddy sea floor, and were trapped by sediment that buried them and preserved their soft bodies.

The paper appears in the May 26 issue of the journal Nature.

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