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If living long and prospering is a measure of success, then the arthropods are life's winners. These are the most common form of life: insects, spiders, crustaceans and centipedes, to name but a few.

And now scientists have their hands on the remains of one of the first ever. It lived 480 million years ago, and it was big and strange.

The fossil was discovered by a Moroccan collector, Ou Said Ben Moula. He gave it several years ago to scientists who spent hundreds of hours scraping away its rocky casing. The "thing" that emerged is ... well ... a man-sized, swimming centipede? A 7-foot lobster without claws?

"It is one of the very biggest arthropods that ever existed," says Yale paleontologist Peter Van Roy. In fact, he says, it was the biggest animal of any kind on the planet, at the time.

Van Roy spent 500 hours preparing the fossilized creature. It's called an anomalocaridid, he says, and evolved at a special time during the Ordovician geological period. Scientists call this period, when the variety of life forms in the ocean exploded, the Great Ordovician Biological Diversification Event.

"(It was) the biggest diversification in marine animal life that we've ever known," says Van Roy, and it took place across 25 million years. The "diversification" turned out to be a bonanza for this creature, because a lot of this new life was plankton. Up until then, anomalocaridids were smaller. This version (Aegirocassis benmoulae) evolved a way to eat the plankton. It developed a comb-like appendage to scoop up the tiny creatures, the way whales do now. Van Roy also discovered that the creature had developed pairs of flaps on its body that later evolved into arthropod limbs.

Becoming a filter feeder did pose some risks, he says.

"If you're filter-feeding, of course, you probably are not going to be able to defend yourself," Van Roy observes. "You're not going to have, like, big fangs or anything. So, one way of escaping from predation is just by growing so massive that there's ... simply nothing else that can tackle you." The creature was, he says, three times as big as anything else alive at the time.

Van Roy describes the creature in this week's issue of the journal Nature and credits Ben Moula with finding it. It was one of the first mother arthropods, Van Roy says, and its progeny changed our world.

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