Fossil of Ancient 'Sea Monster' Sheds Light on Evolution of Arthropods

 / Updated  / Source: Live Science

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A 480-million-year-old fossil is giving paleontologists new insights into a sea monster called an anomalocaridid, which is an ancestor of modern-day arthropods such as lobsters and scorpions.

The 7-foot (2-meter) long fossil reveals that the extinct giant had two sets of legs — not one, as researchers previously thought. It also had a filter-feeding system that likely allowed it to consume plankton, the researchers found.

The researchers named the species Aegirocassis benmoulae after its discoverer, Mohamed Ben Moula, who found the fossil in southeastern Morocco in 2011. [See photos of anomalocaridid fossils and illustrations]

Image: An illustration of the anomalocaridid, a giant filter feeder that lived about 480 million years ago
An illustration of the anomalocaridid (Aegirocassis benmoulae), a giant filter feeder that fed on plankton and lived in the Early Ordovician about 480 million years ago. The animal measured about 7 feet (2 meters) in length, and is one of the largest arthropods that ever lived.Marianne Collins, ArtofFact

The fossil was "dirty and dusty" when the study's lead researcher Peter Van Roy, a paleontologist at Yale University, got it into the lab. Van Roy was cleaning the specimen when he realized it had two sets of flaps on each body segment — indicating that the creature had two sets of legs.

"I was totally shocked" to see the two sets of legs, Van Roy told LiveScience. "For a week on end, I actually went back to the specimen every day just to look at it again, to make sure that I wasn't seeing things."

The fossil helped researchers place the anomalocaridid within the arthropod family tree, because it gave researchers an unfettered view of the beast whose anatomy has stumped paleontologists for ages, he said.

Researchers first identified anomalocaridid fossils in the 19th century, but the creature is so odd-looking — with a whalelike head, bristly appendages and segmented body covered in flaps — that some people thought the fossilized body parts belonged to several different animals instead of just one, Van Roy said.

Researchers finally pieced the animal together in a 1985 study published in the journal Philosophical Transactions B. But parts of its anatomy remained a mystery.

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