Giant Extinct Sea Creature Closes Evolutionary Gaps

Written by on March 13, 2015 in Marine Life

Reaching seven feet in length, recently discovered fossils of a giant, extinct sea creature are providing scientists with key information about the early evolution of arthropods — the most species-rich and morphologically diverse animal group on Earth.

Screenshot from Dr Peter Van Roy’s YouTube video describing Aegirocassis benmoulae.

Screenshot from Dr Peter Van Roy’s YouTube video describing Aegirocassis benmoulae.

The new animal, Aegirocassis benmoulae (named after its discoverer, Mohamed Ben Moula), had modified legs, gills on its back, and a filter system for feeding. It was found in southeastern Morocco and is about 480 million years old.

“Aegirocassis is a truly remarkable looking creature,”Yale University paleontologist Derek Briggs, co-author of the study, said in a news release. “We were excited to discover that it shows features that have not been observed in older Cambrian anomalocaridids — not one but two sets of swimming flaps along the trunk, representing a stage in the evolution of the two-branched limb, characteristic of modern arthropods such as shrimps.”

Arthropods first appear in the fossil record 530 million years ago and include familiar animals like horseshoe crabs, scorpions, lobsters, butterflies, and ants. All of these animals share a hard exoskeleton, as well as bodies and legs that are made up of multiple segments. Having multiple segments allows arthropods to adapt to many different environments, which has contributed to their success as a group throughout history.

Scientists have long studied arthropod legs, but understanding how they evolved to have two distinct branches has been difficult. One particular group, anomalocaridids, has been considered central to the answer. Until now, it was assumed that anomalocaridids, which are long-extinct, had lost their walking legs altogether and only had one set of flaps per trunk segment. However, the discovery of A benmoulae suggests a different story.

This animal represents a stage before upper and lower branches were fused into the double-branched limb of the arthropods that we know. The new fossils show that anomalocaridids had two separate sets of flaps per segment. The upper flaps were equivalent to the upper limb branch of modern arthropod legs, while the lower flaps were similar to walking limbs, adapted for swimming.

A benmoulae is interesting ecologically, as well. Most anomalocaridids were active predators that grabbed their prey. The new animal, however, had an intricate filter-feeding system that allowed it to passively harvest plankton from the water.

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About the Author

About the Author: Emily Tripp is the Publisher and Editor of She holds marine science and biology degrees from the University of Miami's Rosenstiel School of Marine and Atmospheric Science and a Master of Advanced Studies degree in Marine Biodiversity and Conservation from Scripps Institution of Oceanography. When she's not writing about marine science, she's probably running around outside or playing with her dog. .


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