Ancient Fossil of Giant Sea Creature Hints at Early Arthropod Evolution (VIDEO)

A giant, extinct sea creature may be a key piece of evidence for the early evolution of arthropods. The newly discovered fossil shows it had modified legs, gills on its back and a filter system for feeding.

The new species is called Aegirocassis benmoulae. It stretched to a staggering seven feet, which ranks it among one of the largest arthropods that ever lived. The fossil was found in southeastern Morocco and dates back nearly 480 million years.

"Aegirocassis is a truly remarkable looking creature," said Derek Briggs, co-author of the new study describing the animal, 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 have been the most species-rich and morphologically diverse animal group on Earth since their appearance in the fossil record about 530 million years ago. This group includes animals such as horseshoe crabs, scorpions, lobsters, butterflies, ants and beetles. Their success is largely due to how their bodies are constructed. They have a hard exoskeleton that is molted during growth, and their bodies and legs are made up of multiple segments. These segments can be modified for various purposes, which means these animals can adapt to a wide range of environments.

Modern arthropod legs have two branches, and each is highly modified to cater to a specific function on that leg. Understanding how these double-branched limbs first evolved has been a major question for scientists.

A long-extinct group of arthropods, called the anomalocaridids, is considered central to the answer. All of these creatures had a head with a pair of grasping appendages and a circular mouth surrounded by toothed plates. Their segmented bodies carried lateral flaps that could be used for swimming. While scientists believed that these animals had only one set of flaps per trunk segments, the new fossil reveals that the new species had two separate sets of flaps per segment.

The findings reveal a bit more about this ancient group. Not only does the fossil resolve the debate on where anomalocaridids belong on the arthropod tree, but also clears up problematic aspects of their anatomy.

The findings are published in the journal Nature.

Want to learn more? Check out the video below, courtesy of YouTube.

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