New fossils reveal details about giant lobster ancestor

NEW HAVEN, Conn., March 11 (UPI) -- Scientists are quickly gaining a greater understanding of a giant, extinct sea creature known as Aegirocassis benmoulae, thanks to series of newly discovered fossils in Morocco.

At seven feet long, the newly discovered sea creature was one of the largest on Earth; it dates to roughly 480 million years ago. Named for its discoverer, an amateur Moroccan collector named Mohamed Ben Moula, the animal is part of a group of ancient sea creatures known as anomalocaridids.

Anomalocaridids, which first appear in the fossil record about 530 million years ago, are a group of arthropod ancestors characterized by a pair of grasping appendages extending from the head, a round mouth with toothed plates, and a long, segmented body with a single set of dorsal flaps used for swimming.

For several years, paleontologists have been trying to fill in the evolutionary gap between anomalocaridids and arthropods, the most species-rich and morphologically varied group of animals on the planet. The group -- which boasts horseshoe crabs, scorpions, spiders, lobsters, butterflies, ants and beetles -- is characterized by a hard exoskeleton, molted during growth, as well as segmented bodies and legs.

This setup allowed arthropods to adopt to all kinds of ecological circumstances, and is responsible for their diversity across the globe and evolutionary timeline.

In their most basic form, modern arthropod legs have two branches, each serving separate biological functions. None of this, however, explained the transition from the single dorsal fins of the anomalocaridids to the jointed legs of modern arthropod.

But the Aegirocassis benmoulae appears to provide an answer.

A close examination of the new fossils showed that the animal actually had two sets of dorsal flaps. The two separate appendage-like flaps serve as a period of transition between earlier anomalocaridids and the double-segmented limbs of modern arthropods.

"It was while cleaning the fossil that I noticed the second, dorsal set of flaps," lead study author Peter Van Roy, an associate research scientist at Yale, explained in a press release. "It's fair to say I was in shock at the discovery, and its implications. It once and for all resolves the debate on where anomalocaridids belong in the arthropod tree, and clears up one of the most problematic aspects of their anatomy."

The discovery prompted scientists to re-examine older anomalocaridids. In doing so, researchers located the same separate row of flaps on several other specimens.

Aegirocassis benmoulae also marks a branching off point from a dietary perspective, marking an ecological change in the seas. Whereas older anomalocaridids were active predators, pinching predators with their forward appendages, Aegirocassis benmoulae used its arms to funnel and filter plankton into its mouth. It was an ideal strategy for an ocean that was suddenly teeming with the tiny sea creatures.

"Giant filter-feeding sharks and whales arose at the time of a major plankton radiation, and Aegirocassis represents a much, much older example of this -- apparently overarching -- trend," Van Roy added.

The new study was published in the journal Nature.