Giant Sea Creature Sheds Light on Arthropod Evolution

Newly discovered fossils of a giant, ancient sea creature dating back 480 million years ago is helping to shed light on the evolution of arthropods, according to new research.

The long-extinct monster, named Aegirocassis benmoulae, measured at least seven feet long (2 meters), making it one of the largest arthropods that ever lived. But despite its size, the findings indicate that A. benmoulae was actually a gentle giant.

"Its feeding appendages [were] built for filtering plankton, not grasping prey. This is in contrast to older [anomalocaridid] species, some of which are interpreted to be the apex predators of their time," John Paterson, an associate professor of paleontology at the University of New England in Australia, who was not involved in the study, told Live Science.

Anomalocaridids are a group of arthropods that haven't been around for some time now, the youngest one living around 480 million years ago, and are key to better understanding arthropod evolution.

Arthropods in general are the most diverse and species-rich group of animals on Earth, made up of many familiar, modern-day creatures such as horseshoe crabs, scorpions, spiders, lobsters, butterflies, ants, and beetles. The reason arthropods - which have been around since 5320 million years ago - have stuck around for so long is due in large part to the way their bodies are constructed.

Interestingly, they have not one but two sets of legs on each of its body segments that are modified for different purposes, whether it's locomotion, sensing its surroundings, respiration, or copulation. This allows them to adapt to different environments. However, how these unique double-branched limbs evolved has largely remained a mystery - that is, until now.

The recent discovery of A. benmoulae in southeastern Morocco, with its modified legs, gills on its back, and filter system for feeding, is helping to bridge the gap in arthropod evolution.

"Aegirocassis is a truly remarkable looking creature," paleontologist and co-author of the study, Derek Briggs, from Yale University, said in a statement. "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."

The upper flaps were equivalent to the upper limb branch of modern arthropods, while the lower flaps represent modified walking limbs, adapted for swimming. What's more, it turns out these flaps were present in older anomalocaridids, but scientists just hadn't realized it.

These findings, reported in the journal Nature, show that anomalocaridids represent a stage before the fusion of the upper and lower branches into the double-branched limb of modern arthopods.