Researchers are shedding light on the evolution of arthropods, thanks to the fossils of a 480-million-year-old giant sea creature. Named for Mohamed Ben Moula, who discovered it in southeastern Morocco, the Aegirocassis benmoulae was at least seven feet in size — making it one of the largest arthropods to have ever lived.

Yale paleontologist Derek Briggs co-authored a study on the "truly remarkable-looking creature" for the journal Nature, detailing the extinct animal's distinct characteristics.

"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," said Briggs.

The earliest fossils of arthropods date back 530 million years. Arthropods make up the most morphologically diverse and species-rich animal group on the planet, including creatures like butterflies, crabs, spiders, barnacles and beetles. They have thrived due to the structure of their bodies — hard exoskeletons that can be molted away during growth, and legs and bodies that can be segmented to suit separate functions.

Basic modern arthropods have legs with two branches. How their limbs evolved has always baffled researchers, but anomalocaridids — an extinct arthropod group — were considered key to solving the problem. Anomalocaridids look positively alien, and until now, were believed to have only a single set of flaps for every trunk segment and lack legs.

While cleaning Aegirocassis benmoulae fossils, Van Roy, an associate research scientist at Yale, noticed a second, dorsal set of flaps. This prompted the re-examination of other older anomalocaridids, and it was confirmed that they did in fact have two independent sets of flaps for every trunk segment. The upper flaps are identical to upper limb branches of today's arthropods, while the lower flaps are akin to modified limbs for walking with swimming capability.

Researchers also pointed out that the Aegirocassis benmoulae is notable in its dissimilarity to other anomalocaridids, which actively preyed on other organisms, grabbing victims with spiny limbs on their heads. By contrast, the Aegirocassis had a filter-feeding part that let it consume plankton from oceans.

The results of the study place anomalocaridids in the stage of arthropod evolution prior to the fusion that produced the double-branched limb of their descendants. 

Allison Daley from the University of Oxford also contributed to the study.

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