About 480 million years ago, the seven-foot-long Aegirocassis benmoulae swam about in the sea. The lobster-like sea creature used its flaps — swimming devices on its belly and back — and long segmented body to get around. And unlike many of its fellow relatives, it ate plankton. This means that it had to swim with its appendages outstretched in front of its head, dragging a net-like "sieve" made of long downward-directed spines through the water.

"It would have dwarfed anything else at the time, being twice as big as the next biggest animal — at the very least," says Peter Van Roy, a paleontologist at Yale University and a co-author of a Nature study published today that contributes entirely new information about Aegirocassis benmoulae. "They were absolutely massive."

illustration by Marianne Collins, ArtofFact

A lot of what you’ve just read wasn’t known until recently. When the Aegirocassis benmoulae's relatives — a group of marine animals called anomalocaridids — were first discovered, paleontologists looked their spiny appendages and thought they represented the body of a shrimp. Some also believed that their toothed mouths were jellyfish, and their complete bodies were actually described as sea cucumbers.

Then, in 1985, a group of researchers figured it out. The bits and pieces that people were associating with various other species actually belonged to a single group of early marine animals called anomalocardids — large sea creatures that are ancestors to all arthropods, a category of animals composed of everything from lobsters and crabs to scorpions and ants. "Nevertheless, the affinities of anomalocaridids remained enigmatic: it would take almost another decade before they were finally recognized as arthropods," Van Roy says.

In 2011, things started coming together. Before that, paleontologists thought that Aegirocassis benmoulae was a predatory animal, much like the majority of anomalocaridids that had come before it. But the man who found the Aegirocassis benmoulae fossils — a local collector in Morocco by the name of Mohamed Ben Moula who doesn’t have paleontology training — changed all that. "With a big smile, he walks over to me and says, ‘you know, I have something to show you,’" Van Roy says. Ben Moula showed him a beautifully preserved appendage with "very delicate filter-feeding morphology; it had all these fine spines," Van Roy says. "It was shocking."

To show their gratitude, Van Roy and his team named Aegirocassis benmoulae after the Moroccan collector.

In the time of the Aegirocassis benmoulae, during the the Ordovician period, plankton ecosystems were changing and many new species of plankton were popping up. This likely spurred the emergence of this large plankton-eating anomalocaridid. And going from predation to plankton-filtering is part of a larger evolutionary theme, Van Roy says. "Exactly the same thing happened much, much later in sharks and whales," he says. "They started out as predators, and because of a major plankton diversification suddenly you get extremely big filter-feeding animals, like whale sharks and blue whales." Researchers also think Crocodiles went through a similar period in which a large plankton-eating species of crocs emerged. "They reached massive sizes," he says. "And our animal is the oldest example of this."

The researchers also had no idea that Aegirocassis benmoulae — or any of other anomalocaridids, for that matter — had ventral flaps. No one thought they had fin-like swimming devices on their bellies, Van Roy says. "They were believed to lack limbs on the body." This might seem like a small oversight, but it’s actually quite significant from an evolutionary standpoint; it’s the missing link that explains how some crustaceans — like shrimp and lobsters — ended up with limbs that have two branches. These branches are involved in everything from gas exchange, to swimming, walking and grasping, and transferring sperm.

Biramous limbs, Zhang, X.-l., et al. (2007)

The fact that anomalocaridids had two sets of flaps — instead of just one set on their backs — supports the idea that the two-branched limbs we see in most crustaceans arose from the fusion of the dorsal and ventral flaps. "Because these structures are not yet joined together at their base in anomalocaridids, this confirms that anomalocaridids represent a very early stage in the evolution of arthropods," Van Roy says. It's one of the most exciting discoveries he has ever made.

"When I discovered these flaps, the implications for the evolution of limbs and for anomalocaridids… I thought ‘jeez, is this really true?,'" Van Roy says. "And so for weeks, I went back to the fossils every day just to make sure that ‘yes the flaps are there, I’m not being crazy, I’m not seeing things.’"

He wasn’t crazy or seeing things. Eventually Van Roy and his team of researchers looked at other, older anomalocaridid fossils and discovered that the flaps had simply been overlooked. "When we went back [to the older fossils], we saw that they actually also had this second set of flaps."

It’s important to note that although Aegirocassis benmoulae was the largest animal on Earth at the time of existence, it isn’t the largest arthropod to ever exist. That title goes to the sea scorpion. It was eight feet long.

Sea Scorpion. [Credit: Ryan Somma/ FLickr]

Now that the researchers have published their findings, Van Roy says that he plans to move on to other types of arthropods. But he doesn’t think he will leave anomalocaridids behind; there is far too much knowledge to gain from them. "There’s still new material to study, and much more to learn from the specimens that have already been studied — this study isn’t finished by a long stretch."

Update: This story has been changed to more accurately refer to Van Roy as a paleontologist.

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