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What Sea Monsters Looked Like Half a Billion Years Ago

Even in the earliest epochs of life on Earth, filter-feeders were huge.
​Reconstruction of Aegirocassis benmoulae. Image: Marianne Collins, ArtofFact

Paleontologists have discovered the remains of a giant oceanic creature named Aegirocassis benmoulae, which lived an astonishing 480 million years ago. A detailed analysis of the ancient seafarer, which belongs to the extinct group Anomalocarididae, appears today in Nature.

"They were the largest animals during the Cambrian and Ordovician," Yale paleontologist Peter Van Roy, the lead author of the paper, told me. "In fact, Aegirocassis benmoulae is one of the largest arthropods to have ever lived, reaching a length of at least 2.1 meters."

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"It would have dwarfed anything in the Ordovician seas, and is much bigger than any arthropod alive today," said Van Roy. Arthropods are an incredibly diverse group of invertebrates comprising some 80 percent of described species today, including spiders and insects.

Reconstruction of Aegirocassis benmoulae. Image: Marianne Collins, ArtofFact

Most anomalocaridids were active carnivores, but Aegirocassis reached its enormous size by feeding on the tiniest prey in the oceans: plankton. Like so many other evolutionary Goliaths, including the all-time heavyweight champion known as the blue whale, these animals gained its bulk as a filter-feeder.

Given the sheer number fossils these animals left behind, the strategy seems to have been a very successful one. In addition, the new specimens are clear proof that the Ordovician world supported an abundant plankton, capable of supporting this thriving population of filter-feeders.

"From an ecological point of view, the presence of giant filter-feeding anomalocaridids indicates that planktic ecosystems some 480 million years ago were already very rich, highly developed and complex," said Van Roy. "It takes a lot of food to sustain such a large animal, and even more to sustain a population of these animals."

The filter-feeding appendage of the Aegirocassis fossils. Image: Peter Van Roy

But these animals are extraordinary for reasons beyond size and success. The rise of these primordial plankton-eaters foreshadows the development of other giant filter-feeders down the evolutionary line.

"That is one of the interesting points of our study," Van Roy said. "There seems to be an overarching trend of giant filter-feeders evolving from predators at the time of a major plankton diversification. Our anomalocaridid represents the oldest example of that overarching trend."

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That's all without getting into the most obvious feature that sets anomalocaridids apart: its utterly bizarre morphology. Though these filter-feeders occupied a familiar ecological niche, there is something inherently alien about their appearance. Their heads sported a large, net-like appendage for trapping plankton, and their bodies were adorned with idiosyncratic ventral and dorsal flaps. This latter feature has clarified longstanding mysteries about arthropod evolution.

"Until now, it had been believed that anomalocaridids had only one set of flaps per trunk segment," Van Roy told me. This difference has distinguished the family from most other arthropods, which have two flaps that are each specialized for a particular function such as locomotion, respiration, copulation, or ion exchange, among others.

The flaps, visible at the top of the image, were preserved three-dimensionally. Image: Peter Van Roy

Using the new specimens, however, Van Roy and his colleagues have overturned the assumption that anomalocaridids were single-flapped, and thus recontextualized the history or arthropod evolution.

"The fact that the flaps are still separate in anomalocaridids indicates that classical branched arthropod limbs only arose later, through the fusion of both structures," Van Roy said. "This confirms that anomalocaridids represent a very early stage in arthropod evolution."

As is the case with so many fossil beds, they never would have been preserved in such gorgeous condition if not for an unusually disruptive event on the ancient seafloor—a storm, for example.

"The preservation of soft tissues is exceptional, and requires some very specific conditions to be met," Van Roy said. "These conditions are only met extremely rarely, and hence soft tissue preservation is very rare."

"Our anomalocaridids are even more exceptional because they are preserved in their original three dimensions—all specimens known previously were flattened," he added.

It's truly amazing to think that an event that occurred 480 million years ago has gifted us this rare glimpse into the murky Ordovician world, where Earth's first megafauna evolved.