Aegirocassis benmoulae straining plankton from the Ordovician sea.
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Art by Marianne Collins.
Aegirocassis benmoulae straining plankton from the Ordovician sea.

Strange Fossil Filter Feeder Was an Ancient Survivor

Paleontologist Jakob Vinther pointed to a rust-colored boulder sitting on the black lab table. “What do you think that is?”, he asked. I hadn’t a clue. I was used to looking at bones – often really big saurian bones – and I couldn’t pick out any endoskeletal signs in the stone. It was mostly the fossil’s size that struck me. Whatever it was, the specimen was almost as big as me. I shrugged and opened my mouth to hazard a guess, hoping that some shot in the dark would get me close to the answer, but Vinther spoke before I did. “That”, he said, “is a giant anomalocaridid.”

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Side view of a Aegirocassis benmoulae fossil. From Peter Van Roy, Yale University.

There’s no standardized common name for these early animals. The closest is a literal translation of the name – “anomalous shrimp” – that was coined by paleontologist Joseph Frederick Whiteaves over a century ago. But even that title has more to do with history than actual identity. Paleontologists Simon Conway Morris and Harry B. Whittington later discovered that the fossil Whiteaves identified as a crustacean tail was actually a curled feeding appendage of a bug-eyed arthropod that used those spiny arms to stuff small prey into its shutter-like mouth. These animals became known as the anomalocaridids, and they were among the largest, strangest creatures to swim through Earth’s Cambrian oceans.

These anomalous invertebrates were so bizarre that they seemed to be perfect illustrations of Stephen Jay Gould‘s central point in his masterpiece Wonderful Life. Along with other oddballs, the anomalocaridids appeared to die out by the end of the Cambrian, 485 million years ago. They were an early evolutionary success story that was cut off by total extinction, but, had they survived, they would have altered the course of life on Earth. It might have even changed what we find frightening. If the anomalocaridids had survived to the present day, Gould wrote in jest, “Why not a Steven Spielberg film with a crusty seaman sucked into the cylindrical mouth of a sea monster, and slowly crushed to death by multiple layers of teeth lining a circular mouth and extending well down into the gullet?”

But what paleontologists have started to discover is that the anomalocaridids – and other contemporaneous weirdos – were not simultaneously snuffed out at the end of the Cambrian. Some, like a rather cute little anomalocaridid named Schinderhannes, lived about 80 million years after the close of the Cambrian. And while not quite as geologically young, the giant Vinther introduced me to was another survivor. Named Aegirocassis benmoulae by paleontologists Peter Van Roy, Allison Daley, and Derek Briggs, the ancient invertebrate is a sign that the evolutionary aftershocks of the Cambrian Explosion were still felt long after the event’s initial spark.

Aegirocassis – a reference to a mythological Norse giant and the Latin word for shield – swam through seas that covered what is now Morocco between 485 and 443 million years ago. Along with a slew of other creatures, it was part of what paleontologists have dubbed the Fezouata Biota – a combination of more archaic Cambrian creatures that lived alongside more familiar lineages that formed the basis for what we think of as “modern” oceans. In particular, the anatomy of Aegirocassis shows that anomalocaridids weren’t technically arthropods, but the flaps along their bodies represent equivalent parts to arthropod limbs. In other words, anomalocaridids like Aegirocassis might provide a look at parts of the body plan that evolution co-opted and modified in true arthropods.

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A close-up of the great appendage of Aegirocassis. From Van Roy et al., 2015.

The newly-named fossil also adds to paleontologists’ understanding of our evolving oceans. At over six feet long, Aegirocassis was one of the largest anomalocaridids. Despite facile headlines calling this creature a “sea monster” and “Frankensquid“, though, Aegirocassis was only terrifying to the tiny. The invertebrate’s great appendages carried rows of fringe-like spikes, reminiscent of a whale’s baleen, that sieved plankton from the water. This adds Aegirocassis to the growing number of filter-feeding anomalocaridids, and, as Van Roy and colleagues write, the invertebrate’s anatomy is a sign of sea changes.

The giant individual Vinther showed me wasn’t a loner. The paleontologists who have sifted through the Ordovician rock have found at least thirteen individuals, and probably more. Some of these might represent shell moults left on the bottom as groups of Aegirocassis sloughed off their old armor, much like some undersea arthropods do today. But, in addition to their size and anatomy, this anomalocaridid abundance is another sign that plankton were flourishing during this time, allowing filter-feeders to become larger and more numerous than ever before. As otherworldly as Aegirocassis may seem, the plankton-sucker helps mark the origin of ocean life as we know it.


Van Roy, P., Daley, A., Briggs, D. 2015. Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps. Nature. doi: 10.1038/nature14256