Behold Aegirocassis benmoulae, an extinct creature that swam through the Earth's oceans some 480-million years ago. Discovered by paleontologists in Morocco, it possessed modified legs, gills on its back — and a filter system for feeding. Remarkably, it's the oldest known animal to fill an ecological niche now occupied by whales.
The creature, which measured about 6.5 feet (2 meters) in length, belonged to the family of anomalocaridids, the early ancestors of modern crustaceans, spiders, and insects. It would have been one of the largest creatures to exist at the time, and is now considered the oldest example of gigantism in a freely swimming filter-feeder.
The discovery of A. benmoulae is remarkable from an ecological standpoint. Most other anomalocaridids were active predators that seized their prey with their spiny head limbs. But this sea creature had head appendages that evolved into an intricate filter-feeding apparatus. As it swam through the water, this gentle giant harvested plankton from the oceans.
Giant filter-feeding sharks and whales appeared at a time when plankton populations were much more pronounced. But A. benmoulae emerged much earlier than this.
Also, the well-preserved fossil shows that the animal had pairs of swimming flaps along its body, which were likely precursors of the double-branched appendages seen in modern crustaceans.