Aegirocassis was a giant filter-feeding arthropod that swam the Early Ordovician seas of what is now Morocco around 480 million years ago. Growing over 2 meters long, it was one of the largest animals of its time, gliding through the water like a living submarine. Despite its shrimp-like appearance, Aegirocassis was a distant cousin of the famous Anomalocaris, but unlike its fierce relatives it was a gentle giant: its head bore a pair of huge flap-like nets made of bristle-covered spines, which it swept through the water to catch plankton and tiny drifting animals. It belonged to the hurdiids, a group of early arthropods, and its fossils are so well preserved that scientists could even see the fine filtering hairs on its feeding limbs. Its body was covered in broad, rounded swimming flaps that rippled along its length, propelling it slowly through the ancient ocean in search of its next mouthful of microscopic food.