It is thought to be the oldest giant filter-feeder ever discovered.
Dr Allison Daley, from Oxford University, who co-led a team studying Aegirocassis writing in the journal Nature, said: ''This would have been one of the largest animals alive at the time.
''These animals are filling an ecological role that hadn't previously been filled by any other animal. While filter-feeding is probably one of the oldest ways for animals to find food, previous filter-feeders were smaller, and usually attached to the sea-floor. We have found the oldest example of gigantism in a freely swimming filter-feeder.''
A. benmoulae was named after the Moroccan fossil hunter who discovered it, Mohamed Ben Moula.
The three-dimensional fossil, exposed using tiny needle-like tools to chip away the surrounding rock, is exceptionally well preserved. In contrast, other anomalocaridid fossils have been flat, like pressed leaves.
''Without these 3D remains, we may never have got the insight into these animals' anatomy that we did,'' said Dr Daley.
The fossil shows that Aegirocassis had pairs of swimming flaps along its body, which were likely to be precursors of the unique double-branched appendages seen in modern crustaceans.
It's the latest in a line of bizarre fossils unearthed.
Fossil hunters searching for ancient relics found the skeleton of a 40,000-year-old woolly mammoth in the North Sea.
The team of archaeologists, salvagers and palaeontologists trawled the waters off the east coast of Britain at a depth of 100 feet.
North Sea Fossils, who are based in Urk, Netherlands, include an expert they call "Mr Mammoth" and are in search of the remains of extinct animals in the dark depths.
Bones of animals including woolly rhinos, Irish elks and parts of the male skeleton of an 11-foot tall woolly mammoth, including its skull and tusks, have all been brought up and collected.
The ancient sinkhole is believed to have opened up 25,000 years ago, trapping a large number of unsuspecting creatures who fell into it over the course of thousands of years and whose remains were preserved in the cool, dark conditions.
The 2014 find in Natural Trap Cave, at the base of the Bighorn Mountains in northern Wyoming, had gone unexplored for more than 30 years, and scientists had no idea of the scale of the remains they would find when they began digging at the end of last month.
