Anomalocaris, the Cambrian apex predator

Anomalocaris canadensis was a swimming animal roughly 40 centimetres long, with a pair of spiny grasping arms at the front, a round mouth ringed with plates, stalked eyes and a row of flaps along each side. In 1892 Joseph Whiteaves of the Geological Survey of Canada described isolated arms from the Burgess region, mistaking them for the tail of a shrimp, hence the name meaning 'abnormal shrimp'. The ring-shaped mouth was later named Peytoia and taken for a jellyfish. Only in the 1980s did Harry Whittington and Derek Briggs pull the pieces together into one animal.²

Anomalocaris belongs to the radiodonts, a group of early arthropod relatives that appear in Chengjiang about 518 million years ago and survive into the Devonian. Some radiodonts were filter feeders, such as the Ordovician Aegirocassis, about 2 metres long. Others hunted soft prey. A radiodont eye from the Emu Bay Shale of Australia, described in 2011, had some 16,000 lenses. Whether Anomalocaris could break trilobite shells has been debated, because its mouth plates may have been too soft. Either way, it shows that active, sighted predators were in the Cambrian seas and helps explain why armour, spines and burrows spread so quickly.¹³