The largest ‘spider’ ever?

Fossil of Megarachne; head and jaws to right. Photo: M Gray © Australian Museum.

A 300 million year old, half metre long, fossil arachnid, Megarachne servinei, was originally described as a spider, but is now thought more likely to represent another type of spider-like ancient arachnid. Its unique features include the enormous size, massive shovel-like jaws and ribbed, shield-like covering over the abdomen. An arachnid of this size must have fed on large prey like cockroaches and giant millipedes. But why did this massive predator need such an impressively armoured body - were there even bigger arachnid predators about?