If coral reefs are cities of the sea, then those that exist today are but scattered hamlets compared to those that thrived in the Devonian (419 to 358 million years ago). Today the Great Barrier Reef, the largest of the remaining corals spans an area some 340 thousand square kilometres, but back in the Devonian many were three million square kilometres — an area the size of India — and hosted enormous diversities of life, so strange to our eyes that it looks like something from an exoplanet. Ocean floors crept with vast populations of trilobites of exotic specialisation such as Erbenochiles with their tower-like eyes, and roving groups of Mimetasters; which looked like something between a starfish and a crab. Waters teemed with jawless fish with heads shaped like disks and spears, and when its inhabitants did resemble something we might recognise they had unusual adaptations, such as the shark-like Stethacanthus whose dorsal fin resembled a giant anvil. Also spiralling through the tenebrous depths were great shoals of lungfish, thought to be distant ancestors of us and all other amphibians and reptiles, birds and dinosaurs. And pursuing them and all life were ferocious apex predators — the Placoderms — some of them 10 metres long, their malevolent heads encased in bony armour.