You wouldn’t have recognized the trilobites’ Cambrian seas as belonging to your planet, even though your planet once held those same waters. In this Ordovician period, forty million years later, life is beginning to look like a modeller’s rough maquettes of something you’d see out of a submarine’s portholes. In this timeline there are fish, though weak-chinned and jawless things, armoured like tanks. There are coral reefs. There are things that look like lobsters, although they are more like scorpions.

And there are giants in these seas. If you were there, in your submarine, you would see them looming like blue shadows against the gloom of the deep water, or snuffing out the sparkling daggers the sun casts through the waves. Like whales, you might think, having no other basis for comparison. Indeed they are, though: slow, vast, twenty, thirty, fifty feet long, perfectly poised within the water.

Most of that length is shell, straight or spiralling like the horn of a gigantic unicorn. They ghost ponderously through the water point first, as though steeling themselves to ram an enemy below the waterline. In your world they are a fleeting memory, supplanted by jawed fish, sea scorpions and other swifter creatures—but their relatives are still with you: the squid, the octopi. In these seas, the long, pointed shell glides past and, at the open end, you see an eye the size of the very porthole you gaze out of, huge and detached in its regard. It presides over a parliament of writhing arms, with which it picks over the sea floor or snatches prey from the water. An orthocone.