Here, in these Silurian seas, the fish still look armoured and alien. And they have good reason to go kitted out in mail. This is a story about top predators.

The humble scorpion relatives which lived on the sea floor in the shadow of the orthocones have stepped up their game in this timeline. Some are tiny, some bigger than you are. They are fierce, active hunters, keen of eye and equipped with a Swiss Army knife of tools for shelling fish and molluscs. In your world they had their time and were gone before they might trouble your nightmares. But this is not your world; this is theirs.

There are metrics of success that would place ants, beetles or bacteria at the top. However, we are all naturally biased to value large, fierce animals, those that can adapt to change and exploit opportunities. The eurypterids, sea scorpions, are all of that, and their realm is littoral, all the world a beach. They live both sides of the tidal divide, emerging from their burrows to fish the waters when they are in and sieve the sand when the waves soak away.

Soon land becomes an interesting place too, as a more congenial atmosphere rides in on the back of the first crude forests. A grab bag of sea life evolves to move inland—not least to escape these damn scorpion monsters that patrol the beaches, like murderous border guards. The eurypterids follow on their ever-stronger legs, progressing from a dragging crawl to a spider-quick scuttle, their appalling cutlery drawer of arms held high for death and dismemberment. We would recognize them as the dominant animal group of their world, but for a long time they are beasts of very little brain. Sometimes ferocity is enough.

In the Permian days of mass extinction, this timeline changes gear. Countless species of eurypterids vanish. Of the survivors, one is of particular note. This descendant of the sea scorpions has colonized much of the world, but only in the narrowest of strips. And, now the world has contracted to a single continent, there is far less in the way of beachfront real estate for the ambitious predator.

As their world shrinks, their populations become denser. They live in submerged burrows side by side, jostling elbows and clashing claws. There are fights, and this becomes a real threat to the species because they are spectacularly well equipped to tear apart large shelled animals, most definitely including each other.