One subspecies learns to live with its neighbours. Its members negotiate with one another, assert dominance or back down. They recognize individuals, assess their fitness, remember the results of previous clashes. Their brains become more convoluted, shaped by memory creation and decision-making. Soon enough the strong amongst them are not merely driving off the weak, but co-opting them for more efficient hunts, sharing morsels from their kills to feed their new minions. As the world recovers, they are the one species to have thrived through the great dying. Appropriately, for they are the greatest killers.

Their coastal communities spread, and for another fifty million years they are always on the point of becoming simply predators, losing the fragile meniscus of society that necessity imposed on them.

A dominant culture emerges. By now they are far more terrestrial than aquatic, breeding on land and laying hard eggs, letting their hatchlings battle for survival in the nursery pens. They are not scorpion-shaped any more. Instead they are hunched over vulnerable lung-like membranes, tails curled beneath them, backs still a column of armoured segments. Stalked eyes bob and flick over a melange of appendages that are limbs, mouthparts and weapons: blades, pincers, fangs, spears and clubs in constant chittering motion.

By the time the dinosaurs of your world are glancing anxiously up at the sky in case that dot has grown larger since yesterday, there is only one eurypterid way of life across all the globe: a fierce one. The territoriality that once marked individuals is now espoused by communities. Each clawed polity knows only two states: war today or preparation for war tomorrow. A fight for resources is subsumed into a fight for us against them.

They fall readily into a kind of feudalism. A warrior class rules and a declawed underclass serves, dependent on a nobility who possess a literal right to bear arms. The eurypterid states harness sinew and plant material to launch sharp flints at one another. They smith copper, then bronze, then iron. Every technological development is achieved over a mound of corpses, every advantage is turned towards the subjugation of a neighbour. Dynasties come and go; empires rise and fall.

Later, they have an industrial age of sorts, discovering the uses of fossil fuels made from the crushed and compounded remnants of their own long-ago ancestors. Their societies are already pre-adapted for factory lines. A century later, and the world is at war.