In this world, the early orthocones did not simply rest on their laurels before giving way to your piscine ancestors. They grew immense and cunning, brains outpacing their huge bodies, their organs rearranging to fit an ever-increasing cerebral volume. The evolutionary pressures on them are readily apparent. They must keep a leash on their unruly pack of limbs, and they must master a skinful of chromatophores—at first used for crude camouflage, then for brute signalling between courting couples, and at last for primitive speech. The early orthocones slide silently past one another like billboards, flashing threats, warnings, queries. Later species will begin to sound off at one another, generating gas within their shell chambers and mantles and agitating it against their septa. They’ll produce basso profundo groans that can be detected across a hundred leagues of ocean by another of their long-lived, lonely kind. Over ten thousand years, the ocean fills with their conversational flatulence. They speak constantly but meet only to mate, a process steeped in slow, graceful ritual: the intertwining of arms, the rippling of patterns on the skin. A dance of days for one clutch of eggs, which will hatch into three score of miniatures carried off on the tide, never to know their parents. Yet the sea resounds with the speech of their kind, and they learn it over their long childhoods. The few that reach adulthood join in a select global community of hermit seafarers.

And what of our own ancestors? By the time the orthocone descendants have reached a level of self-awareness we might recognize, the fish, scorpions and the rest have had their chance to become fierce and dangerous. Vertebrates come to the land very late in this world, and still without jaws.

Even as they do, the continents of Earth are clasping together in a single mass like the jaws of a trap. The world is heating up, volcanos belching out poisons to darken the sky from pole to pole. The great dying time that falls on all Earths descends, with the inevitability of a clock striking midnight, to strip land and sea of their biodiversity.

The orthocones perceive that their world is changing, and they debate and discuss, their deliberations rolling back and forth from shore to shore of their world-ocean. They are towering intellects by now. Knowing one another only by their conversations, they have been choosing mates by the quality of their slow, reverberant repartee for thousands of years. A depth of intellect is like a peacock’s tail or the blue backside of a mandrill, for them: infinitely attractive. Countless generations have selected for intelligence and gone on to deliver a newer generation, slightly brighter still. By the time the extinction clock strikes, they are perhaps the most complex intellects any timeline will ever see.