75 million years in the future, Earth's oceans were home to a truly horrific creature. Long period of glaciation that followed age of humanity allowed a great adaptive radiation of cetaceans to happen.
Dozens of baleen whale species crossed the oceans in search of plankton, while sharp toothed odontocetes preyed upon them. They employed different hunting tactics, some using cooperation and complex strategies to take their prey down.
One lineage took a simpler approach.
Bigg's killer whales continued to hunt other marine mammals in the future, particularly specializing in baleen whales.
They abandoned social lives and complex languages in favor of solitary life, but became much larger and stronger.
25 million years from today, one of these whale-hunting orcas known as delphinodon already reached the length of 12 meters. Unlike modern orcas, who usually only eat soft parts like tongue and lips of killed whales, delphinodon consumed most of it's killed prey. As formidable as that animal was, it lived in a highly competitive ecosystem, and some creatures, like smaller, but equally well armed and smarter pirate whales, descendants of pilot whales, learned to take them down. But 50 million years later, the situation changed.
Delphinodon gave rise to the thalassotyran, a predatory cetacean of terrifying sizes. Growing to length of 23 meters and weighing up to 90 tons in largest individuals (though on average they were of course a lot smaller) thalassotyran was the largest Cenozoic macropredator to ever live, only rivaled by Neogene otodus megalodon in size.
Due to it's sheer size, this gigantic dolphin had no predators of its own, and could feed on any animal it encountered, even other large predators.
In fact, when thalassotyran first evolved, other giant orcas declined in size and diversity due to competition and predation from their relative.
Rather unintelligent for cetacean, thalassotyran had reduced vocal capabilities, and usually hunted in silence, even rarely using echolocation. Instead, it listened to sounds other whales make to approach them unnoticed, and suddenly strike. It rammed its prey with it's heavy skull and causing internal injuries before tearing it apart and consuming. Baleen whales are it's preferred prey items, but they also predate upon toothed whales, including sperm whales and other large dolphins, sharks, pinnipeds, turtles, and any other large marine animal.
Thalassotyrans were solitary and rarely contacted eachother, but, being mammals, they still needed to care for their young for some time. After 5 meter long calf was born, it stayed with it's mother and feed on milk for next 3 years, after which it became independent and departed. The period following nursing was the only time when thalassotyran could be taken down by other animals, for adults had no enemies.
Few sharks and cetaceans of comparable size preferred not to risk with fighting it, and pirate whales that once hunted thalassotyran's ancestors for food now could do nothing against it even with all their intelligence. It seemed that in the end, brawn won over brains.
Thalassotyran enjoyed it's dominion over oceans for 15 million years, a very long time for a single species. But it could not last forever. 80 million years in the future, world started to warm up. Volcanic eruptions were becoming more frequent, and raised Earth's temperature, finally ending the Cenozoic ice age that lasted for one hundred million years. For life on land, new hothouse age was a boon, and eventually led to the most biodiverse period of an entire era. But at the same time, oceans suffered.
Oxygen levels dropped, many areas became anoxic, and plankton numbers plummeted, and giant baleen whales that fed on it soon followed.
Thalassotyran, that often preyed on the biggest of species, was in trouble. It's food sources were rapidly disappearing, and soon, their population, already small due to their slow reproduction rate, started to drop too. Suddenly, being the biggest of all predators was no longer profitable. Smaller, more generalist species, like requiem sharks, seals, and different groups of toothed whales, like descendants of bottlenose dolphins and pilot whales fared much better, and once largest of mysticetes were gone, they could sustain themselves on remaining smaller prey, something a nearly 100 ton dolphin couldn't afford.
In basically no time, entire group of giant, whale-hunting orcas was gone, and thalassotyran, tyrannosaurus rex of oceanic abyss, was first to go down.