What terrestrial trait makes cetaceans so successful in an aquatic lifestyle?

Question

Intuitively, I would expect that an organism that has lived in the ocean for millions of years and can breathe underwater should outcompete any newcomer that is not fully adapted for oceanic life. It makes sense that there may be some semiaquatic animals that can outcompete fish in semiaquatic niches, such as seals, crocodiles, and diving birds. And I could see there being short-term instances of odd species that manage to colonize an isolated area that happens to lack competition.

But dolphins and whales are extremely successful across the globe, have been around for millions of years, and a large part of their niche overlaps with the much older and also hugely successful sharks, yet they manage just fine. A common explanation of this is "intelligence" but sharks are now known to be pretty intelligent, and even if they weren't, why wouldn't they be?

This isn't the first time this has happened, either. Aquatic reptiles like ichthyosaurs, plesiosaurs, and mosasaurs, and even modern sea turtles, represent hugely prolific and long-lasting groups of air-breathing, fully-aquatic animals (sea turtles come on land to lay eggs, but this seems more like a flaw than an advantage), suggesting that there is something in particular about land-dwelling that gives animals returning to the sea a natural advantage not found in their piscine counterparts.

Is there anything specific about evolving on land - some particular trait that doesn't generally evolve in aquatic animals - that cetaceans retained after returning to the sea, and that makes them particularly successful in the aquatic lifestyle?

Answer 1

Thermoregulation and echolocation both depend on evolutions that occurred on land and seem to be less accessible to fish than to the ancestors of mammals. The heat capacity of water is much greater than air, so maintaining an elevated temperature in body tissues is both more difficult and less necessary. Still, some fish lineages have independently evolved partial thermoregulation. From the article Warm eyes provide superior vision in swordfishes:

In these animals, warming the central nervous system and the eyes is the one common feature of this energetically costly adaptation. In the swordfish (Xiphias gladius), a highly specialized heating system located in an extraocular muscle specifically warms the eyes and brain up to 10 degrees C-15 degrees C above ambient water temperatures.

Depending on diving depth, temporal resolution can be more than ten times greater in these fishes than in fishes with eyes at the same temperature as the surrounding water. The enhanced temporal resolution allowed by heated eyes provides warm-blooded and highly visual oceanic predators, such as swordfishes, tunas, and sharks, with a crucial advantage over their agile, cold-blooded prey.

Echolocation in mammals is possible due to the mammalian ear, which is summarized in a previous answer. From Ultrasonic hearing and echolocation in the earliest toothed whales:

The advent of echolocation is thought to be a key innovation that supported exploitation of a vast pelagic biomass—vertically migrating organisms, especially cephalopods—and explosive diversification of odontocetes

Answer 2

Advantages of air breathing over gills? Keeping in mind water-breathing animals could still evolve lungs.

Not being vulnerable to parasites and diseases of the gills, which are obliged to have blood supply close to the water.

Able to colonise water regardless of salinity, able to survive warm water.