A long time ago the 60ft long Megalodon roamed the oceans, fast forward to 2.6 million years ago and they went extinct (and yes they definitely went extinct don't let the discovery channel fool you). But lets say that Megalodon never went extinct how might they affect the oceans ecology? As in would whales be as large as they are today?
Did you know that Great White Sharks are terrified of orcas? like proper poop-their-ocean terrified. Tagged Great Whites who hear the click-click of a pod of hunting orcas dive to the deepest point they can and stay there for hours.
This is probably one of the reasons Megalodon went extinct*: eaten by the ancestors of orcas.
Baleen whales got to their current sizes right around the time Megalodon went extinct but there is no evidence that predation kept them small, you would expect the opposite: a race to giant size so that they become untouchable by sharks. Instead the race to gigantism seems to have been driven by changes in climate and ocean currents that meant more food was available.
So what would change if Megalodon hadn't gone extinct:
- slightly fatter orcas
- baby whales might try to grow even faster, mom-whales might be a bit more protective.
- other large sea creatures like elephant seals will employ strategies that their smaller cousins employ today to dodge Megalodon.
But long story short: Megalodon didn't enjoy a ecological niche that isn't filled these days by modern large sharks or orcas so there is no reason to expect any widespread changes in the overall ecology of the oceans, no matter how much a 10-metre shark speaks to the imagination.
* Megalodon extiction seems to have been one of those perfect storms: climate change, geological upheaval (Panama closing), astronomical events and the rise of a class of intelligent predators that know the trick to eating sharks.