How Did Marine Life Recover After the Asteroid That Killed the Dinosaurs?

POV Travel
Jul 31
9 min read

You have heard this story. A rock the size of a mountain struck the sea off what is now Mexico, the sky went dark, and the dinosaurs died.

It is a good story. It has a villain, a moment, and a clean ending. It is also, we would suggest, told with rather more confidence than the evidence supports, and the part everyone skips is the part that ought to worry us.

Because the deeper catastrophe was not on land. It was in the ocean, and it happened for a reason so simple it sounds too small to be lethal. The lights went out. In the sea, the lights are everything.

And then the ocean came back. Slowly. Wrongly. Into something that was not what it had been. And a great many people now cite that recovery as a reason not to worry about what we are doing to the sea today.

We would like to look at what actually happened, because the reassurance is not in the rock.

How did marine life recover after the asteroid that killed the dinosaurs?

Quick Answer

Around sixty six million years ago, roughly three quarters of species on Earth died. In the ocean, the killing mechanism was the loss of sunlight, which collapsed the plankton that everything else depends on, followed by acidification that dissolved shells.

Basic productivity returned within thousands of years. A functioning ecosystem, with its full architecture of predators and diversity, took on the order of millions.

Whether the impact acted alone is genuinely disputed. Enormous volcanic eruptions were underway at the same time, and scientists still argue about how the blame divides. What is not disputed is that the ocean that emerged was a different ocean. It is the one we swim in.

The sea before

Picture the ocean of the late Cretaceous, because almost none of it survives.

Mosasaurs, huge marine lizards with the jaws of a crocodile and the body of a shark, ruled the shallows. Plesiosaurs moved on four great flippers. Ammonites, coiled relatives of squid, drifted in thousands of species and had done for hundreds of millions of years.

Beneath them, the animals that made all of it possible. Vast populations of plankton in the sunlit upper layer, turning light into living matter. Some built shells of calcium carbonate, and when they died their shells sank and accumulated. The white chalk cliffs of Europe are made of them, an ocean of microscopic lives compressed into stone.

Sharks were there too. Already ancient. Already successful. Already survivors of earlier catastrophes.

Remember that. It matters at the end.

Why the ocean died

The killing mechanism was the darkness.

Sunlight drives the sea. Plankton capture it, and everything else eats them or eats something that did. The entire structure rests on that single foundation of light.

When the atmosphere filled with dust and sulphate aerosols, photosynthesis in the ocean collapsed. Not diminished. Collapsed. The plankton died in staggering numbers, and everything above them began to starve from the bottom up. The aftermath is sometimes called the Strangelove ocean, a sea so stripped of surface life that its chemistry changed, the ordinary signature of a living, productive ocean simply absent from the rock.

Then acidification. Sulphur fell as acid rain, and the sea turned hostile to the very organisms trying to build shells in it.

The ammonites, after hundreds of millions of years, ended. The great marine reptiles ended. Three quarters of species on the planet did not come through.

The part of the story that is not settled

Here is where the tidy version begins to fray, and we think you should know about it.

At the same time as the impact, on the other side of the world, one of the largest volcanic events in the history of the planet was underway. The Deccan Traps in India erupted across an immense span, releasing carbon dioxide and sulphur dioxide on a scale that alters global climate and ocean chemistry by itself.

Some researchers argue the volcanism was already stressing the biosphere, and the asteroid finished off a world that was failing. Some argue the impact did essentially all of it. Some argue the impact itself intensified the eruptions. The debate is decades old, it is conducted by serious people, and it is not over.

We are not going to pretend to adjudicate it. We raise it because the single cause story, one rock, one afternoon, one extinction, is repeated everywhere as settled fact, and it is not settled at all.

Notice, too, which version is more comfortable. A rock from space is nobody's doing. A slow poisoning of the atmosphere by carbon dioxide and sulphur, unfolding over an extended period, weakening a biosphere until a shock it might otherwise have survived becomes fatal, is a story with a rather different resonance in our own century.

We are not suggesting anyone suppressed anything. We are suggesting that stories which absolve tend to travel further than stories which implicate, and that this is worth noticing every single time.

Who lived, and why

The pattern of survival is where this stops being palaeontology and starts being a warning.

Being large was fatal. Big predators need constant food, and when the food chain fails, the animals at the top starve first. The mosasaurs were superbly adapted to an ocean that stopped existing.

Being specialised was fatal. Depend on one prey, one habitat, one way of living, and you have nowhere to go.

What came through, again and again, were the small, the flexible, the generalists, and above all those that could eat detritus, the dead matter drifting down from above. When nothing is growing, the animals that consume the dead inherit the sea.

Life in the deep, sustained by falling organic material rather than direct sunlight, weathered the crisis better than life at the surface. The dark, the cold and the patient did better than the bright and the fast.

And some sharks made it. Not all. But the group persisted, as it has persisted through every mass extinction since long before the dinosaurs existed. When we describe sharks as the oldest survivors in the ocean, this is what we mean. They were here for this. They were here for worse.

The lie of comfort

Now the part that is routinely misunderstood, and it is the most important thing in this article.

Plankton began recovering within thousands of years. If you define recovery as life restarting, life restarted quickly. Life is extraordinarily difficult to kill outright.

But a functioning ecosystem is not the same as the presence of life. Rebuilding a complex ocean, with its predators and prey, its diversity, its stable food webs, its ecological roles filled, took vastly longer. The fossil record indicates not thousands of years but something on the order of millions before comparable complexity returned, and different parts of the system healed at wildly different rates.

Sit with that ratio. The wound was inflicted in an afternoon. Scar tissue formed within a few thousand years. Genuine healing took millions.

This is why we flinch when the recovery is offered as consolation. Life bounced back. Nature is resilient. The ocean healed. Every word of that is true and the conclusion drawn from it is false.

Resilience is not a reason for comfort. It is a description of how long the consequences last. The ocean recovered on a timescale against which the entire history of human civilisation is a rounding error. Every person who will ever be born to anyone now alive will live and die inside the wound.

A different ocean, and why you exist in it

What eventually filled the empty sea was not a restoration. It was a replacement.

With the marine reptiles gone, the role of great ocean predator lay vacant. Over tens of millions of years, others took it. Sharks diversified into forms that had never existed. Bony fish radiated spectacularly, and the modern diversity of fish traces to that long aftermath. Much later, mammals returned to the water, and the ancestors of whales began the journey that produced the largest animal that has ever lived.

None of it could have happened had the ocean not been emptied first. The whale exists because the mosasaur does not. The humpback off the South African coast, the strange little pygmy right whale in the southern seas, occupy an ocean that a catastrophe cleared for them.

There is nothing benevolent in this. Nothing was owed to the survivors and nothing was intended. Catastrophe destroys a world and hands the wreckage to whoever can use it. Which means the ocean you swim in is not the ocean that ought to be there. It is the ocean that was left.

The mechanisms are running again

We are going to state this plainly and then leave it with you.

Acidification, the chemistry that dissolved shells sixty six million years ago, is underway again, driven now by carbon dioxide rather than impact sulphur. Plankton communities are shifting. The ocean is warming.

And the apex predators, the animals the fossil record identifies as the very first to fail, are being removed at a rate of tens of millions a year. We wrote elsewhere about what happened when the great whites vanished from False Bay, and about the fear that let it pass almost unremarked.

The rock record tells us exactly what this combination does to a marine ecosystem. It also tells us how long the repair takes.

Something will survive. Something always does. The small, the flexible, the generalists, the eaters of the dead. Whether the animals we find beautiful are among them is a separate question, and the fossil record is unsentimental about it.

How POV Travel sees the great extinction

We cannot take you to the Cretaceous. We can put you in the water with the survivors.

That is what a shark is. When you drift alongside a blacktip at Aliwal Shoal, or watch a mako materialise out of the blue off Cabo San Lucas, you are looking at an animal whose lineage was already ancient when the sky went dark, and which came through an event that annihilated three quarters of life on this planet. It kept going. It is still here. It has outlasted every dominant predator the ocean has produced.

It may not outlast us, which would be an extraordinary sentence to have to write.

We think that fact should change what you feel when a shark passes you. Not fear. Something nearer to deference. These animals have seen this before.

We question. We teach. We leave you with an opinion of your own.

So take the rock record, which is not ours and cannot be accused of an agenda, and read it yourself. The ocean has been emptied before. It took millions of years to become what it is. The same chemistry is running again, and we are removing the survivors.

What you make of that is entirely your business. We would simply rather you had the evidence.

Frequently Asked Questions

What killed marine life after the asteroid?

Chiefly the loss of sunlight. Dust and sulphur stopped photosynthesis, plankton collapsed, and the ocean food web starved from the bottom up. Acidification then dissolved the shells of many marine organisms.

Did the asteroid act alone?

This is genuinely debated. Enormous volcanic eruptions, the Deccan Traps, were underway at the same time and released climate altering gases. Researchers still argue about how much each contributed, and the single cause account is not settled.

Which marine animals survived?

Small, flexible generalists, especially those able to feed on dead organic matter. Deep sea and sea floor communities fared comparatively well. Sharks survived, as they had survived earlier mass extinctions.

How long did the ocean take to recover?

Basic productivity returned within thousands of years, but rebuilding a complex ecosystem with full diversity and stable food webs took on the order of millions. Recovery of life is not recovery of an ecosystem.

Does this mean the ocean will recover from what we are doing?

Life will persist. The ecosystem we recognise may not, and the fossil record suggests the timescale for genuine recovery runs to millions of years. Resilience describes how long the damage lasts, not how quickly it heals.

Meet the survivors for yourself

Everything on this page changes the moment you are actually in the water. Drift among oceanic blacktip sharks in the warm currents of Aliwal Shoal, or hang in the blue off Cabo San Lucas as a mako, the fastest shark in the ocean, cuts past you. No cage, no bait, no adrenaline theatre. Just the animals as they truly are, met on their terms, in small groups, with people who understand them.

Explore the expeditions: Swim With Sharks & Marine Life →