These were the real-life embodiment of mythical sea monsters, with all the ferocity and grandeur we might imagine. Cruising around were sixty-five-foot-long ichthyosaurs, some as long as eighty-five feet. They looked like blue whales with elongate, tooth-filled jaws. Other ichthyosaurs were roughly the size and proportions of bottlenose dolphins. Excalibosaurus had a rapier-like rostrum, as swordfish do today, and presumably used it in a similar way to slash through shoals of fish. Tylosaurs looked like enormous modern-day orcas, up to twice their size, and may have hunted like them too, subduing prey by ramming into it with their bony snouts and then tearing it apart with razor-sharp teeth.
Plesiosaurs looked like archetypal incarnations of the Loch Ness monster, with a streamlined body, tiny head, and two pairs of long flippers, which they paddled in elegant undulations to fly underwater. Many had phenomenally long necks, some measuring more than twenty feet and taking up two-thirds of their body length. It’s tempting to imagine plesiosaurs using their necks to strike out at prey, like a coiled snake, or to grab pterosaurs flying above the waterline. In fact, the plesiosaurs’ abundant neck vertebrae were likely quite stiff and didn’t flex from side to side. These reptiles may have floated horizontally in the water, dipping their necks below their body to rake fish shoals with a snarl of intermeshed teeth or to root out prey in the seabed.
Swimming through Mesozoic seas were reptiles that looked like monitor lizards, others like giant newts, salamanders, or crocodiles, and huge, long sea snakes with little legs and gently bulging bellies. This was also the era when another group of reptiles, the sea turtles, first evolved, including the biggest ever to exist, the two-ton Archelon, which grew to fifteen feet long and would have needed four king-size mattresses to stretch out on.
In all, reptiles retraced their ancestral past and took to the ocean on at least a dozen separate occasions. No one knows for sure what drew them all down to the sea—one idea is that the land was getting crowded with other animals while the ocean offered plenty of space and prey—but clearly reptiles learnt to swim many times over. Their bodies underwent extreme adaptations to enable them to live underwater. Fossils of ichthyosaurs show that their arms gradually became shorter and their hands longer with more fingers—all the better to act as large, swimming flippers. Some ichthyosaurs evolved eyes the size of ten-pin bowling balls, bigger than those of any other animal extinct or alive, which gave them excellent vision as they hunted in the dim waters of the twilight zone. Thalassodraco (“sea dragon” in Greek) looked like a dolphin with a huge ribcage, accommodating enormous lungs that let it take great breaths and stay longer underwater.
Occasional food remains preserved in their stomachs and the arrangement and shapes of their teeth tell us Mesozoic marine reptiles had a varied animal-based diet. The most terrifying of all were those that filled an ecological niche that until then had been empty. Hyper-carnivores—predators that eat other predators—hadn’t existed until the Mesozoic. It’s evident that swimming reptiles were in the habit of eating one another. A fossilised Diandongosaurus, a close relative of plesiosaurs, has been found with its hind left flipper missing, likely bitten clean off by a fellow reptilian predator. And a sixteen-foot ichthyosaur swallowed most of a thirteen-foot thalattosaur, another Mesozoic reptile, shortly before it died, as palaeontologists saw when they found a fossil within a fossil.
Not all the swimming reptiles were spine-chilling carnivores. The oldest known plant-eating marine reptile, Atopodentatus, had a strange hammerhead skull and may have had an unusual two-step mode of feeding. With its chisel-shaped teeth, it likely scraped at seaweeds on the seabed. It also had a row of needlelike teeth, which formed a mesh and could have sieved fragments stirred into the water column.
And not all these swimming reptiles were giants. Many of the early ichthyosaurs were salmon-sized, including one called Cartorhynchus, which had a stubby snout, pebble-shaped teeth for crunching snails and clams, and big flippers with flexible wrists that would have let it move about on land. In life it would have looked rather like a small seal that basked on the shore and dived in the water to feed.
Throughout the 190-million-year span of the Mesozoic, an ebb and flow of marine reptiles occurred, as some went extinct and new forms kept evolving. To begin with, they mostly stayed near the shorelines fringing the coasts of the supercontinent Pangaea, which had formed when other continents collided with Gondwana and spanned both hemispheres. Then, as Pangaea began to break apart, the continents and oceans we know today began taking shape. Volatile seams in the earth’s crust opened, and the new Pacific and Atlantic Oceans were born. Marine reptiles swam along the seaways that opened up between continents and soon were living all across the global ocean.
The Mesozoic was more than just an exciting time for its assortment of swimming reptiles. The reign of sea dragons was part of an oceanic revolution that shaped much of life on earth in ways that continue into the present day. The procession of predators triggered an evolutionary arms race as their prey found means of survival and escape, which in turn caused new life forms and lifestyles to flourish.
A major battleground was the seabed, where reptiles as well as bony fishes, sharks, ammonoids, and crabs were all busy searching for shelled creatures and crushing them in their powerful jaws or claws. The prey responded in many different ways. Clam-like bivalves began a long game of hide-and-seek that still goes on today, as they escaped downwards, burrowing into the seafloor and sprouting long breathing tubes.c Sea lilies, umbrella-shaped relatives of starfish, escaped the feeding frenzy by drifting off into open waters. Snails evolved thicker, spinier shells; some also took flight from the dangerous seabed, evolving tiny wings and flitting off into the plankton as sea butterflies. Other snails fled the oceans altogether. The Mesozoic reptiles that went back to the sea and evolved into terrifying sea monsters were part of the forces that drove snails into fresh water and ultimately onto land.
Predators evolved countermeasures to keep up with their prey. They developed excellent vision and camouflage, improving their chances of spotting prey from farther away and sneaking up undetected. And predators evolved new ways of breaking through tough-shelled defences. If you ever find a seashell with a neat, round hole, it contained a creature that was killed and eaten via a mode of attack that first evolved in the Mesozoic, when predatory snails gained the ability to drill through the shells of other molluscs and suck out their soft insides.
Ocean food webs had become a scramble of animals eating and getting eaten. In 1977, mollusc expert Geerat Vermeij proposed this ferocious struggle should be called the Mesozoic marine revolution. Since then, signs of the revolution’s effects have been noted in many other animals, but only in 2021 did this transition come into view across the entire fossil record. Earlier fossil studies found obvious signs of the five prehistoric mass extinctions—it’s not too difficult to spot the annihilation of more than 90 per cent of all life. But the influence of mounting predation in the Mesozoic was more gradual and didn’t clearly show up. With advances in computing power and new ways of analysing enormous fossil data sets, researchers can now extract patterns that were previously hidden from human eyes and minds. Scientists at Umeå University in Sweden built what amounts to an interactive digital map of the entirety of ancient ocean life through space and time. It shows that the Mesozoic marine revolution was indeed a life-shaping force equally as powerful as mass extinctions. Dramatic global catastrophes have caused ocean-wide changes in biodiversity, and so too did the prolonged shift that took place under the influence of marine reptiles and all the other predators roaming through Mesozoic seas.
Today the only reptiles that live a fully oceanic life are sea turtles and sea snakes. All the other fish-like, whale-like, and lizard-like Mesozoic lineages went extinct. The last of the ichthyosaurs were gone by around one hundred million years ago, likely because they weren’t evolving quickly enough to adapt to an intense period of climate change. Meanwhile, the plesiosaurs made it to the end of the Mesozoic, then died along with three-quarters of life on earth, extinguished by another mass extinction. That moment, sixty-six million years ago, is the most infamous planetary disaster, one that captures people’s imaginations more than any other and shapes the popular view of extinction. An asteroid hit the earth, the skies darkened, and the dinosaurs’ days were numbered. A story often told is that once dinosaurs were out of the way, mammals had their chance to arise from their shadows. It’s a dusty old theory, challenged by many lines of evidence in the fossil record. And yet, the idea persists that mass extinctions pave the way for new life to rebound—that devastation is a prerequisite for renewal.
Machine learning tools are helping scientists construct another powerful new view of biodiversity through time. By programming computers with artificial intelligence algorithms, biologist Jennifer Cuthill from Essex University (UK) and colleagues have held millions of fossils in their virtual hands and tracked subtle shifts in life from the Cambrian to the present day. Their method detected the same five mass extinctions that everyone else has found, plus seven other major extinction events. They also pinpointed seventeen times in the past when life flourished and evolution surged into action, which they called mass radiations. Critically, they found that mass extinctions and mass radiations don’t tend to pair up in the fossil record. There are only two examples of a notable extinction followed right away by a great radiation. On fifteen other occasions, radiations happened nowhere near an extinction event. Rather, they were often associated with evolutionary innovations, such as the development of eyes or the movement of different forms of life onto land. There is little evidence that mass extinctions are a force of creative destruction.
The pace and rhythm of evolution in deep time is not simply driven by dramatic extinctions; life is far more responsive and complex. A catastrophe isn’t needed to steer evolution down new avenues and sculpt biodiversity in novel ways. A tangle of other elements is constantly in play, as more gradual and less immediately lethal environmental changes take place. And these intricate responses are evident in the ways ocean life changed through the third great chapter of life, the Cenozoic,d beginning sixty-six million years ago and leading up to now.
When the aftermath of the dinosaur-killing extinction had subsided, roughly ten million years into the Cenozoic, the earth was in the grip of fearsome global warming. There was no ice at the poles. Forests flourished in Antarctica. Flying lemurs leapt through the trees on Ellesmere Island in far northern Canada. The situation suddenly escalated fifty-six million years ago, when the deep sea released a colossal burp of the potent greenhouse gas methane, perhaps because gradual heating had caused seabed sediments to destabilise. Global temperatures spiked by ten degrees Celsius in the ocean surface and five degrees Celsius in the deep sea. The ocean became more acidic, a mass extinction swept through deep-sea ecosystems, species moved towards the poles, and the earth became a tropical hothouse.
Many scientists consider this to be the closest analogue to anthropogenic climate change, and while certainly the most recent such event, it wasn’t as extreme as what’s happening now. In comparison, changes in ocean chemistry and the rate of carbon release are an order of magnitude higher today because of humankind. Back then, temperatures continued to rise until forty-nine million years ago, when the climate dramatically changed course. Carbon dioxide levels in the atmosphere began to fall, thanks in part to the chemical weathering of continental rocks,e which accelerated when India collided with Asia, building the Himalayan mountain range. Temperatures steadily dropped, and the earth, recovering from its fever, began to descend into an ice house, setting the scene for the climate we live in now.
The world was also beginning to look much like it does now. The continents had drifted into their approximate current locations, except for some subtle but important tectonic shifts that were still to come. Around thirty-four million years ago, the southern tip of South America pulled away and left Antarctica on its own. This allowed an oceanic current to begin swirling endless clockwise loops around the continent, effectively isolating it from warmer waters in the rest of the ocean. It also happened that the Antarctic continent had come to sit right over the South Pole, putting it in the perfect place to get very cold very fast, and it was soon covered in a giant sheet of ice.
Not until much more recently, around 2.7 million years ago, did the seas at the North Pole begin to turn to ice. The story of how this happened is more complex and disputed than Antarctica’s deep freeze. But likewise, continental movements were involved. The gap was gradually closing between North and South America. Where the Central American Seaway had provided a direct connection between the Pacific and Atlantic, now the land bridge of Panama lay in the way, preventing the two oceans from directly mixing. This had profound effects on global ocean circulation and climate, including intensifying the Gulf Stream, which flows from the Caribbean Sea into the northeast Atlantic. The closure was also linked with the strengthening of the global conveyor belt, the ocean circulation that flows around the whole planet today. By pushing moisture-laden air northwards, the Gulf Stream likely played a part in boosting snowfall over Greenland and North America, increasing the size of glaciers that were forming in the Northern Hemisphere as the earth entered a prolonged and volatile ice age.
In the Cenozoic, the ocean settled into its present-day configuration and welcomed the arrival of many life forms that still prevail. A recurring theme during this era was the return of tetrapods to the ocean. Yet again, four-footed land animals gave in to the irresistible lure of the sea, many times over. The main protagonists this time weren’t reptiles, as in the Mesozoic. Instead, it was the turn of mammals to go back to their aquatic roots. Seven different types of mammals dipped their hooves, paws, and feet in the water and evolved ways to survive at sea, in time becoming integral parts of ocean ecosystems.
Around fifty million years ago, an assortment of prehistoric mammals walked the shores of the Tethys Ocean, an ancient water body that used to connect the Atlantic and Indian Oceans across northern Africa. Among them were animals that trotted on long, thin legs and had something of the wolf in them. These were archaeocetes, the beginning of a long animal dynasty that led up to modern cetaceans—the whales and dolphins. They were animals with even-toed hooves, a group that today includes giraffes, camels, antelopes, llamas, sheep, pigs, and cows. Modern cetaceans’ closest living relatives are hippopotamuses.
Proto-whales went through a series of evolutionary stages, as shown by elegant fossils, at first amphibious then increasingly oceanic. They followed parallel lines of evolution leading them to resemble many of the giant marine reptiles that swam before them. Their legs turned into flippers, and their tails sprouted powerful, wide flukes, although they didn’t swing sideways, like the tails of swimming reptiles, but up and down, a throwback to the gait of their terrestrial forebears; contrast a reptilian gecko, which sashays along flexing its spine from side to side, with a cheetah, which bends its back up and down as it runs. It took roughly ten million years for whales to become fully pelagic, with nostrils on the tops of their heads (i.e., blowholes) and inner ear bones adapted to hearing well underwater. But the cetacean lineage really took off a while later.
A key point in the evolution of whales was when Antarctica broke away from South America, and the global climate shifted into an ice house. This was when the two main cetacean lineages—the baleen whales (mysticetes) and the toothed whales (odontocetes)—diverged from each other. With Antarctica on its own, a current now whirled around the continent, which not only sent temperatures falling but also triggered deep mixing of the Southern Ocean, stirring nutrients into upper layers of the sea. Consequently, oceanic ecosystems around the world became enormously productive. Plankton proliferated, and the new group of filter-feeding whales with hairy baleen plates in their mouths had plenty to eat. Around that time, toothed whales began to interrogate the ocean with beams of sound, hunting for prey with their new echolocating powers. From these early odontocetes, a lineage of smaller cetaceans separated and began filling seas and rivers with dolphins, porpoises, narwhals, and orcas.f
At the time when the proto-whales were starting to wade and swim, a different group of mammals were splashing along the shores of the Tethys Ocean. Sea cows began as pig-size relatives of elephants with stumpy legs that walked on land and wallowed in the shallows. Also known as sirenians, they spread across the planet and evolved into assorted species of dugongs and manatees, which mostly lived along warm and tropical coasts and spent their days chewing on seagrasses. Some moved into the North Pacific rim and fed on the giant kelp forests that started growing there around thirty million years ago as the ocean was cooling. They shared their kelp forest home with yet another gang of aquatic mammals—desmostylians, stocky, hippo-size animals with two pairs of short, goofy tusks sticking out of their mouths. Like many other herbivorous marine mammals, they had thick, heavy bones that acted as ballast to help them sink while they foraged underwater, especially important when their rotund bellies filled up with digestive gases produced by their plant-based diet. Desmostylians existed for only a narrow window of time, between thirty-three and ten million years ago. All the North Pacific sea cows also went extinct, including, most recently, the biggest ever to evolve. Steller’s sea cow grew up to thirty feet long, several times the size of living sea cows. This giant was named after the German explorer Georg Wilhelm Steller, who encountered it in the 1740s while shipwrecked on the Commander Islands, off Kamchatka, Russia. By then the species was already on its way out, driven into decline by the changing climate during the last ice age. When sea levels dropped during glaciations, much of its kelp habitat became fragmented. Human hunters played a part in the Steller’s sea cows’ annihilation, killing the last of them for their oil and for their meat, which apparently tasted like corned beef.
Outliving the kelp-dwelling sea cows and desmostylians are mammals that much more recently took to the ocean. Sea otters evolved in the North Pacific from weasel-like ancestors only in the past two to three million years, and at up to a hundred pounds, they’re the heaviest mustelids but the smallest marine mammals.
Farther south, seagrass meadows along the South American coast were grazed by giant swimming sloths. Five species of Thalassocnus were related to the giant ground sloths that lumbered around the pampas grasslands. These aquatic sloths grew up to eleven feet long from snout to tail, longer than a bison, and they bounded along the seabed, steering with their tails like beavers. Giant sloths swam about for fewer than five million years, a brief moment in the ocean’s history. When the Central American Seaway closed, the changing ocean currents and falling temperatures killed off swaths of seagrasses along the South American coasts. With little to eat and no blubber to keep them warm, the furry, swimming sloths came to an end.
The same climatic shift caused a major stir among the cetaceans too, with the arrival of conditions that favoured outrageously big whales. When the Northern Hemisphere froze over, sea levels dropped, and patches of rich seasonal food developed in the ocean. Smaller baleen whales were confined to the coasts, where the seawater was draining away, and many of them went extinct around three million years ago. Meanwhile, much bigger whales were thriving. They set off on long journeys, cruising between high-latitude feeding and low-latitude, tropical breeding grounds. Today, enormous whales migrate every year to feed in the Arctic or Antarctic, the fifty-foot humpbacks and grey whales, the eighty-foot fin whales, and the largest animals known to have existed, the hundred-foot blue whales.
The Cenozoic also saw several other mammal groups take to the ocean. Pinnipeds originated in the Arctic at least twenty-four million years ago from a similar group of ancestors to the sea otters. Indeed, early on they looked a lot like three-foot-long otters, but their fossilised teeth give away that they were in fact seals. Three groups of pinnipeds have since diverged and moved onto shores around the world: the seals, sea lions, and walruses.g In the Arctic, walruses and seals haul out on the sea ice to rest and raise their young, all the while watching out for polar bears, the mammals that most recently took to the sea. Polar bears diverged from brown bears and began prowling the Arctic ice within the past half million years.
For seals in Antarctica, there are no polar bears to fear, and they share the Southern Ocean with another gaggle of vertebrates that also dived into the Cenozoic ocean and became Antarctic specialists. Direct ancestors of modern-day penguins evolved twenty million years ago on temperate coasts and islands of Aotearoa (New Zealand)h and Australia. From there they moved south and took advantage of Antarctica’s circumpolar current to disperse around the continent, gradually adapting along the way to the freezing climate and diverging into new species. First to evolve were the biggest living species, which stand waist-high to a human: the emperor and king penguins. Then came gentoos, chinstraps, and Adélies.i All of them eat krill, the finger-size swimming crustaceans that evolved at a similar time and became the staple diet of most Southern Ocean animals. Later, around eleven million years ago, the Antarctic current strengthened and propelled penguins northwards to warmer climes, including back to Aotearoa and even as far as the equatorial Galápagos Islands, but they’ve not made it as far as the Arctic. Penguins remain resolutely Southern Hemisphere species.
The many new arrivals in the Cenozoic ocean joined a mix of marine life that had survived from earlier epochs and was busy shifting into a modern guise. During the Cenozoic, modern day coral reefs took shape. Previously, reefs had been built by various kinds of corals as well as sponges and shells. Then, after millions of years of coming and going through multiple mass extinctions, the scleractinian corals finally gained a firm footing in the ocean, and their diversity steadily increased. Individual corals are tiny, flower-like polyps that construct around themselves exoskeletons of limestone, hence their other name, the stony corals. In great colonies, they form the main foundation for tropical reefs today.
The Cenozoic also saw a huge proliferation of fish species, in particular the group known as the spiny-rayed fishes because they have sharp bony spines in their fins.j Flatfish, tuna, swordfish, rockfish, mackerel, and cod are just some of the many abundant spiny-rays in the ocean today. Coral reefs are brimming with spiny-rayed fishes, among them the butterflyfish and angelfish, parrotfish, pufferfish, and groupers. Seahorses evolved in the Cenozoic when seagrass meadows spread between Australia and Indonesia. Their unusual upright posture, horses’ heads, and curling, gripping tails were all adaptations seahorses evolved that camouflage them and help them navigate through the grass blades.
While bony fishes with spiny rays were doing very well in the Cenozoic, another group of fishes, with bendy, cartilaginous skeletons, were not so lucky. Sharks had been swimming through the ocean for hundreds of millions of years but almost went extinct nineteen million years ago. This near-extinction was discovered only in 2021, from a study of their toothlike scales, called denticles, which dropped down onto the deep seabed and became trapped in sediments. Close to three-quarters of shark species disappeared, and it’s not obvious why. Fossils suggest that sharks have never been as abundant as they once were, but within a few million years they began to regain their diversity and to get a lot larger. The megatooth shark lineage was made up of species that gradually got bigger through the Cenozoic, with the unrivalled giants evolving around sixteen million years ago. Megalodonsk had seven-inch-long teeth and ten-foot-wide jaws and from head to tail were between fifty and sixty feet long, roughly two full-size school buses parked bumper to bumper. The species went extinct 3.6 million years ago, shortly after the arrival of a newcomer, the great white shark. It’s possible great whites pushed their competitors towards extinction by hunting the same prey as young megalodons.
At odds with the popular idea that sharks are prehistoric creatures that haven’t changed much in millions of years, new species have evolved relatively recently in the Cenozoic. The youngest belong to a group known as the walking sharks, which live in shallow seas of Papua New Guinea, Indonesia, and northern Australia. These three-foot-long sharks spend their days resting on the seabed, then set off at night to hunt, using their pectoral and pelvic fins as rudimentary legs, strutting along in a gecko-like gait. With their leisurely strolling and reluctance to swim, these sharks never move too far under their own steam. The most westerly species arrived in its current location in Indonesia not by swimming but by riding on its home island. That’s how sedentary these sharks are—they use the geological forces of tectonic drift to get around. This means walking-shark populations are easily cut off from one another, between one island and the next, increasing the chances of splitting into separate species. All nine known species of walking sharks evolved in the past nine million years, and the youngest species split apart only two million years ago.
The view we now have back through deep time tells us that life on earth and in the ocean has always been turbulent and ever changing. Life has been sculpted by life itself and steered by powerful external forces that shift currents and continents and alter the climate. Now, in the present moment we’ve reached in the Cenozoic era, a single global force has come into effect. The Anthropocene, the age of humans, is underway.
Homo sapiens has been modifying the earth for millennia, by hunting other animals, felling forests, burning vegetation, and domesticating animals and crops. But only in recent times have humans been altering the planet on such vast scales and in so many ways. The word Anthropocene is intended to embrace the accumulating and accelerating ways human activities have become dominant agents of global change, from chemical pollution to climate change, from the mass rearing of livestock to the rampant destruction of biodiversity.
When exactly the Anthropocene began is a rather esoteric matter, one that a team of geologists, the Anthropocene Working Group, has been considering since 2009. In 2023, they proposed that a lake in Canada serve as the marker for the start of the Anthropocene, with the idea it will be identifiable thousands of years in the future. Crawford Lake, near Toronto, is six acres in area and almost eighty feet deep, and contains layers of undisturbed sediment that gently sink down into it, including key molecules that indicate when human activities shifted gear to a world-altering scale. A thin layer laid down in 1952 contains plutonium particles from the hydrogen bomb tests that drifted around the world. There are also spherical particles of carbon released by the widespread burning of fossil fuels, and nitrates released by the flood of chemical fertilisers. For Crawford Lake to be officially adopted will require three more committees of geologists to vote in favour, but already the idea of the Anthropocene has taken hold.l
A critical part of anthropogenic change is the endangerment of thousands of species that is driving the earth’s sixth mass extinction. Humans have already caused countless species to disappear, ranging from documented extirpations, like that of Steller’s sea cow, to the hidden extinctions as species slip away when entire ecosystems are demolished. One way to get a sense of the scale of what’s happening now is to compare current extinction rates against the background rate at which species have steadily been lost from the fossil record in aeons gone by when there wasn’t a mass extinction raging. Whichever way you slice the data, the rate of extinction is now far higher, perhaps a hundred times, than during those peaceful intermissions of the past. A contemporary mass extinction is in full swing.
Messages from deep time warn that there will be no creative destruction, no wiping clean of the slate to make way for a flourishing new era of biodiversity. New radiations of species will happen in time, but they’re unlikely to be the direct result of Anthropocene extinctions. There will be no swift recovery of lost species, and certainly not on timescales relevant to human lives. Following even the milder mass extinctions of the past, biodiversity has taken hundreds of thousands of years to recover. If our collective impact comes close to the worst extinctions, life on earth will be diminished for millions of years to come. Decisions made now in the Anthropocene will set changes in motion that will alter the path of life far beyond the likely lifespan of humanity itself.
There’s no telling who the survivors in the long term will be and what kind of new living worlds they will build together in the ocean. If the past is anything to go by, probably predators and prey will continue to exist, as will animals swimming around with hydrodynamic bodies and quite possibly flippers and tails. There’s no telling which group of animals might take on those roles, any more than we could have guessed that an ocean filled with trilobites scurrying and paddling around would be replaced by a realm ruled by giant swimming reptiles, then much later by a similar-looking crowd of mammals.
Perhaps another group of vertebrates will fill niches vacated by the sixth mass extinction; maybe birds will become the next underwater giants, or perhaps an age of oceanic amphibians will dawn and frogs, toads, and salamanders will follow their ancestors back to the seas. Or it could be the turn of invertebrates to become the ocean’s dominant predators once again. Octopuses or squid could rise up and diversify into blood-chilling apex predators or gentle filter feeders. Reefs too could keep growing along the edges of continents and islands, built by new kinds of builders; maybe a different group of corals, shells, or seaweeds will take on that role, or maybe sea stars or sea urchins will press their hard skeletons together in great colonies and shoulder the ramparts of new reefs. Until the ocean enters a post-Anthropocene era, we can only guess what will come next.
