A Cretaceous coastal landscape (detail). © Karen Carr Studio Inc and courtesy the Sam Noble Museum, Oklahoma


The dinosaurs didn’t rule

When we think of changes in Earth’s history as changes of dynasty we miss out on understanding how life really works

by Riley Black + BIO

A Cretaceous coastal landscape (detail). © Karen Carr Studio Inc and courtesy the Sam Noble Museum, Oklahoma

The worst day in the entire history of life on Earth happened in the northern springtime. On that day, the last of the Age of Dinosaurs, a roughly seven-mile-wide chunk of rock that had been hurtling towards our orbit for millions of years slammed into Earth’s midsection and immediately brought the Cretaceous to a close. The consequences were so dire that survival in the hours immediately following impact was merely a matter of luck.

Of course, life wasn’t totally extinguished on that day 66 million years ago. Some species survived, emerging into a transformed world. We can’t help but draw our own history to this specific moment, the dawn of the Age of Mammals, when fuzzy beasts could finally flourish. Dominant dinosaurs suffered a stroke of cosmic bad fortune, and our mammalian kin inherited a planet where they would no longer have to fear death in reptilian jaws. The image is of a great ecological cast change, different players continuing the evolutionary story. It’s a very appealing distortion.

The entire reason we so often fixate on the supposed dominance of the dinosaurs is because we now see ourselves in that position. For more than a century, the decimation of the ‘ruling reptiles’ has been taken as a cautionary tale of what could happen to us – not all that different from pundits who cry that the United States is set to topple like the Roman Empire. The narrative becomes one of power, influence and longevity, one group of organisms above all others deciding the course of entire ecosystems over the span of millions of years. Mass extinctions become examples of winners and losers. Where Tyrannosaurus rex and family faltered, the story goes, our mammalian relatives were victorious. The story says more about the way we interpret the past than what actually transpired; by creating a fairly-tale out of a distant prehistoric event, we’ve inflated our sense of importance in the world.

We are not bound to that view. We created the image of tyrannical dinosaurs ruling Earth. We can just as easily deconstruct it. The process requires returning to the mass extinction of the past, not looking for the victorious and the vanquished but considering how entire communities of living things change in the face of unimaginable disaster.

Prior to the disaster at the end of the Cretaceous, all of Earth’s mass extinctions were protracted, grinding transformations defined by species disappearing faster than new ones could evolve. Some of those extinctions, caused by active and erupting volcanoes, and the climate-altering gases they belched out, took more than a million years to unfold.

About 75 per cent of all known species went extinct in a geological snap of the fingers

The last day of the Cretaceous was different, a cataclysm of unfathomable speed and violence. The flying pterosaurs, the coil-shelled ammonites and all dinosaurs but birds vanished, not to mention deep losses to surviving groups of creatures such as lizards and mammals. No species could have prepared for what was to come, even if they had somehow been granted foreknowledge of the calamity. Within minutes of impact, the ground under the feet of dinosaurs in ancient Montana began to shake from seismic shockwaves emanating from the strike. Only a few hours later, tiny chunks of rock, glass and other debris thrown into the atmosphere by the strike began to rain down all over the planet. No single particle had much of an effect, but together the millions of tons of byproduct produced by the impact created so much friction that the result was a horrific heat pulse – hot enough to cause dry forest tinder to burst into flame. Earth’s temperature was set to broil, turning the last non-avian dinosaurs into what could be described as Cretaceous chickens in the oven. The mammals, birds, lizards and other meek creatures that would survive that first day did so by finding shelter underground, little more than a few inches of soil or water shielding them from the global conflagration.

And that was merely the first day, followed by three years of a biting, impact winter that would almost bring photosynthesis to a halt and test the limits of biological resilience. About 75 per cent of all known species went extinct in a geological snap of the fingers.

We often leave the story on the morning after the devastation ebbed, with some bewhiskered mammal sticking its twitching nose out of a dinosaur skull’s eye socket to take in a new dawn free of reptilian horrors. It’s a satisfying story. More than 66 million years removed from the last of those fantastic saurians, we often fill in the gaps with our expectations and assumptions. The asteroid was our ancestors’ deliverance, and through the aeons they pulled themselves up by their primordial fuzzy bootstraps to claim their own dominance over Earth. Dinosaur menageries in museums become bittersweet tributes to creatures that prowl our imaginations yet would have easily erased the possibility of our existence if they had been allowed to continue to keep their clawhold on the planet. Dinosaur decimation was a prerequisite for us to be here and interrogate their bones.

Considering one form of life dominant over others is worse than nonsensical. It’s a form of biological chauvinism that says everything about what we project on to nature and nothing about reality. As Stephen Jay Gould pointed out in his book Full House (1996), we may as well concede that Earth has always been in the Age of Bacteria with animal, plant and fungal life being rare anomalies by comparison.

According to this mythos, first dinosaurs ruled, then mammals did, with each evolutionary dynasty powered by some special character to outrace and outcompete other lifeforms, becoming incredibly diverse and widespread. In that iconography, there is no better example of dinosaurian prowess than T rex. Since the time the dinosaur was named in 1905, it’s been taken as the culmination of more than 150 million years of carnivorous innovations. Its very name, ‘king of the tyrant lizards’, feeds us this perception. Nevertheless, we can look at the gleaming, serrated smile of T rex and challenge conventional wisdom: what would this dinosaur have been without its prey species? And what would a Cretaceous magnolia tree be without a bumbling beetle covering itself in pollen at the heart of the tree’s flower?

T rex existed as part of an ecosystem, both shaped by and shaping the world around it. The dinosaur could even be said to have been an ecosystem unto itself, a living animal that harboured parasites and bacteria in and outside its body (just like us). The dinosaur was large, impressive and no doubt ferocious, but it was also a living thing at the intersection of various ecological connections. To say the dinosaur ‘ruled’ anything is ridiculous, a form of fossiliferous individuality that ignores broader communities. We’ve often ignored these threads in favour of simplicity, as if each surviving species were pitted against each other in a neverending battle for survival.

The extinction of T rex and all other dinosaurs save for the beaked birds was not a frivolous disappearance. It wasn’t the equivalent of a prehistoric apartment the dinosaurs cleared out to let mammals redecorate. A vast array of animals that shaped the world around themselves, as they also shaped the evolution of other species, suddenly vanished. The loss of the dinosaurs and the good fortune of the mammals had deeper ecological consequences for the fate of flowering plants, leaf-eating insects and various other forms of life that often make up the background of these stories. We’ve entertained the idea of shifting power between dynasties for far too long. How the asteroid changed the world isn’t a tale of shifting dominance, but how communities cope in the aftermath of disaster.

Every choice a dinosaur made altered the landscape in some fashion

Let’s take another look at the world after the impact, not in the heat of asteroid-triggered extinction but as life began to entwine in new ways. Try to put your mind back to the forest primeval, about a million years after the impact, approximately 65 million years ago. You’d likely hear the squawking of birds, the chattering of mammals and the trill of insects in a forest the likes of which the world has never seen before. These woods grow thick, flowering plants for the first time in their history forming the core of these humid glades, rather than conifers. Tree limbs spread wide and entangle with each other overhead, broad leaves shading the understorey far below.

Aside from the odd oldtimer crocodile, no animal in this environment gets larger than the size of a German shepherd. That fact alone has fundamentally changed the world. Prior to the impact, the average dinosaur weighed about three and a half tons and was roughly the size of a small African bush elephant. Such immense animals browsed and grazed bushels of vegetation at a time, trampled pathways through the forests, pushed over trees, and left plenty of chlorophyll-packed dinosaur pats to keep prehistoric dung beetles busy. Every choice a dinosaur like the three-horned Triceratops or shovel-beaked Edmontosaurus made altered the landscape in some fashion, from busting up rotting logs inhabited by invertebrates to creating shallow ponds in areas where they frequently churned the soil. Big dinosaurs kept the forests open and clustered together, their appetites and footfalls altering the shape of the forest itself. But now they are all gone, leaving forests to grow thick and tall.

The rise of those very forests relied on the few dinosaurs that survived. Birds were just another form of feathery dinosaur that evolved alongside their relatives since the late Jurassic, about 150 million years ago. Some kept their ancestral teeth, short nubs perfect for gripping crunchy insects or the occasional small lizard. But others evolved to be herbivores, losing their teeth entirely and evolving muscular gizzards to help them break down seeds, nuts and other sturdy plant parts.

Because these birds were very small compared with the average non-avian dinosaurs, they were able to find shelter in the crevices of the world, shielding many of them from the heat pulse. And during the impact winter that followed, when much of the world had been denuded of vegetation and small insect morsels, the beaked birds dug into the seed banks held safe in the soil. Beaked birds survived while any carnivorous survivors vanished, and the herbivorous birds would end up spreading the seeds they had survived on. Some of the seeds and nuts were busted and broken inside the birds’ digestive tracts, but others surely passed unscathed and were deposited with a gift of guano to begin reseeding the early Cenozoic woodlands. Such changes might have been swamped by the activities of the larger dinosaurs just a million years earlier, but now birds could plant a new kind of forest. And our furry ancestors certainly benefitted from these sweeping changes. Dinosaurs provided the foundation for the so-called Age of Mammals not by stepping aside, but by inadvertently helping to grow an entirely novel garden.

During the tens of millions of years prior to impact, ancient beasts did not shiver in the shadow of the dinosaurs as if waiting for an end to the sharp-toothed nightmare. Mammals and their close relatives evolved into a stunning array of forms during the Triassic, the Jurassic and the Cretaceous. There were ancient equivalents of flying squirrels, aardvarks, otters, squirrels and more that evolved right alongside the ‘terrible lizards’. The very first primates even evolved around the same time as Triceratops, a shrew-like animal called Purgatorius that scampered through the trees embodying the form of these earliest members of our own mammalian family. And while their small size is often overplayed – most mammal species alive today are mouse-size, after all – the diminutive statures of prehistoric mammals helped them find hiding places on the fateful day the asteroid struck. Many perished, but those that survived witnessed a world devastated by fire and brought nearly to a standstill by cold, living off the planet’s crumbs until the forests grew back.

A million years after impact, then, the world’s dense forests offered the surviving mammals a greater array of habitats than ever before. Mammals might take up a living searching for fruit and insects in the branches of the canopy, clamber along tree bark and branches in search of succulent leaves, chase down prey along the surface of the soil, or even burrow into the dirt itself. Competition for space and food is certainly part of the story, but primarily as a nudge for mammals to open up new niches and ecological interactions. The field was so open that some mammal lineages began to increase in size extremely rapidly, their bodies evolving larger on the surfeit of nutrition these forests offered.

Palaeontologists are only just beginning to understand what transpired in the first 10 million years or so after the Cretaceous came to a close. The earliest parts of this time, known as the Palaeocene, are preserved only in patches around the planet, and fossil evidence is sparse. What palaeontologists find interesting and which organisms gain the most attention have roles to play, too. The most mundane discovery about the life of T rex is more likely to get press attention and public interest than a new, strange Palaeocene bird or mammal. Some of these creatures have been known for more than a century but are only just beginning to be understood now as living things rather than static objects in museum drawers. We assumed that the story of what happened after the impact would be straightforward, as simple as survivors filling a void left by saurian giants. We were wrong.

Our actions are cutting through life’s web, affecting entire communities and ecosystems

Just as we have projected our hopes and worries on to the dinosaurs, the emerging image of changing, entangled communities ripples outward to our own time. We are living through an ecological crisis of our own making. The loss of every species, whether documented by science or not, is not just another tally of biodiversity’s losses. When a species vanishes, it leaves a void in its ecosystem. The way those living things uniquely interacted with the world vanishes, nudging adjustments in the ecosystem that once hosted the species. The extinction of a plant might alter nutrient cycling in a patch of forest of what plants a herbivore eats. The disappearance of a carnivore might make prey populations more vulnerable to disease if another predator doesn’t take up its role. A large herbivore’s population crashes and forests grow differently, some plants losing a means to disperse their seeds and others growing thicker in the absence of large feet trampling down trails through the woodland.

Evolution and extinction are bound together in these small, often-invisible interactions between species, the connections that continually shape the unique nature of life on our planet. In our present moment, we are not only playing a role in which lineages will survive and which will disappear. Our actions are also cutting through life’s web, affecting entire communities and ecosystems that will test the resilience of more species than we’ll ever count.

The history of life on Earth cannot be encapsulated as a balance sheet of losses and gains through time. Nor can our present moment be understood as different groups of creatures ceding the way for each other as life climbs the rungs of progress. The reality, like life itself, is messy. Comprehending what transpired 66 million years ago – or even in this moment – requires that we look beyond the details of what we can discern from a given species in isolation. Every fossil bone we uncover and carefully cradle in a museum grew from nutrition derived from other forms of prehistoric life; and those food sources, in turn, built their tissues from plants that took up essential components from the soils, enriched by the decay of yet other creatures that came before. Wherever we find life, one existence touches another, enmeshed and setting the conditions for what might appear tomorrow.