Let’s clear something up right away. Dinosaurs aren’t extinct. Not entirely. Every magpie, pigeon, penguin, and ostrich alive today – every single bird – is a dinosaur. They’re all descendants of small, toothy, feathery dinosaurs that hopped and fluttered around from the Jurassic era onwards, meaning that birds are dinosaurs in the same way that bats are mammals. The Archaeopteryx laid out under glass at London’s Natural History Museum was the first of its ilk, and the only reason today’s birds seem so different is because the last of their close dinosaurian relatives trailed off into extinction about 66 million years ago.

But why did the most spectacular dinosaurs become extinct? Why don’t we have to worry about descendants of velociraptor tipping over garbage cans, and why can’t I feed a baby sauropod at the petting zoo? What happened to all those dinosaurs that inspire our dreams and fuel our nightmares?

We don’t know why most dinosaur species are extinct. That’s because the dinosaur timespan ranged over about 170 million years. It’s an easy figure to type, but almost impossible to truly understand. It’s long enough that the entire “age of mammals” could fit within the “age of dinosaurs” about two and a half times, and during this time dinosaurs were present the world over. Species came and went as evolution spun off new variations and extinction eventually picked them off, and, unfortunately, the fossil record doesn’t offer enough resolution to pinpoint why this or that species eventually vanished.

Facebook Twitter Pinterest The Land That Time Forgot, 1975: ‘What happened to all those dinosaurs that inspire our dreams and fuel our nightmares?’ Photograph: Michael Ochs Archives/Getty Images

The very last non-avian dinosaurs are a different story. Their ranks included some of the most famous and beloved dinosaurs – Tyrannosaurus, triceratops, ankylosaurus, pachycephalosaurus, and more. And even though the global climate was starting to cool and warm seas that once washed over vast expanses of Earth’s continents were beginning to recede, there was no sign that dinosaurs were about to exit the evolutionary stage. They had already been through millions upon millions of years of shifting continents, fluctuating climates, and one mass extinction, around 200 million years ago, that removed the competition and allowed dinosaurs to flower into the fantastic forms that have so tenaciously sunk their claws into our imagination.

Individual species might succumb to the environmental alterations, but dinosaurs, as a group, had the potential to keep flourishing.

It’s fun to think about what history would have been like if this had been the case. In fact, the animation studio Pixar has run with the concept for an entire movie, The Good Dinosaur, due out later this year. The reason they’re able to do so is because the end of the age of dinosaurs came in a geologic instant that irrevocably changed the course of evolution for the entire planet.

Even though the scenario was scientific heresy when first proposed in 1980, the idea that an immense asteroid sparked the dinosaur decimation has become canon. Unbeknown to the dinosaurs, a chunk of extraterrestrial rock about six miles wide was hurtling towards Earth. The bolide smacked into what is now Mexico’s Yucatán peninsula, leaving a geologic scar 110 miles wide and signalling the curtain call for the Cretaceous. No devastation witnessed in the course of human history can even get close to what happened. The immediate impact triggered enormous tsunamis that raced over nearby shores, and as hot debris sent skyward landed around the world, forests began to burn. But the worst effects were not immediate.

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Just prior to the asteroid impact, an area in prehistoric India called the Deccan Traps started pouring out an immense volume of molten rock. These eruptions spewed tons of greenhouse gases into the air, to which the products of the impact only added. Dust from the initial impact and soot from wildfires turned the sky black and blotted out the sun. The sky remained dark for years, depriving vegetation of sunlight. As the plants withered, so did the herbivores, and, in time, the carnivorous dinosaurs followed. And even though dinosaurs are the most famous victims of the catastrophe, they were not the only ones. The disaster also marked oblivion for various other forms of life, from coil-shelled cephalopods called ammonites and the giant marine reptiles that fed on them in the seas, to the leathery-winged pterosaurs that soared through the air.

Even lineages that we typically think of as survivors, such as mammals and lizards, lost lineages and suffered significant extinctions.

No one knows how long the extinction took. Through the lens of deep time, it’s practically instantaneous. There are places in the American west where you can even see the band of iridium-rich rock that marks the separation between a world ruled by dinosaurs and the one that allowed mammals to prosper. And, even now, no one really knows why the dinosaurs were knocked from dominance.

Much of what we know about the end-Cretaceous mass extinction comes from western North America. That’s because this expanse holds the continuous sequence of rocks documenting life before, during, and after the impact. Paleontologists and geologists are starting to examine sites elsewhere around the planet that hold similar records, but, for now, how the extinction played out is framed by what happened in a relatively small part of the world. We’re trying to comprehend the big, global picture by looking through a pinhole. And given that North America was relatively close to the site of impact and more affected than most other continents by the climate and habitat shifts going on at the end of the Cretaceous, what happened to the last of North America’s dinosaurs might not reflect what happened worldwide.

Facebook Twitter Pinterest Lambeosaurus cretaceous fossil: ‘Unbeknownst to the dinosaurs, a chunk of extraterrestrial rock about six miles wide was hurtling towards Earth.’ Photograph: Alamy

Yet, even within the best-studied deposits, we don’t yet have a good answer for why the spectacular and varied non-avian dinosaurs died out while some of the avian dinosaurs – birds – survived. What separated survivors from the dead at the end of the Cretaceous is one of the most elusive questions in paleontology. Many of the survivors were relatively small and either lived in water or were able to burrow underground. This may have shielded them from the heat and the fires, at least. But there were small non-avian dinosaurs alive during this time, and some of the surviving groups still lost diminutive members of their lineages. Try to make any hard-and-fast rule for survivorship during this catastrophe and you’ll wind up with an array of exceptions.

Extinction today is just as confounding. Think about what the fossil record we’re creating is going to look like 66 million years from now. Future paleontologists, if some other species spins off such experts, will see a pulse of extinction leading up from the end of the Ice Age to the present. They’ll probably conclude that our species was to blame, and rightly so, but will they be able to parse how we killed a particular species? That we’ve eliminated species because we viewed them as threats to livestock (Tasmanian tiger), clear cut their habitats (Macoun’s shining moss), and ate too many and introduced competitors to their environments (blackfin cisco), just for starters. Presence and absence is easy to tally, but the factors that lead to any extinction are difficult to tease out unless you witness the decline yourself.

We have a good handle on the extinction trigger for the non-avian dinosaurs – an enormous asteroid – but how that fleeting collision translated into a landscape dotted by dead saurians hasn’t been unravelled yet. We must continue to try. Perhaps one day we’ll be able to figure out what made the difference between death and survival during those intense years of dust and use that knowledge to help today’s imperilled species gain some resistance to extinction.

But even if we never gain such an understanding, it’s important to keep looking back. We may feel comfortable with our place as the dominant species on the planet, but the fate of the dinosaurs is a reminder that extinction and survival can rest on a tipping point we may never see coming.