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Experts in the early twentieth century carried on this fatalistic assumption. Dinosaurs were big, bizarre, and anatomically extravagant. The question wasn't why they died out. The real mystery was how they could have persisted for so long, especially when the clearly superior mammals were waiting in the wings to take charge.

Our mammalian conceitedness held on for decades. Even when the disappearance of the dinosaurs became a more legitimate question, the explanations were most always delivered in such a way that the dinosaurs themselves were to blame. The great, trundling reptiles laid eggs and cared little for their young, so mammals feasted on dinosaur omelets. (At this point researchers had paid no attention to the admirable parental oversight of alligators or snakes.) Or dinosaurs clearly invested so much energy and growth into becoming huge, spiky, and strange that they simply ran out of nebulous vital juices. How could a ten-ton rhino look-alike, studded with three horns and a bony collar around its neck, compete with the up-and- coming mammals? The mental capacities of dinosaurs were famously small, to boot. A cold-blooded reptile like a 'Stegosaurus' or 'Ceratosaurus' was perfectly suited to a lush world of sweltering jungles and dim-witted prey, but the lazy dinosaurs simply did not care to innovate, or even be open to the possibility. And if this is all sounding a little corporate to you, it should come as no surprise that these ideas proliferated during America's great industrialization; "going the way of the dinosaur" is still a phrase used to tar competitors in financial circles.

In time, scientists began to accept the fact that animals do not have internal timers that regulate when species are "born" or "die" according to some cosmic clock, and the ideas about the expenditure of evolutionary energies was misplaced. There had to be some natural explanation. Refining the geological timescale made the question all the more puzzling. Dinosaurs did not represent a primitive lull as the world waited for the rise of mammals. Non-avian dinosaurs persisted for over 150 million years before abruptly disappearing at what seemed to be their apex. There had to be a reason.

Almost everyone had an opinion. Maybe the climate got too hot. Or maybe the climate got too cold. Perhaps some terrible disease ripped through their populations, or sea level rise ruined their favored habitats. Specialists from other fields chimed in, too. An opthalmologist proposed that dinosaurs had terrible cataracts, meaning that the impressive headgear of dinosaurs like the crested shovel-beak 'Parasaurolophus' and the many-horned herbivore 'Styracosaurus' had evolved as the world's first sunshades. An entomologist spitballed that early caterpillars ate vegetation at such a voracious rate that there was no green food left, meaning that soon after there was no meat, either. Or maybe the time was simply right for mammals. Dinosaur diversity at the end of the Cretaceous seemed low compared with what it had been 10 million years prior. Maybe, after tens of millions of years, mammals started to flex their muscle a little bit and carve out more of the landscape for themselves.

The problem was that many experts focused on dinosaurs alone when the real devastation cut much deeper. Yes, an army of very hungry caterpillars could have denuded Cretaceous forests at a terrible rate, but that explanation did not explain why the flying pterosaurs of the air or the broad, flat rudist clams of the sea went extinct 66 million years ago, much less species of armored amoebas called forams that precisely track the extinction even though their witness testimony to the disaster will never be a cover story. Everyone was so tensely focused on the dinosaurs that the larger pattern was obscured even as experts continued to tabulate the Cretaceous body count.

It was only in the late twentieth century, when the signature of mass extinctions began to coalesce for paleontologists focused on the comings and goings of ancient mollusks and arthropods, that the fate of the dinosaurs started to take on a new gloss. The invertebrate record showed a sharp uptick in extinction at the end of the Cretaceous. The forams and armored balls of algae called coccoliths documented a sudden and horrible event. This is when dinosaurs disappeared, too. Something awful must have happened. Now the question was what.

Experts searched for a compelling cause to explain the devastation. At first, it seemed that some terrestrial trigger was to blame. At the end of the Cretaceous, right when the dinosaur record seems to evaporate in rock strata worldwide, the planet was changing. Sea levels dropped. The climate shifted. Volcanic rifts in the Earth's crust emitted tons upon tons of greenhouse gases into the atmosphere.

It seemed as if dinosaurs simply couldn't keep up with the Red Queen's evolutionary race; they fell behind as mammals kept the adaptive beat. But this story didn't quite fit either. Paleontologists working on the comings and goings of ocean mollusks and other invertebrates didn't see a slow changing of the guard. Better fossil sampling and revised statistical techniques affirmed that life at the end of the Cretaceous was weathering the changes perfectly fine. Then suddenly life suffered a major shock. Something terrible had clearly befallen Earth's biota. The answer didn't come from fossils themselves but from the rock that entombed them.

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