In the wake of our asteroid near-miss, people keep claiming that an asteroid wiped out the dinosaurs. But we all know it was the Cybermen. Or time-traveling hunters. Here are science fiction's best dinosaur-extinction theories.
Time travelers hunted them to extinction:
"Big game hunters from the future may have wiped out the dinosaurs," suggests Arthur C. Clarke, in the course of explaining one theory of time travel. (That you can travel millions of years in either direction, but not any closer to your own time, because then you run into your own timeline.)
Also in the early Isaac Asimov story "Big Game," aka "The Hunted," an explorer hears a drunken story about time travel and what really killed the dinosaurs off. A similar idea plays out in A Gun For Dinosaur by L. Sprague DeCamp and "A Sound Of Thunder" by Ray Bradbury, but I don't think it actually results in the dinosaurs' extinction in those instances.
There's a weird-as-heck twist on this idea in the story "One Giant Step" by John E. Stith. In an alternate future, the super-intelligent descendants of the dinosaurs invent time travel. So they travel back in time 65 million years to meet their dinosaur ancestors. "That's one small step for a reptile, one giant step for Reptilia." But one of the three travelers, Ektor, has smuggled back some bombs, which he uses to wipe out all his own ancestors, because of all the suffering and mass extinction of other species the reptile overlords have caused. "Reptiles were not meant to rule the Earth... let some other species take over," he announces before detonating.
And commenter QuantCoates points out that 2000 A.D.'s strip Flesh demonstrates conclusively that stegosaurus-riding cowboys from the future actually killed the dinosaurs.
Aliens did them in:
The Cybermen, Doctor Who's second worst enemies, turn out to have killed the dinosaurs - and the Doctor's math-whiz sidekick Adric - in "Earthshock." It's all because Adric wasn't quite fast enough with those logic puzzles, actually.
In Animorphs, Megamorphs #2: In The Time Of Dinosaurs by K.A. Applegate, the Animorphs travel backwards in time to the dinosaur era. There, they meet two warring alien races, the Mercora and the Nesk. The Nesk divert a comet so that it slams into the Earth, aiming to destroy the Mercora. But instead, the comet merely wipes out the dinosaurs, and provides power for the Animorphs to return to their own time again.
That's also the premise of the story "The Dreams A Nightmare Dreams" by Harlan Ellison. (And in the essay "Revealed! What Killed The Dinosaurs! And You Don't Look So Terrific Yourself," Ellison also suggests it was actually television and other mass culture that wiped them out.)
And then there's the Futurama episode, "The Why Of Fry." Fry asks the Giant Brain at the center of the Infosphere what killed off the dinosaurs, and it replies, "Meeee!"
Cthulhu, an alien from way back, takes credit for wiping out the dinosaurs in Neil Gaiman's short story "I, Cthulhu." (Thanks to commenter m_faustus for pointing this out.)
A superior species emerged:
The classic movie Reign Of Fire explains everything. Dinosaurs died out because dragons evolved as a superior life form, and the poor dinos just couldn't compete. If only the dinosaurs had had Christian Bale on their side.
Writer Jeff Hecht also advances this theory in his story "Extinction Theory," which ran in Asimov's Science Fiction in 1989. It postulated that "the evolution of intelligent dinosaurs was the real cause of the mass extinction at the K/T boundary."
They did it to themselves:
In the Jim Henson Productions show Dinosaurs, which is basically The Simpsons starring dinosaurs, we follow a whole dinosaur civilization... but the show ends on a downer note. The dinosaurs abuse their own environment and cause the extinction of several species they need to survive... so they try to cool the planet down, seeding some clouds with special rain-causing bombs. This overshoots and causes an Ice Age, wiping the poor dinos out.
A similar fate befalls the advanced dinosaur civilization in the novel Toolmaker Koan by John McLoughlin, according to commenter EllenRose.
They didn't die out after all.
Once again, Doctor Who comes to our rescue. In the stories "The Silurians," "The Sea Devils" and "Warriors Of The Deep," we learn that some of the dinosaurs had a super-intelligent civilization, which left no trace. The super-smart dinosaurs went into suspended animation, bringing some of their regular dinosaur brethren with them.
The Lost World by Arthur Conan Doyle has explorers visiting a hidden land where dinosaurs still survive. Which, presumably, helped to inspire King Kong, Land Of The Lost, and countless others "dinosaurs still hanging out" tales.
And then there's the always-reliable Star Trek: Voyager, which introduces the Voth, a race of super-intelligent hadrosaur descendents in the episode "Distant Origin." Before the extinction happened, the dinosaur people left Earth and wound up in the Delta Quadrant. Here's the end of the episode, with TOS music added randomly:
The cartoon DinoSquad, according to Alasdair, tells of two Velociraptors who decide to wait out the dinosaur extinction by hanging out in a cave... for 65 million years. Just, you know, chillin'. At some point, they develop telepathic powers (like one does) and when they finally emerge, they have the power to convince people they're actually human. Oh, and one is good and the other is evil.
The dinosaurs survive the rise of homo sapiens - who start killing them off, in the Anonymous Rex series by Eric Garcia. So the dinosaurs live amongst us in secret, wearing fake human suits, and carry on their dinosaur culture in secret. (Thanks to ItMoons for pointing this one out!)
And of course, the novel Dinosaur Wars: Counterattack tells the story of what happens when the dinosaurs come back from outer space - and they want their planet back.
Or maybe it really was an asteroid:
That's the theory advanced in Armageddon, where a similar fate awaits humans. Or the novel In The Shadow Of Omen, where an asteroid is directed to smash into Earth on purpose. Or Shiva Descending by Gregory Benford and William Rotsler. There's also the Night Of The Comet explanation, as commenter Se7a7n7 points out: a comet turned the dinosaurs to red dust... and it's coming back.
Additional reporting by Alasdair Wilkins.