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Today, all the geologic and fossil evidence at our disposal points to the most likely theory of dinosaur extinction: that an astronomical object (either a meteor or a comet) smashed into the Yucatan peninsula 65 million years ago. However, there are still a handful of fringe theories lurking around the edges of this hard-won wisdom, some of which are proposed by maverick scientists and some of which come from creationists and conspiracy theorists. Here are six alternative explanations for the extinction of the dinosaurs, ranging from reasonably argued (volcanic eruptions) to just plain wacky (intervention by aliens).01of 06
Starting about 70 million years ago, five million years before the K/T Extinction, there was intense volcanic activity in what is now northern India. There is evidence that these "Deccan traps," covering about 200,000 square miles, were geologically active for literally tens of thousands of years, spewing billions of tons of dust and ash into the atmosphere. Slowly thickening clouds of debris circled the globe, blocking sunlight and causing terrestrial plants to wither - which, in turn, killed the dinosaurs that fed on these plants, and the meat-eating dinosaurs that fed on these plant-eating dinosaurs.
The volcanic theory of dinosaur extinction would be extremely plausible if it weren't for the five-million-year gap between the start of the Deccan trap eruptions and the end of the Cretaceous period. The best that can be said for this theory is that dinosaurs, pterosaurs, and marine reptiles may well have been adversely impacted by these eruptions, and suffered an extreme loss of genetic diversity that set them up to be toppled by the next major cataclysm, the K/T meteor impact. There's also the issue of why only dinosaurs would have been affected by the traps, but, to be fair, it's still not clear why only dinosaurs, pterosaurs, and marine reptiles were rendered extinct by the Yucatan meteor.
The world was rife with disease-creating viruses, bacteria, and parasites during the Mesozoic Era, no less than it is today. Toward the end of the Cretaceous period, these pathogens evolved symbiotic relationships with flying insects, which spread various fatal diseases to dinosaurs with their bites. For example, a study has shown that 65-million-year-old mosquitoes preserved in amber were carriers of malaria. Infected dinosaurs fell like dominoes, and populations that didn't immediately succumb to epidemic disease were so weakened that they were killed off once and for all by the K/T meteor impact.
Even proponents of disease extinction theories admit that the final coup de grace must have been administered by the Yucatan catastrophe. Infection alone couldn't have killed all the dinosaurs, the same way that bubonic plague alone didn't kill all the world's humans 500 years ago. There's also the pesky issue of marine reptiles. Dinosaurs and pterosaurs could well have been prey for flying, biting insects, but not ocean-dwelling mosasaurs, which weren't subject to the same disease vectors. Finally, and most tellingly, all animals are prone to life-threatening diseases. Why would dinosaurs and other Mesozoic reptiles have been more susceptible than mammals and birds?
A Nearby Supernova
NASA/ESA/JHU/R.Sankrit & W.Blair/Wikimedia Commons/Public Domain
A supernova, or exploding star, is one of the most violent events in the universe, emitting billions of times as much radiation as an entire galaxy. Most supernovae occur tens of millions of light years away, in other galaxies. A star exploding only a few light years from Earth at the end of the Cretaceous period would have bathed the planet in lethal gamma-ray radiation and killed all the dinosaurs. It's hard to disprove this theory since no astronomical evidence for this supernova could survive to the present day. The nebula left in its wake would long since have dispersed across our entire galaxy.
If a supernova did, in fact, explode only a few light years from Earth 65 million years ago, it wouldn't only have killed the dinosaurs. It would also have fried birds, mammals, fish, and pretty much all other living animals, with the possible exception of deep-sea-dwelling bacteria and invertebrates. There is no convincing scenario in which only dinosaurs, pterosaurs, and marine reptiles would succumb to gamma-ray radiation while other organisms managed to survive. In addition, an exploding supernova would leave a characteristic trace in end-Cretaceous fossil sediments, comparable to the iridium laid down by the K/T meteor. Nothing of this nature has been discovered.
Andy Hay/Flickr/CC BY 2.0
There are actually two theories here, both of which depend on supposedly fatal weaknesses in dinosaur egg-laying and reproductive habits. The first idea is that, by the end of the Cretaceous period, various animals had evolved a taste for dinosaur eggs and consumed more freshly-laid eggs than what could be replenished by breeding females. The second theory is that a freak genetic mutation caused the shells of dinosaur eggs to become either a few layers too thick (thereby preventing the hatchlings from kicking their way out) or a few layers too thin (exposing the developing embryos to disease and making them more vulnerable to predation).
Animals have been eating the eggs of other animals ever since the appearance of multicellular life over 500 million years ago. Egg-eating is a basic part of the evolutionary arms race. What's more, nature has long since taken this behavior into account. For example, the reason a leatherback turtle lays 100 eggs is that only one or two hatchlings need to make it into the water to propagate the species. It's unreasonable, therefore, to propose any mechanism whereby all the eggs of all the world's dinosaurs could be eaten before any of them had a chance to hatch. As for the eggshell theory, that may conceivably have been the case for a handful of dinosaur species, but there is absolutely no evidence for a global dinosaur eggshell crisis 65 million years ago.05of 06
Changes in Gravity
Most often embraced by creationists and conspiracy theorists, the idea here is that the force of gravity was much weaker during the Mesozoic Era than it is today. According to the theory, this is why some dinosaurs were able to evolve to such gargantuan sizes. A 100-ton titanosaur would be much more nimble in a weaker gravitational field, which could effectively cut its weight in half. At the end of the Cretaceous period, a mysterious event - perhaps an extraterrestrial disturbance or a sudden change in the composition of the Earth's core - caused our planet's gravitational pull to increase drastically, effectively pinning larger dinosaurs to the ground and rendering them extinct.
Since this theory is not based in reality, there's not much use listing all the scientific reasons that the gravitational theory of dinosaur extinction is complete nonsense. There is absolutely no geological or astronomical evidence for a weaker gravitational field 100 million years ago. Also, the laws of physics, as we currently understand them, do not allow us to tweak the gravitational constant just because we want to fit the "facts" to a given theory. Many of the dinosaurs of the late Cretaceous period were moderately sized (less than 100 pounds) and, presumably, would not have been fatally afflicted by a few extra gravitational forces.06of 06
Toward the end of the Cretaceous period, intelligent aliens (who had presumably been monitoring the Earth for quite some time) decided that dinosaurs had a good run and it was time for another type of animal to rule the roost. So these ETs introduced a genetically-engineered supervirus, drastically altered the Earth's climate, or even, for all we know, hurled a meteor at the Yucatan peninsula using an inconceivably engineered gravitational slingshot. The dinosaurs went kaput, the mammals took over, and 65 million years later, human beings evolved, some of whom actually believe this nonsense.
There is a long, intellectually dishonorable tradition of invoking ancient aliens to explain supposedly "unexplainable" phenomena. For example, there are still people who believe that aliens constructed the pyramids in ancient Egypt and the statues on Easter Island - since human populations were supposedly too "primitive" to accomplish these tasks. One imagines that, if aliens really did engineer the extinction of the dinosaurs, we would find the equivalent of their soda cans and snack wrappers preserved in Cretaceous sediments. On this point, the fossil record is even emptier than the skulls of the conspiracy theorists who endorse this theory.
Poinar, Geroge Jr. "An ancient killer: ancestral malarial organisms traced to age of dinosaurs." Oregon State University, March 25, 2016.