One fact that paleontologists have always agreed upon is that the dinosaurs are all dead. Beyond that, there's been plenty of room for disagreement—especially concerning the cause of the giant reptiles' extinction. Here are 10 of the most unusual theories.

1. Rickets!


According to a 1928 report in the Science News-Letter, Harry T. Marshall, a pathologist at the University of Virginia, speculated that the dinosaurs died of rickets after clouds of dust obscured the sun and cutoff their supply of ultraviolet (UV) light. (Rickets—caused by deficient levels of vitamin D, calcium and phosphates—is the weakening or softening of the bones, which can lead to deformities.)

Marshall argued that the UV-deprived metabolism of the dinosaurs could not produce sufficient levels of vitamin D. Moreover, he suggested that ferns and other fodder, "lacking ultra-violet energy," would cease producing the nutrients that could counter rickets. Over a period of just a few generations, he claimed, the dinosaurs limped into oblivion.

2. Poison!


In 1967, Seil Koch of the U.S. Geological Survey, published a paper suggesting that selenium, which is found in metal sulfide ores, slowly poisoned the dinosaurs:

Selenium was deposited throughout the world from volcanic lava and gases in Mesozoic time. The decay of selenium-bearing rocks probably produced seleniferous vegetation. These toxic plants were eaten by the herbivorous dinosaurs, whereas the meat-eating animals were poisoned by the selenium ingested and retained in the fatty tissue and bones of foraging animals.

The survival of the mammals into Cenozoic time possibly can be explained. Mammals, having higher intelligence than the reptiles, may have had the ability to distinguish between toxic and nontoxic plants. Today, cattle avoid eating seleniferous vegetation if nontoxic vegetation is available.


3. Fungus!

In 2008, three Chinese researchers proposed that the comet impact 65 million years ago initiated the following chain reaction of events: Global wildfires led to large-scale deforestation, which provided an abundant nutritional source for vegetation-decaying fungi.


As more fungi grew, their spores were dispersed around the world, leading to the growth of even more fungi. Eventually, the density of the fungal spores overcame the immune systems of the dinosaurs, causing disease. Other fungal spores penetrated dinosaur eggs, preventing them from ever being born.

4. Cataracts!


In his 1982 book, The Last Dinosaurs, British ophthalmologist L.R. Croft attributed the extinction of the mighty reptiles to cataract blindness associated with increasing global temperatures. Croft proposed that many of the structures on the heads of dinosaurs—such as the Triceratops—were to protect the eyes from solar radiation. (The New Scientist magazine, apparently without any intended irony, called the theory "illuminating.")

5. Winter!


According to a 1967 article written by paleontologist John Cys: "All of the reptiles that survived the mass extinction at the end of the Cretaceous were forms that could readily find protected, insulated sites in which to hibernate with the onset of winter. Dinosaurs, on the other hand, were so large that finding a suitable hibernation site would have been next to impossible."

6. Plants!


Some scientists believed that the ongoing evolution of plant life sealed the fate of the dinosaurs. Their theory: During the Creaceous period, flowering plants and hardwood forests overran the land and supplanted the ginkgoes, cycadeoids and numerous conifers. Their swift success may have brought starvation to herbivorous dinosaurs, which could not adjust to a change of diet. And, as the herbivores starved, so did the carnivores. (A variation of this theory suggests that the new diet caused terminal constipation. Really.)

7. Eggshells!


In the early 1970s, French and German researchers reported finding dinosaur eggs whose shells were thinner than their predecessors. They suggested that the eggshells became so thin that they were too fragile to support the growing embryos.

Such eggshell thinning, they believed, could have been a result of hormonal changes triggered by overcrowding. Whatever the actual cause, it proved to be a local phenomenon.

8. Mammals!


"Some authors maintain that the horned herbivorous types of dinosaurs were in part destroyed by the large carnivorous dinosaurs," noted the Monographs of the United States Geological Survey (1907). "It seems that animals of another race, or hordes of creatures which emigrated from another region, would be more likely to exterminate their predecessors. The mammals fulfill the requirements of a new foe."

Some paleontologists believed that Ceratopsia (herbivorous, beaked dinosaurs) developed their distinctive bony frills to protect the blood vessels in their neck from the weasel-like attack of small but bloodthirsty mammalian quadrupeds. Others suggested that mammals wiped out the dinosaurs by feeding on their unhatched eggs and helpless infants.

9. Nudity!


In 1965, Loris Russel, the Director of the Royal Ontario Museum of Paleontology, published a paper arguing that the dinosaurs were doomed by their lack of insulation. Although Russel theorized that the dinosaurs were warm blooded, he believed that their lack of fur or feathers made them vulnerable to cold weather. "Each winter, mild as it would have seemed to us, saw more and more individuals and species failing to survive. Occasional, more severe winters would have accentuated the process."

10. Caterpillars!


In 1962, entomologist Stanley Flanders wrote a paper suggesting that the humble caterpillars exterminated the giant reptiles. He based this theory on the observation that, absent a natural predator, the insects were capable of reducing "great areas of lush vegetation to extreme sparseness." Flanders believed that caterpillars arrived on the scene before the emergence of natural enemies to limit their numbers. As such: "The inherent weakness of the reptile was an extraordinary need for an abundance of plant material. Only a few years of plant scarcity could have exterminated it. Hordes of caterpillars, in rapid consumption of fig and breadfruit, laurel and willow, oak and magnolia, could have so restricted the spatial distribution of the giant reptile that either its diet was inadequate or it was unable to avoid its natural enemies."