Smithsonian.com

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What killed the dinosaurs? Paleontologists have been pondering that question since the late 19th century, when they recognized that a mass extinction occurred at the end of the Cretaceous Period, 65 million years ago.

Extinction theories have spanned the spectrum from the inspired to the bizarre. For instance, one popular explanation held that small mammals ended the reign of the giant reptiles by feasting on dinosaur eggs. While this theory might inspire a degree of pride (“Hey, our warm-blooded, furry ancestors totally smacked-down the dinosaurs!”), it doesn’t stand up to scrutiny. As the University of California Museum of Paleontology notes, “No egg-eaters could eat all of the dinosaurs’ eggs; they would eat themselves into extinction if they did (they would have no more food).”

In contrast to the egg-eating hypothesis, many other dinosaur extinction theories share a common theme: the recognition that a sudden change in the environment must have occurred. In this first installment of an ongoing look at extinction theories, one such hypothesis emerged in 1928. According to a 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.

Although the rickets theory never quite caught on, Marshall’s work was not entirely forgotten. More than seventy years later, writing in the journal Paleobiology, Stanford University’s Charles Cockell gave Marshall credit for introducing the concept of UV radiation to the study of mass extinctions. Cockell, however, took an opposing view and suggested that excessive UV levels, caused by periodic depletions of the ozone layer, might have been responsible for mass extinctions of other creatures, including plankton and coral reefs.

So, a tip of the hat to Harry Marshall, who looked at the demise of the dinosaurs in a whole new light.