The extinction of the dinosaurs has long been a mystery. Generation after generation of paleontologists have proposed different mechanisms that could have sent the dinosaurs into oblivion. Today much of the debate over their extinction centers around the damage done by a large hunk of rock from outer space that struck the earth about 65 million years ago, but it can be fun to look back at some other hypotheses that were abandoned by scientists years ago.
One of my favorite discarded explanations for the extinction of the dinosaurs involves the concept of "racial senescence." During the time that it was being considered, the late 19th and early 20th centuries, paleontologists were unsure about what caused evolution. Not everyone agreed that natural selection was evolution's primary mechanism, and many scientists thought that evolution might be driven by internal forces that put organisms on predetermined evolutionary trajectories.
Racial senescence fit nicely into the idea that evolution had a set direction. Some scientists thought that species, like individual animals, had a lifespan. The evolution of a new species would be its birth and extinction would be its death. While the death of a species would ultimately be caused by environmental causes, the reason that they could not be adapted further was because they had become too "old."
Scientists thought they could see the signs of this "evolutionary old age," such as increases in size, the loss of characteristics possessed by their ancestors, or an increase in the number of spines, horns, or spikes on the body. This last trend, in particular, was based upon work with extinct invertebrates carried out by Charles Emerson Beecher, but the same marks of "degeneracy" seemed to mark dinosaurs, as well. Many were quite large, some appeared to be toothless, and varieties like Triceratops and Stegosaurus were very ornately ornamented. Clearly dinosaurs were ripe for extinction, and had been for a long time. This led paleontologist Richard Swann Lull to comment that "the marvel is, not that died, but that they survived so long."
Even then, though, it was known that some dinosaurs became extinct before others and not all the dinosaurs could be fit within these "degenerate" trends. What scientists had discovered did not neatly fit into the idea of racial senescence, and ultimately the idea was discarded when paleontology was combined with genetics, population biology, and other disciplines in the formation of the "modern" evolutionary synthesis in the 1940s and 1950s. There were no internal forces driving evolution or extinction; natural selection was the key to understanding both natural phenomena. Just what evolutionary pressures did in the dinosaurs, though, is still being debated.