Glyptodont

©AMNH / Denis Finnin

Darwin was delighted by the armadillos he saw scurrying about in Argentina. But most intriguing was the striking similarity of these small, armored mammals to some of the fossils he was unearthing. One such fossil was a glyptodont, an immense shelled animal that looked like a giant armadillo. In fact, to Darwin, many ancient, extinct species seemed to be giant versions of living species.

Why, Darwin wondered, had so many species gone extinct, only to be replaced by similar ones? And not just once, but again and again? Perhaps the newer species were better suited to the changing environment, he reasoned. All around him, he was seeing evidence of slow, gradual, geological changes. But if Earth's changes were slow and gradual, what did that mean for the changes in species? Back in England, Darwin would ask himself: Over long periods of time, could older species have evolved into new ones?

Armadillos for Breakfast

Though Darwin was charmed by the armadillos' behavior, he was equally interested in having a fine meal.

"In the morning we had caught an armadillo, which, although a most excellent dish when roasted in its shell, did not make a very substantial breakfast and dinner for two hungry men."

Armored Giant

Among Darwin's most dramatic finds was the "armour of a gigantic armadillo-like animal, the inside of which...was like a great cauldron." What were these ancient, extinct animals? It was difficult to tell, since many bones were often mingled together. At one site, Darwin at first thought the shell of a glyptodont went with the skeleton of a Megatherium--a giant sloth that in fact had no armor. He shipped them all back to London for more seasoned scientists to sort out.

Darwin tried to figure out each fossil's age from the age of the rocks and shells around it, and his new quest topped even his former passion:

"There is nothing like geology. The pleasure of the first days, partridge shooting...cannot be compared to finding a fine group of fossil bones, which tell their story of former times with almost a living tongue."