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The most famous of them was Megatherium, an elephant-size ground sloth that had already been named and described by the French anatomist Georges Cuvier on the basis of one set of fossils found in Paraguay. Living sloths are native to Central and South America, and only there; Megatherium shared many of their anatomical traits but was far too large for climbing trees. Darwin's finds also included at least three other giant ground sloths, an extinct form of horse, and a protective carapace of small bony scutes fitted closely together, remnant from some big beast that must have strongly resembled an armadillo. He was already familiar with flesh-and-blood armadillos, having eaten those shucked, ducky ones with his ostrich dumplings. He had also watched local gauchos kill armadillos and roast them in the shells. Of the 20 species of living armadillo, all are confined to the Americas and several are common on the Pampas; the roasted animals may have been six-banded armadillos (Euphractus sexcinctus), plentiful thereabouts and reputed to taste terrible, which might not have dissuaded those unfussy gauchos, who sometimes lived off the land for weeks. "Like to snails, all their property is on their backs & their food around them," Darwin wrote, referring to the cowboys, not the armadillos.

A month later, 30 miles up the coast from Punta Alta, Darwin discovered another fossil-rich sea cliff, this one rising 120 feet and marking a place called Monte Hermoso. There he unearthed the stony remains of several gnawing creatures, which variously put him in mind of an agouti, a capybara, and a smaller South American rodent, the tuco-tuco, except that again, in each case, the match between fossil and living species was close but not identical. Still later and farther south on the Argentine coast, he excavated a third set of mammal bones, which, to an anatomist who eventually examined them, suggested an extinct form of camel. That creature became known as Macrauchenia. The camel family includes two wild South American species, the guanaco and the vicua, as well as their domesticated forms, the llama and the alpaca. Darwin was well aware that living guanacos inhabited that area, having shot one himself just days earlier.

These discoveries, analogies, and juxtapositions went into his memory and imagination, to ferment there as the voyage continued and for years afterward. Meanwhile the fossils themselves were crated up for shipping back to England, mostly to the care of John Stevens Henslow, the gentle botanist who had been Darwin's mentor at Cambridge.

"I have been lucky with fossil bones," he told Henslow in a letter. He mentioned the giant rodent, the ground sloths, and the section of bony polygonal scutes, commenting on the last: "Immediately I saw them I thought they must belong to an enormous Armadillo, living species of which genus are so abundant here." And he added: "If it interests you sufficiently to unpack them, I shall be very curious to hear something about them."

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