01 February 2011

Fossil Vertebrate of the Month: Megatherium americanum

FVOTM is back from its extended holiday vacation, and because February 12th is Darwin Day, this month's vertebrate is an animal that played a crucial role in the development of evolutionary theory: the giant ground sloth Megatherium americanum.  The species would have been familiar to Darwin before he ever departed on the Beagle: it had been named in 1796 by no less a figure than Baron Georges Cuvier and its size (comparable to that of a modern elephant) and bizarre combination of traits (such as teeth without enamel and claws that the animal evidently walked on) had made it immensely popular.  Darwin himself uncovered fossils of Megatherium - as well as the hippo-like ungulate Toxodon - at Bahia Blanca, south of Buenos Aires.  While it is impossible to pinpoint exactly where or when Darwin first began to understand the patterns that he would later use to support natural selection, his recognition that Megatherium shared many features - including its apparently aberrant teeth and claws - with modern tree sloths certainly represents a milestone, as the great scientist would himself acknowledge in the opening lines of his epochal On the Origin of Species: "When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent.  These facts seemed to me to throw some light on the origin of species - that mystery of mysteries as it has been called by one of our greatest philosophers."

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