The skulls and bones of extinct mammals were the crowning glory of Charles Darwin’s fossil collecting in South America, not only for him but for the eager recipients of the cargoes he sent home. They were the discoveries that made his name known beyond his immediate circle.
When the first consignment of fossil bones arrived at the Royal College of Surgeons in 1833, the puzzled curator, William Clift, recorded them as apparently “from a Mr Darwin at Rio de la Plata.” Only a few months later, however, after the specimens had been exhibited in Cambridge, Darwin’s friend Frederick William Hope wrote to tell him that his “name was in every mouth.” Even more significantly, Darwin later credited the fossil mammals as one of the two main factors that led him to embrace the reality of evolution.
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The most complete fossil of any mammal discovered by Darwin was an almost entire skeleton, later recognized as another new species of ground sloth by Richard Owen, a professor at the Royal College of Surgeons who described and named Darwin’s fossils. It appears to have been found on the beach, partially embedded in loose sand, and Darwin concluded that the whole mass had fallen from the cliff.
The skeleton comprised the skull, vertebral column, ribs and limb bones down to the claws, “all nearly in their proper relative positions,” including even the kneecaps. This remarkable discovery was first noted by Darwin on September 1, 1833, and must have been made at some point during his second phase of collecting at Punta Alta, Argentina, the previous week.
image: https://thumbs-prod.si-cdn.com/CDrkx0lzxDG98de3aybOFDLoA2M=/1024x596/filters:focal(342x441:343x442)/https://public-media.smithsonianmag.com/filer/7e/56/7e566e6e-6ca1-4ca4-9189-e2ee2672849d/scelidotherium-colour-version-2.jpg
The ground sloth <em>Scelidotherium</em>, with a long, low body and snout. Recent evidence suggests it may have been a burrower.
The ground sloth Scelidotherium, with a long, low body and snout. Recent evidence suggests it may have been a burrower. (Smithsonian Books)
Darwin quickly recognized the significance of finding an articulated skeleton; whereas odd bones might have been washed out of earlier deposits or fallen in from above, a complete skeleton embedded in sand demonstrated conclusively the contemporaneity of the living animal with the ancient deposit in which it was found. “Gran bestia all nonsense,” he exclaimed in his notebook, referring to a local legend, for it was perfectly clear that the remains were ancient and not those of a mysterious creature that still roamed the Pampas.
He wrote to his sister Caroline that he had discovered the skeleton of an animal “of which I do not think there exists at present on the globe any relation.” Later he wondered if it might represent the same species as the mandible whose four teeth he had previously illustrated, subsequently named Mylodon darwinii. On detailed comparison, however, Owen confirmed it as a different genus, smaller and with a more elongate skull, and named it Scelidotherium leptocephalum.
Another sloth species discovered by Darwin was found in November 1833 during his two-week excursion across present-day Uruguay. It was part of the back of a skull, later named Glossotherium by Owen. The find was made in the same stream, the Sarandi, where a larger, more complete skull of the celebrated mammal Toxodon had been discovered. It is not quite clear whether Darwin himself found the Glossotherium specimen at the stream, or obtained it from the finder together with the Toxodon, although the former seems more likely. At any event the specimen, which Darwin described as from “an animal rather larger than the horse,” was remarkable for its superb state of preservation; he wrote that it appeared “so fresh that it was difficult to believe [it had] lain buried for ages under ground.”
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<em>Glossotherium,</em> the third new species of ground sloth discovered by Darwin, named by Owen for its supposedly long tongue.
Glossotherium, the third new species of ground sloth discovered by Darwin, named by Owen for its supposedly long tongue. (Smithsonian Books)
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Darwin’s Fossils: The Collection That Shaped the Theory of Evolution

Richly illustrated with photos from the fossil collection and line drawings produced when Darwin was alive, Lister’s work is an essential acquisition for every library prizing quality books on evolution.