What Was Significant About the Fossils Darwin Found

The skulls and bones of extinct mammals were the crowning celebrity of Charles Darwin’due south fossil collecting in Due south America, non simply 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 commencement 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, nevertheless, after the specimens had been exhibited in Cambridge, Darwin’s friend Frederick William Hope wrote to tell him that his “name was in every rima oris.” Fifty-fifty more significantly, Darwin after credited the fossil mammals as i of the ii primary factors that led him to embrace the reality of evolution.

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The almost consummate fossil of any mammal discovered by Darwin was an almost entire skeleton, afterwards recognized as some other new species of basis sloth past Richard Owen, a professor at the Royal College of Surgeons who described and named Darwin’southward fossils. It appears to accept 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 about in their proper relative positions,” including even the kneecaps. This remarkable discovery was kickoff noted past 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.

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 to a higher place, a consummate skeleton embedded in sand demonstrated conclusively the contemporaneity of the living creature 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 articulate that the remains were ancient and not those of a mysterious creature that all the same roamed the Pampas.

He wrote to his sister Caroline that he had discovered the skeleton of an animal “of which I do not think in that location exists at present on the globe whatever relation.” Afterwards he wondered if it might represent the same species as the mandible whose iv teeth he had previously illustrated, afterward named
Mylodon darwinii. On detailed comparison, however, Owen confirmed it as a different genus, smaller and with a more elongate skull, and named information technology
Scelidotherium leptocephalum.

Another sloth species discovered past Darwin was found in November 1833 during his two-week excursion across present-solar day Uruguay. Information technology was part of the back of a skull, later named
Glossotherium
by Owen. The detect 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 whatsoever event the specimen, which Darwin described equally from “an animal rather larger than the equus caballus,” 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 footing.”

Not but was the advent of the bone fresher than any of his other fossil finds, information technology preserved delicate parts that are ordinarily cleaved away in ancient remains. This included the tympanic bone, one of the tiny ear basic. Its preservation in identify in the skull atomic number 82 Owen to praise “the care and attention devoted to his specimens by their gifted discoverer.”

Darwin wanted to know more.
He held a slice of the bone in the flame of a spirit-lamp, finding that it not only burnt with a small flame, only “exhaled a very stiff animal odour.” He sent a piece to Trenham Reeks at the Museum of Economical Geology in London, who had undertaken chemical analyses of several of his rock samples, request what percentage of animal matter information technology contained. By this he meant organic fabric aside from os mineral, and the reply was 7 percent. Nosotros would at present recognize that around a quarter of the original protein content had been retained. The remarkable state of preservation of this skull, and its different appearance from others in Darwin’s collection, brand it very likely to accept fallen from a college, later level in the river bank than the

Toxodon

and glyptodont remains constitute nearby.

Having but a fragment of skull at his disposal, Owen was characteristically cautious and identified it as an edentate without specifying to which group it belonged. A large attachment surface for the bone supporting the natural language, and a broad pigsty for the nerve supplying the latter, led him to reconstruct a very large tongue and to devise the name
Glossotherium
(tongue-brute). He afterwards abased the name, considering the skull to belong the aforementioned species as the jaw that he had named
Mylodon darwinii.

Information technology is now recognized as being distinct, still, and so Owen’s name has been reinstated and the species is known as
Glossotherium
robustum. Owen considered the animal might have been an insect-eater, breaking open termite nests like an anteater, merely information technology is now known to have been herbivorous in its habits. Its wide muzzle suggests unselective bulk-feeding on grasses and depression-growing herbs.

In 2017, based on collagen protein extracted from Darwin’s
Glossotherium
skull, a radiocarbon date of around 12,660 years ago was obtained. This is one of the latest known records of the genus, close to the fourth dimension of its extinction.
Glossotherium robustum
had an estimated body weight of around ane.5 tonnes [1 ton U.S.].

In spite of this, a remarkable contempo suggestion is that
Glossotherium
and/or
Scelidotherium
may accept constructed large burrows to escape predation or unfavorable weather. Several lines of bear witness back up this thought. Start, several large “fossil burrows” accept been discovered, especially in the expanse effectually Buenos Aires, their diameter of 3 to four.5 anxiety matches the body width of these species. 2d, the forelimb bones of these animals appear modified for very powerful movements like digging. Third, claw marks have been found on the inside of some of the burrows, forming pairs of grooves that match closely the claws of the large 2nd and third digits seen in these species. These sloths would exist by far the largest animals known to couch in this way—and one of the fossil burrows is more than 130 feet in length.

Darwin’due south discovery on this journey of four genera of large basis sloths,
Mylodon, Glossotherium, Scelidotherium
and
Megatherium
was remarkable, and as well serendipitous in that the area in which he was collecting happened to be the simply region where all four could accept been found together.
Mylodon
is distributed in the southern one-half of the continent,
Glossotherium
in the northern one-half, and
Scelidotherium
in the middle. The genus
Megatherium
is widespread, simply
M. americanum
is known mainly from Argentina. Only in the Pampas region and La Plata basin exercise they overlap. The differing forms of their skulls, and teeth and limbs show how several species could take co-existed in the Late Pleistocene, using different food and habitat resources.

For Darwin, the relationship of the extinct giant sloths to the living species was one of the examples that led him to his “police of succession of types,” whereby there was an analogousness between the past and present inhabitants of a particular region (in this instance South America). This general pattern was one of the key factors that ultimately persuaded him of the reality of development.

Darwin’s Fossils: Discoveries that Shaped the Theory of Development
by Adrian Lister, a research leader in the world science department at the Natural History Museum, London, is published past Smithsonian Books.

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