The skull of Toxodon

The giant fossil mammals that inspired Charles Darwin's theory of evolution

In the 1830s, a young Charles Darwin made a series of discoveries in South America: the mysterious remains of extinct mammals.

They were to revolutionise his worldview, impacting the naturalist's understanding of extinction, and helping to persuade him of the reality of evolution.

His discoveries included four different species of giant ground sloth (some of the largest land mammals ever to have lived), a gomphothere and the remains of an extinct horse.

Many of Darwin's fossils survive, at the Museum and elsewhere. Interdisciplinary teams at the Museum are now digitising these specimens to allow scientists across the world to study them in greater detail.

This bone belonged to Macrauchenia, a three-toed South American ungulate that lived during the last ice age

This bone belonged to Macrauchenia, a three-toed South American ungulate that lived during the last ice age

 

Research Leader Adrian Lister has written about the specimens in his new book, Darwin's Fossils.

He says, 'It is little recognised that fossil-hunting was one of Darwin's main pursuits while on the Beagle voyage.

'The fossil mammals from South America, collected years before he arrived in the Galápagos Islands, were a key factor in his acceptance of evolution. We are fortunate indeed that many of these specimens, of huge importance to the history of science, survive at the Natural History Museum'.

The voyage of the Beagle

Darwin found these fossils during his five-year voyage on HMS Beagle, which sailed around the world between 1831 and 1836.

Captained by Robert Fitzroy, the voyage sailed from England to complete a survey of the coasts of South America.

Darwin was employed as a naturalist, and although most of the expedition was spent at sea, Darwin made many excursions on land, collecting specimens and observing the continent's plants and animals. These collections and records provided the material that helped develop his thinking around geology, extinction and evolution. 

The skull of Toxodon, an extinct South American mammal, discovered by Charles Darwin

The skull of Toxodon, an extinct South American mammal, discovered by Charles Darwin

 

The remaining collection of Darwin's South American mammal fossils includes about 100 bones and fragments. They are between 10,000 and 500,000 years old.

One of the strangest is the skull of Toxodon platensis, which belonged to an extinct, giant species of mammal first discovered by Darwin in present-day Uruguay.

The skull was nearly the size of an elephant's. Darwin bought it for a shilling and sixpence, about £7.50 today. Later it was discovered that some giant, rodent-like teeth that had puzzled Darwin belonged to the same creature. He was thrilled at the idea of this 'rhinoceros-sized rodent' and regarded it as one of the most valuable finds of his voyage. 

The skull became the type of a new genus and species. We now know that Toxodon platensis is not related to rodents. Instead, it is a member of a group of extinct South American hoofed animals called notoungulates.

Today it ranks among the Museum’s most treasured specimens.

Digitising this collection will mean that more scientists can have access to this material. It also enables us to digitally reconstruct fragments of bone. Digitisation may enable scientists to apply new research techniques, undreamt of by Darwin, to his material. 

An artist's impression of how Toxodon might have looked

An artist's impression of how Toxodon might have looked. Image by Mauricio Antón.

 

For instance, carefully managed destructive sampling can now be considered, in order to extract DNA from the fossils. This kind of destructive work could never be considered if a digital surrogate did not exist.

Pip Brewer, Senior Curator of Fossil Mammals, will be leading the digitisation.

She says, 'These specimens are extremely valuable and some are very fragile. There is regular demand to view them by visitors to the collection, due to their incredible historical and scientific significance.

'It is therefore a priority for the Museum to fully document, conserve and produce 3D digital surrogates of these important specimens in order to support their long-term preservation, public engagement and scientific use.'

A Toxodon bone being laser scanned

The bones were scanned in the Museum's Imaging and Analysis Centre

 

A missing Megatherium

Dr Lister's book explores the story of Darwin's fossil-hunting adventures of the 1830s, the pioneering science behind the fossils he found, and how these remarkable discoveries played a crucial role in forging his revolutionary ideas.

As well as finding extinct mammals, Darwin worked out how coral reefs and atolls formed, he collected and explained marine fossils high in the Andes, and he discovered a fossil forest that now bears his name.

In the course of writing his book Lister uncovered many remarkable fossil survival stories.

One of them is the story of Megatherium, an extinct giant ground sloth. Darwin found a skull on the Argentinian coast near Buenos Aires.

He shipped the skull to the Royal College of Surgeons in London, where it was examined by anatomist Richard Owen, the future founder of the Natural History Museum. Owen cut two cross sections from the giant skull (a common practice at the time).

No one knew what became of the vertical section until Adrian Lister and Pip Brewer identified it in Darwin's home - Down House in Kent - in 2017. Darwin presumably had kept it as a souvenir and had it taken to Down House in the 1840s, where it had lain unrecognised until now.

A bone from Glossotherium, a prehistoric ground sloth

A bone from Glossotherium, a prehistoric ground sloth

 

A link to modern species

Another species of ground sloth is now called Glossotherium. New technologies mean fresh scientific discoveries continue to be made about this animal.

Dr Lister says, 'Radiocarbon dating now reveals the fossil to be only 12,660 years old - very close to the extinction of the species.

'Museum scientists are hoping to extract DNA from it and sequence its genome, by dissolving the mineral content and isolating tiny amounts of DNA. This will pinpoint the relationship of these ice-age giants to the living sloths, a connection recognised by Darwin and one of the lines of evidence that led him to accept the theory of evolution.'

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