Darwin’s 18 pence

South American Pleistocene beasts were super weird. They owe their peculiar evolution to events that happened deep within the very bowels of the planet, hundreds of millions of years ago. Almost all the land that we know today was squashed together to form one massive, enormous supercontinent called Pangaea around 300 million years ago. Incredibly slowly, it began to be ripped apart by the powerful forces of plate tectonics, spreading the plates across the surface of the earth, like a thin skin riding ontop of the thick yellow custard beneath. Hot, sticky magma rose from below, pushing plates apart at the rate of fingernails growing, creating new oceans. The North of this  huge landmass began to break up first, around 175 million years ago. Around 30 million years later, the South part of Pangaea (named Gondwana) began to painstakingly break apart. Here, the recognisable outline of South America split from Africa, and for the next one hundred and forty million years drifted in isolation. And with it, so did the animals.

Almost all the current continents in one place. The supercontinent, Pangea. (Image from here)

Almost all the current continents in one place. Crashing together around 300 million years ago, the supercontinent, Pangea. (Image from here)

The extinction of the dinosaurs, along with the isolation of this fairly large landmass, allowed the surviving animals to move around freely, adapt to new environments and evolve into some of the most incredible creatures of the recent past. From the wonderfully weird heavily armoured Glyptodonts, to the bonkersly bizarre Macrauchenia which couldn’t decide if it was a camel or an elephant this really was the land of the beasts. The freaky big, hooved Toxodon was no exception.

An old school illustration of Toxodon platensis. (Image from here)

An old school illustration of Toxodon platensis. (Image from here)

The discovery of this giant is all down to one young man exploring the tough terrain of South America. On October 1st 1833, the 23 year old records in his diary the first finding of remains he saw of this creature; “a curious and large cutting tooth.” It baffled him. He wrote to his friend back in England, “I got a tooth which puzzles even my conjectures. It looks like an enormous gnawing one.” He found a few more bones in the next month, until he struck gold: he bought a skull off a boy in Uruguay for 18 pence. The best 18 pence anyone ever spent. This was the skull of a new creature, a large beast that had lived in the not too distant past.

The passionate young man was of course Charles Darwin. At the age of just 22, young Darwin joined the HMS Beagle as the captains companion (in those days, a captain only socialised with someone from a similar class, and with a five year voyage ahead, a captain needed someone just to talk to to keep sane). This was the famous voyage of the Beagle that changed Darwin’s whole life, and his thinking about life. It wasn’t just the Galapagos Islands that got Darwin thinking. The fossils he found in South America played a big part in developing his ideas about evolution and extinction. Darwin found a number of remains of extinct animals from South America, including fossils of giant ground sloths (Mylodon and Glossotherium), mastodons, the weird Macrauchenia, and Toxodon.

Toxodonts were big beasts, with a robust skeleton hinting at a similar appearance to a stocky rhinoceros. Huge extensions of bone coming out of the vertebra (called apophyses) would have attached enormous muscles to the animal, holding up its large body. As with the giant sauropod reconstructions in the very early 20th century, this great beast was always viewed as living in swampy areas, where the large body could only survive in water. The skeleton says otherwise. The larger, long back legs means the head is facing down, closer to the ground: not an ideal adaptation for an animal that spent its time living in swamps. And the sediment where Toxodon fossils have been found hold evidence of a arid, grassy environment, typical of the South American Pleistocene.

A familiar chap posing as a scale next to a wonderful Toxodon. (Image from here).

A familiar chap posing as a scale next to a wonderful Toxodon platensis. (Image from here).

As with many Twilight Beasts, Toxodon was described and named by the rather frightful Richard Owen. Toxodon means ‘bow teeth’ due to the very curved nature of the tooth. These teeth do resemble rodent teeth, but of gigantic proportions! In his original description, Owen does assign these extinct giants to the Rodentia Order. As is often the way with early taxonomy, species are shuffled out from one Order and placed into another based on the latest evidence and fossil finds: they were moved into the extinct Order Notoungulata, which were large hooved mammals living in South America.

The big stocky Toxodon was a common herbivore in South America. It’s head hung low, facing the ground, and it had strong flat teeth; both were perfect for grazing the luscious grasslands, and it is thought they would have browsed low lying bushes. With just four species of Toxodon, they were not the most diverse of creatures, but they were well adapted for living where they did.

The unique creatures evolved in isolation for millions of years at home on a large island landmass. Even when North America joined to South around 3 million years ago, and many beasts moved up or down the newly formed bridge during the Great American Interchange, the Toxodon appears to have stayed put. One tooth recently found in Texas, America, suggests that some of these big beasts may have moved north (although the species wasn’t identified, and other fossils are lacking).

One predator that headed South, and would have enjoyed a juicy Toxodon meal, was Smilodon. The bulky size of Toxodon would have not protected it from this formidable predator, and the sabretooth would have added pressure to population of the big herbivores. But not enough to make them extinct, for the hunter and the prey co-existed for tens of thousands of years. A common story appears when we talk about the extinction of one of our amazing Twilight Beasts: changing environment and humans. The climate was changing for these animals, the grass lands were dying out with warmer climates. These slow reproducing giants were hit hard, and suffered a new threat from humans: arrow heads have been found at several sites with Toxodon remains. For this enigmatic giant, their comfortable lives could not keep up with a sudden changing world.

Written by Jan Freedman (@JanFreedman)

Further Reading:

Charles Darwin’s Beagle Diary online.
Charles Darwin’s correspondence online.

Astolfo, G. M., et al.  (2004). ‘Vegetation changes and megafaunal extinctions in South America: Comments on de Vivo and Carmignotto (2004)’, Journal of Biogeography. 31(12). pp.2039-40. [Full article]

Baffa, O, et al. (2000). ‘ESR dating of a toxodon tooth from a Brazilian karstic cave’. Applied Radiation and Isotopes 52 (5): 1345–1349. doi:10.1016/S0969-8043(00)00093-2

Cope, E. D. (1881). ‘Note on the structure of the posterior foot of Toxodon’, Proceedings of the American Philosophical Society. 19 (108). pp.402-403. [Full article]

Fernicola, J. C., Vizcaino, F, and de Iuliis, G. (2009), ‘The Fossil Mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle’, Revista de la Asociatión Geológica Argentina. 64 (1), 147-59. [Full article]

Lundelius, J. R., et al. (2013), The first occurrence of a Toxodont (Mammalia, Notoungulata) in the United States. Journal of Vertebrate Paleontology. 33(1). pp.229-232. [Abstract only]

Martin, P. S. (2005), ‘Twilight of the Mammoths: Ice Age extinctions and the rewilding of America‘, University of California Press. [Book]

Norma, L. N, Musalem, S., & Cerdeno, E. (2000). ‘A new Toxodont from the Late Micene of Catamarca, Argentina, and a Phylogenetic analysis of the Toxodontidae’, Journal of Vertebrate Paleontology20(3). pp.591-600. [Full article]

Quammen, D. (February 2009), ‘Darwin’s First Clues’. National Geographic: 45. [Full article]

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9 Responses to Darwin’s 18 pence

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  6. Ben's Lab says:

    Yet another good post! My favourite episode of BBC’s “Walking with Beasts” series was the one looking at South America. Like Australia, South America has a special place in my heart. The isolation for so many millenia gave rise to some of the coolest prehistoric animals of all time. I love this blog’s focus on mammals (mostly).

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