When Charles Darwin arrived to South America, he was only 22 years old. He was part of the second survey expedition of HMS Beagle. He was recommended to Captain Robert FitzRoy by John Stevens Henslow, clergyman, botanist, mineralogist, and Darwin’s mentor.

Before this journey, Darwin’s experience with Earth Sciences was limited to one field trip to the North of Wales with famous Adam Sedgwick, one of the founders of modern geology. But Darwin had a special interest in this field of knowledge and shared this interest with Captain Fitz Roy. In fact, his geological findings promoted him for the very first time, to the scientific and public consideration.

During the first two years of the expedition, Darwin collected several fossil mammals from Argentina and Uruguay. He sent all the specimens, to his mentor John Stevens Henslow. The samples were deposited in the Royal College of Surgeons where Richard Owen began its study. Between 1837 and 1845, Owen described eleven taxa, including: Toxodon platensis, Macrauchenia patachonica, Equus curvidens, Scelidotherium leptocephalum, Mylodon darwini and Glossotherium sp.

Previous to this expedition, the first news of “fossils” in South American were reported by early Spanish explorers. These fossils were interpreted as the remains of an ancestral race of giant humans erased from the face of the Earth by a divine intervention.

George Cuvier, in 1796, published the first scientific work about a South American fossil: Megatherium americanum, based on the specimen recovered by Fray Manuel Torres from Lujan, Buenos Aires, Argentina.

On September 23, 1832 Darwin recovered his first fossil at Punta Alta (Buenos Aires, Argentina). He continued collecting fossil from Buenos Aires Province until October, when he moved to Uruguay. Later in 1834, he returned to Argentina and collected his last specimens.

It was in Uruguay where Darwin bought to a local farmer, the skull of a Toxodon used by Owen to establish the genus. He paid 18 pence for it. Darwin described it as “one of the strangest animals, ever discovered…”

Owen bestowed the name because its upper incisors were strongly arched (Toxodon means “arched tooth). He also recognized Toxodon as “A gigantic extinct mammiferous animal, referable to the Order Pachydermata, but with affinities to the Rodentia, Edentata, and Herbivorous Cetacea”. Toxodon was a puzzle that shared the massive skeleton of a rhino and the teeth had a certain resemblance to those of rodents.

Nevertheless, there was no close phylogenetic relationships with the groups mentioned by Owen. Toxodonts were a group of large-sized notoungulates of South American origin, ranging from the late Oligocene to late Pleistocene.

They are now considered among the more derived native notoungulates of South America and share an ancestry with North American condylarths and a recent study, indicates that is quite possible that Toxodonts traveled to North America.

Toxodonts shares a number of dental, auditory and tarsal specializations. They had short hippopotamus-like head with broad jaws filled with bow shaped teeth and incisors, a massive skeleton with short stout legs with three functional toes. The estimated weight is over a tonne.

About the different groups that appeared to be related to Toxodon, Darwin stated: “How wonderfully are the different orders, at the present time so well separated, blended together in different points of the structure of Toxodon ”

Despite the “erroneous” assignments of Darwin, it is quite possible that the observation of these characters supposedly shared significantly influenced his theory on the origin of species.

By the end of the expedition, Darwin was already earned a name as a geologist and fossil collector. He narrated his experiences in his book “Journal of Researches into the Geology and Natural History of the Various Countries visited by H.M.S. Beagle, under the Command of Captain FitzRoy, R.N. from 1832 to 1836″, published in 1839 and later simply known as “The Voyage of the Beagle”.

When Darwin wrote his memories in 1858, he described the expedition in one strong and powerful sentence: “the voyage of the Beagle has been by far the most important event in my life and has determined my whole career”.

References:

Fariña, Richard A.; Vizcaíno, Sergio F.; De Iuliis, Gerry (2013). Megafauna. Giant Beasts of Pleistocene South America. Indiana University Press.

Lundelius, Jr., E., Bryant, V., Mandel, R., Thies, K., Thoms, A. 2013. The first occurrence of a toxodont (Mammalia, Notoungulata) in the United States. Journal of Vertebrate Paleontology. 33, 1: 229-232