The creature first entered the scientific spotlight thanks to Charles Darwin, who stumbled upon its bones during the HMS Beagle expedition.
On November 26, 1834, while travelling in Uruguay, Darwin heard rumours of “giant’s bones” on a nearby farm.
Curious, he rode over, investigated the cache, and purchased the skull of a strange beast for eighteen pence — a bargain for a fossil that would later puzzle the greatest minds of the 19th century.
In his journals, Darwin mused: “Toxodon is perhaps one of the strangest animals ever discovered.” And frankly, he wasn’t wrong.
Once the skeleton was fully reconstructed, it appeared to pull anatomical traits from every corner of the mammalian tree. It was as large and barrel-bodied as a rhinoceros, yet equipped with chisel-shaped incisors reminiscent of oversized rodents — hence its name, meaning “arched tooth.”
Its high-set eyes and nostrils suggested an animal comfortable in water, much like a hippo or manatee. Darwin marvelled at this evolutionary mash-up: “How wonderfully are the different orders… blended together in the structure of the Toxodon!”
For over a century, the lineage of Toxodon remained a scientific enigma. Traditional morphology bounced it somewhere among ungulates, rodents, and even sirenians. Then, in 2015, ancient DNA changed the game.
A groundbreaking genomic study revealed that Toxodon — along with the equally bizarre Macrauchenia — belonged to a lineage known as the South American native ungulates, or SANUs.
These animals were the evolutionary result of South America’s long isolation after the breakup of Gondwana. And here’s the kicker: SANUs are now understood to be distantly related to modern perissodactyls, the group that includes horses, tapirs, and rhinoceroses.
So Darwin’s instincts weren’t far off — the resemblance to rhinos wasn’t just superficial whimsy.
Toxodon and its relatives (family Toxodontidae) appear in the Late Miocene, roughly 9 million years ago, and flourish throughout the Pliocene and Pleistocene of South America.
Their fossils have been uncovered across Argentina, Bolivia, Brazil, Paraguay, and Uruguay, with especially rich deposits in the Pampean region where Darwin first collected his specimens.
These creatures were part of a wider radiation of endemic South American mammals — a remarkable fauna that included giant ground sloths, glyptodonts, terror birds, and litopterns. For tens of millions of years, South America functioned almost like a massive evolutionary island, producing lineages found nowhere else on Earth.
Toxodon itself survived until the tail end of the last Ice Age, vanishing about 12,000 years ago, around the time humans arrived on the continent and climate systems shifted dramatically. Its demise mirrors the fate of many Pleistocene megafaunal giants.
Toxodon stands as a fascinating case study in convergent evolution and the challenges of reconstructing deep-time relationships. Its stocky limbs, massive grinding teeth, and robust skull mark it as a grazer well-suited to tough vegetation, while its semi-aquatic adaptations hint at a lifestyle spent wallowing in wetlands and rivers.
It was, in many ways, a South American answer to the hippo — yet biologically and evolutionarily, it belonged to an entirely different branch of the mammalian tree.
Darwin might have described it as a beautiful blend of mismatched traits, but with DNA in hand, we now see Toxodon not as a puzzle piece forced to fit the wrong box — but as the last great representative of an ancient, isolated ungulate lineage that flourished for millions of years in a continent of evolutionary mischief.
