Skull of Toxodon platensis

Skull of Toxodon platensis, collected by Charles Darwin near Montevideo, Uruguay during the voyage of the Beagle 1832-1836.

Strange mammals' family tree mystery solved

A strange group of mammals lived 10,000 years ago in South America. Some of them had a puzzling combination of rodent, hippo and whale-like features - Charles Darwin called them the ‘strangest animals ever discovered’.

Where these animals, and hundreds of other related, but extinct, species sat in the mammal family tree has baffled many, until now.

Scientists, including those at the Natural History Museum, have revealed the closest living relatives of South America’s ungulates, or hooved mammals, to be horses, tapirs and rhinos.

‘Although the bones of these animals had been studied for over 180 years, no clear picture of their origins had been reached’, said Ian Barnes, Research Leader at the Natural History Museum.


This extinct South American mammal called Macrauchenia is most closely related to horses, tapirs and rhino. © Illustration by Peter Schouten/'Biggest, Fiercest, Strangest'/W Norton Publishers


The team, which also includes scientists from the American Museum of Natural History and the University of York, began by investigating ancient DNA but this had not survived in the fossils.

So, they studied collagen, a structural protein found in all animal bones that can survive for millions of years.

‘People have been successful in retrieving collagen sequences from specimens dating up to 4 million years old, and this is just the start,’ said University of York Professor Matthew Collins, whose lab did the sequencing work.

Chemical structures inside the proteins can be compared between different species, revealing clues about how closely the species are related.  

The team analysed 48 fossils of Toxodon platensis and Macrauchenia patachonica, the very species whose remains Darwin discovered 180 years ago when he visited Uruguay, Argentina, on the Beagle voyage.

‘By selecting only the very best preserved bone specimens and with various improvements in proteomic analysis, we were able to obtain roughly 90 per cent of the collagen sequence for both species,’ said lead author Frido Welker, a PhD student at the Max Planck Institute for Evolutionary Anthropology and the University of York.

The team showed that the closest living relatives of these species were the perissodactyls, the group that includes horses, rhinos and tapirs. This makes them part of Laurasiatheria, one of the major groups of placental mammals.

The research findings support a theory that the ancestors of these South American ungulates came from North America more than 60 million years ago, probably just after the mass extinction that killed off non-avian dinosaurs and many other vertebrates.

This research was published in the journal Nature.