A paper in the new issue of Proceedings of the Royal Society announces the discovery of fossil evidence of the largest rodent yet recorded by science. The fossil reported is a skull, found in Uruguay. Of course, South America is home to the largest living rodent (the capybara, Hydrochoerus hydrochaeris), and it's no surprise that the continent should also have been home to the largest fossil rodent. The reason is that South America was a hotbed of mammalian evolution from about 65 million years ago (Ma), when it separated from North America, until about 3 Ma, when it reconnected with North America via the narrow Isthmus of Panama.

The new species, dubbed Josephoartigasia monesi, is closely related to another large extinct rodent called Phoberomys pattersoni as well as an extant (still living) species known as a pacarana (Dinomys branickii). The image at right, from the paper, shows a reconstruction of the head of J. monesi and a comparison with a pacarana.

P. pattersoni (from Venezuela) is estimated to have weighed about 700 kg, while J. monesi's bulk (extrapolated from the relative size of the skull) must have been about 1000 kg, a full metric ton.

As a continent hosting an independent trajectory of mammalian evolution, South America offers a beautiful contrast to Australia. Both continents were isolated from other continents for tens of millions of years, and their particular blend of mammals (& other species) evolved into unique & interesting forms. The South American experiment ended when it reconnected to North America at 3 Ma. At that time, mammals from North America trooped southward through Panama, and mammals from South America trooped northward. This flux of biodiversity is referred to as the Great American Faunal Interchange, and it dramatically reshaped South America's mammal assemblage. Saber-toothed cats (Smilodon sp.) was one of the turistas from the north, and it likely fed on big fat critters like J. monesi.

Other southward-bound groups included deer, bears, squirrels, rabbits, raccoons, and camelids (which went on to diversify into llamas & their kin). Northward-bound groups colonizing North America were fewer in number: the armadillos, the opossum, ground sloths (now extinct), and porcupines. Today, about 50% of South American mammal species have a North American origin, while only 20% of North American mammals have a South American origin. One of the reasons suggested for this lopsided distribution was that North America had been periodically connected to Eurasia (both east and west) via landbridges during the Cenozoic, and therefore North American mammals had been continually tested against the biggest continent's most competitive species. The North American mammals had already been forced to prove their mettle. South American mammals were living a life of blissful ignorance & luxury, and they experienced a rude awakening when their neighbors from the North came stampeding in. The competition between the two groups resulted in a lot of extinctions, and one of those appears to have been J. monesi. The fossils were found in a sedimentary unit usually interpreted to be about 2 Ma.

The image below shows the sheer size of the J. monesi skull by comparing it to a handheld rat. (Maybe a Norway rat? I can't tell, and the image from New Scientist doesn't specify the species.) It's a pretty extreme difference, of the same order of magnitude as the difference between an elephant and a hyrax.


Evolution can be really fruitful given a sufficient easing of competitive pressure. Island biogeography gives us some weird creatures (Komodo dragons, dwarf elephants, and Homo florensis come to mind as three Indonesian examples) if isolation is maintained over time. This new discovery from South America confirms the larger pattern by showing us that gigantism has happened in the rodent family, too. Discoveries like this (and the recent giant ape reported from China) prompt me to imagine other possible evolutionary trajectories: what about giant bats, or mouse-sized whales? Could evolution produce arboreal deer, or bears the size of Chihuahuas? I welcome your thoughts & creative speculations on potential record-breaking mammals.

Reference: Rinderknecht, Andres, and Blanco, R. Ernesto. The largest fossil rodent. Proceedings of the Royal Society, B. doi:10.1098/rspb.2007.1645 Published online.

For those without a subscription to the Proceedings, you can check out New Scientist's write-up of the research here.

Labels: evolution, fossils