Ratzilla: Extinct rodent was big, really big

Think the rodents you’ve seen in movies are scary? Scientists who’ve analyzed the fossilized remains of an extinct South American relative of guinea pigs say that the ancient bruisers were as large as bison.

Researchers first described Phoberomys pattersoni in 1980 but until recently had only bone fragments and isolated teeth to study. Despite that limitation, scientists suspected that the animals were huge, says Marcelo R. Snchez-Villagra, a paleontologist at the University of Tübingen in Germany.

Now, analyses of newly recovered fossils, including a nearly complete skeleton, have enabled Snchez-Villagra and his colleagues to refine estimates of Phoberomys‘ size. They put it at about 740 kilograms, easily earning the species the title of heavyweight rodent of all time. A disproportion between the front and rear limbs suggests that the creature could rest on its haunches and manipulate food with its front paws like its modern relatives do, says Snchez-Villagra.

The new fossils, which the researchers describe in the Sept. 19 Science, were excavated from 8-million-year-old rocks in northwestern Venezuela. The team also unearthed the remains of crocodiles, fish, and freshwater turtles from the same layer of brown shale, says Snchez-Villagra. These companion fossils hint that Phoberomys led a semiaquatic life and probably grazed on aquatic grasses. Examinations of the sediments suggest that the region was probably a river delta surrounded by brackish wetlands, Snchez-Villagra notes.

Phoberomys belongs to a group of rodents called caviomorphs. Scientists have evidence that the group evolved in South America about 40 million years ago, a time when that land mass was isolated from other continents. Because South America didn’t have any grazing animals such as horses, cows, or antelopes, the caviomorphs seem to have diversified to fill wide-open ecological niches. Living caviomorphs include guinea pigs, chinchillas, and capybaras, which at 50 kg weigh in as the largest living rodents.

Being big can be an advantage for plant eaters, says C. William Kilpatrick, a molecular evolutionist at the University of Vermont in Burlington. In general, heftier herbivores have longer digestive tracts and can extract more nutrition from low-quality leaves and grasses than smaller herbivores with shorter guts can.

While South America was isolated from other landmasses for millions of years, a unique fauna developed. The mammalian predators there during that isolation were marsupials, which were less efficient at hunting than were other predators, such as big cats, that had evolved on other continents. Moreover, most of the South American predators were smaller than those on other continents, a characteristic that could have driven herbivores to evolve into large forms less prone to predation.

As often is the case in evolution, the megarodents’ golden age came to an end. When a land bridge formed between North America and South America between 5 million and 2 million years ago, the fierce predators that invaded South America wreaked havoc on the plant eaters there, especially those too large to burrow underground.

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