When you look at a reconstruction of the skull and brain of Neoepiblema acreensis, an extinct rodent, it’s hard to shake the feeling that something’s not quite right.

Huddled at the back of the cavernous skull, the brain of the South American giant rodent looks really, really small. By some estimates, it was around three to five times smaller than scientists would expect from the animal’s estimated body weight of about 180 pounds, and from comparisons to modern rodents. In fact, 10 million years ago the animal may have been running around with a brain weighing half as much as a mandarin orange, according to a paper published Wednesday in Biology Letters.

The glory days of rodents, in terms of the animals’ size, were quite a long time ago, said Leonardo Kerber, a paleontologist at Universidade Federal de Santa Maria in Brazil and an author of the new study. Today rodents are generally dainty, with the exception of larger creatures like the capybara that can weigh as much as 150 pounds. But when it comes to relative brain size, N. acreensis, represented in this study by a fossil skull unearthed in the 1990s in the Brazilian Amazon, seems to be an extreme.

The researchers used an equation that relates the body and brain weight of modern South American rodents to get a ballpark estimate for N. acreensis, then compared that with the brain weight implied by the volume of the cavity in the skull. The first method predicted a brain weighing about 4 ounces, but the volume suggested a dinky 1.7 ounces. Other calculations, used to compare the expected ratio of the rodent’s brain and body size with the actual fossil, suggested that N. acreensis’ brain was three to five times smaller than one would expect.