Evolution and systematics

Fossils exist from the Pleistocene of Brazil. The two species are well defined and occur at different altitudes (though they do overlap in altitudinal range and are not always easy to distinguish under field conditions). Part of an ancient rodent family with its origins in South America, pacas are one of the few mammal species that successfully moved north after North and South America became connected by the central American isthmus. Possessing a cone-shaped body, hind limbs longer than the forelimbs, and a diet devoted largely to fruit, pacas show a remarkable convergence with the chevrotain (Hyemoschus aquaticus), a deer-like ungulate of the West African rainforests. Like the paca, the chevrotains also have a pattern of strong horizontal bars and blotches along its flanks. Pacas were formerly grouped with the agoutis in the family Dasyproctidae, under the subfamily Agoutinae, but were eventually given full family status because they differed from agoutis in the number of toes on the feet, the shape of the skull, and the patterning of the fur. The family name Cuni-culidae was formerly used instead of Agoutidae.

The name paca comes directly from the Tupi indigenous language, as does the word agouti. The fact that the genus name for this animal is the common English name for another, quite similar-looking animal shows why it is a good idea to have the Linnean naming system as a final arbiter for whichever animal is being talking about.

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