Evolution and systematics

Dinomys branickii is the only surviving species of the family Dinomyidae, genus Dynomys, an ancient and once varied group of South American rodents whose rat-like ancestors probably rafted on vegetation from Africa to South America between 45 and 36 million years ago (mya). The dinomyids were far more diverse in the Oligocene-Pliocene (34 to 1.8 mya). At least eight species of Dinomyidae are known from fossils, and some were the largest rodents ever to exist. The unusually large size of pacaranas recalls but merely echoes creatures like its extinct relative, Telicomys gigantissimus, as big as a rhinoceros, and, rivaling it in size, Artigasia magna. The great size of these and a few other related species gave rise to the family name, Dinomyidae, meaning "terrible mouse."

Pacaranas are hystricognath rodents, within the suborder Hystricognathi, that classification based on peculiarities of their skulls, lower jaws, and jaw muscles, shared among all member species. Hystricognath rodent species live mainly in South America and Africa, plus some species in southern Europe and Asia, and one species of New World porcupine (Erethizon dorsatum) in North America. The suborder is vast, made up of 18 families, which include the African mole rats, dassie rats, grasscutter rats, Old and New World porcupines, capybaras, chinchillas, agoutis, and many others. With few exceptions, hystricognath rodents produce small litters of one or two young; infants are precocial, i.e., born furred, with eyes open, and able to move about. Suborder Hystricognathi can

Machetero Pacarana
Pacarana (Dinomys branickii). (Illustration by Jaqueline Mahannah)

be separated into two infraorders, Caviomorpha, the New World hystricognaths, and Phiomorpha, the Old World hys-tricognaths.

Some taxonomists have tentatively explained the similarities of the American and African hystricognaths as resulting from convergence. Some of the latest studies in genetic and molecular evolution involve the "molecular clock" method, whereby genes from related species are sequenced and the number of mutations in neutral genes are noted, providing an estimation of the time a new species diverged from the ancestral line. As of 2001, molecular clock studies among hys-tricognath species on both sides of the Atlantic strongly support their sharing a common ancestor, and strongly suggest a single colonizing event of African hystricognaths to South America, in the interval of 45-36 mya, that eventually gave rise to the great diversity of New World hystricognaths, including Dinomys branickii (Honeycutt; Huchon & Douzery; Honeycutt, Nedbal & Schlitter). During the interval of 45-36 mya, the Atlantic ocean was about half as wide as it is today, and had ocean currents between the two continents that would have been favorable for a rafting and colonization event from Africa to South America. There is, however, far from universal agreement on this scenario.

The taxonomy for this species is Dinomys branickii Peters, 1873, Montana de Vitoc, Department of Junin, Peru. The pacarana was first described in Western science in 1873 by a Polish count named Branicki, who did field research on South American wildlife for the Warsaw Museum of Natural History. Other common names include false paca, Branick's giant rat, machetero ("cutter" or "chopper"), rukupi, piro, teconi, tecon, lapa rabuda, lapo, lapa cacique, guagua lanuda, and guagua loba.

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