Evolution and systematics

Members of the family Octodontidae are considered to represent some of the most morphologically primitive species of the suborder Hystricognathi. Depending upon the assignment of fossils to the family, the fossil record for the family extends from either the early Miocene or early Oligocene to recent. All recent systematic treatments place the families Octodontidae, Capromyidae, Ctenomyidae, Echimyidae, My-ocastoridae, and Abrocomidae in the monophyletic (sharing a common ancestry) superfamily Octodontoidea. A molecular phylogenetic study in 2003 by Honeycutt and others suggested that the families Octodontidae and Ctenomyidae, commonly known as tuco-tucos, share a common ancestry, followed by an association with a group containing the families Myocastoridae, Echimyidae, and Capromyidae.

Speciation within the family was influenced by environmental changes occurring in the Pliocene and Pleistocene. Within the family, species of octodontids are chromosomally diverse ranging in diploid chromosome number from a low of 38 to a high of 102. The red viscacha rat, Tympanoctomys barrerae, has the highest diploid number, and in 1999, Gallardo and others provided genetic evidence for this species being a tetraploid with a genome size twice that seen in related species. Honeycutt and others provided evidence for relationships among genera within the family. Two major groups were observed, one representing the desert specialists from Argentina, Octomys and Tympanoctomys, and the other containing Octodontomys and a group represented by Octodon, Aconaemys, and Spalacopus, three genera restricted to Chile. The latter two genera contain species adapted for semi-fossorial and fossorial lifestyles, and according to the molecular data, they are each other's closest relatives. Although in 1987 Contreras and others questioned the species-level distinction between O. lunatus and O. bridgesi, estimates of genetic and phyloge-netic divergence between these forms support these taxa as being distinct species. In 1994, Hutterer named a new species, Octodon pacificus, restricted to Mocha Island off the coast of Chile. Evidence from standard karyotypes and morphological comparisons by Gallardo and Mondaca in 2001 and the molecular phylogenetic study of Honeycutt and others support species-level status for the two currently recognized subspecies of Aconaemys, A. fuscus fuscus, and A. fuscus porteri. Mares and others in 2000 described two new genera and species, Pipanacoctomys aureus and Salinoctomys loschalchaleroso-rum, both of which are desert specialists occurring in perisaline

A degu (Octodon degus) emerging from underneath a rock. (Photo by S. R. Maglione/Photo Researchers, Inc. Reproduced by permission.)

shrublands in Argentina. These two new species are most closely related to Tympanoctomys barrerae.

0 0

Post a comment