Evolution and systematics

Over the last several decades, the contents and limits of the subfamily Sigmodontinae have been a center of much debate. However, as of 2001, an agreement has been reached in defining Sigmodontinae to only encompass the predominantly South American complex-penised mice and rats, leaving out of the subfamily those predominantly North American simple-penised mice and rats (e.g., the wood rats Neotoma, the grasshopper mice Onychomys, and deer mice Peromyscus). Now defined, the subfamily Sigmodontinae includes 71 living genera and 373 species. Remarkably, these numbers keep growing as new genera and species are being discovered and identified from both field and museum based work. New sig-modontines are being discovered even in areas that are presumably well known from a mammalogical point of view. For example, Oxymycterus josei was described in 2002 for the first time from areas near the southern coast of Uruguay. These areas have been frequently visited by mammalogists since the early nineteenth century. Sigmodontines include several familiar rats and mice, such as the cotton rats (Sigmodon), and the rice rats (Oryzomys), but the subfamily also encompasses an enormously diverse collection of other rodents practically unknown to most people.

Traditionally, sigmodontine genera have been arranged into different groups, some have been formalized as tribes in zoological classifications. Depending on the relative weight that is given to different trenchant characters, the number and content of sigmodontine groups vary from author to author. In the early 1990s an almost universal adoption of a phyloge-netic approach to delimit these groups caused major reconsiderations on the identity of the groups as well as on their limits and contents. These studies have promoted: the recognition of a previously unnoted group (the abrothricines, mostly confined to the central and south Andes); the subsuming of some main groups within others (e.g., scapteromyines into akodontines); the corroboration of the distinction of some groups (e.g., reithrodontines, wiedomyines); and called into question the nature of at least one group (the thomasomyines) and the composition of most groups. As of 2003, nine main sigmodontine groups are recognized (Smith and Patton, 1999).

Plains harvest mice (Reithrodontomys montanus). (Photo by Stephen Dalton/Photo Researchers, Inc. Reproduced by permission.)

Diversity is not evenly distributed among them. The akodon-tines, oryzomyines, and phyllotines account for more than half of extant genera. The abrotrichines, the ichthyomyines, the reithrodontines, the sigmodontines, the thomasomyines, and the wiedomyines are groups of one to five extant genera each. In addition, there are several extant genera whose phy-logenetic relationships are not clear; these genera cannot be assigned with certainty to any monophyletic group less inclusive than the subfamily. In formal classifications these genera (e.g., Abrawayaomys, Delomys, Irenomys, Juliomys, Phaenomys, Punomys, Rhagomys, Wilfredomys) are generally considered as incertae sedis (of uncertain taxonomic position). Phylogenetic relationships among sigmodontine main groups or tribes are mostly unclear. The only point that appears well corroborated is that the Sigmodontini, composed only of one extant genera (Sigmodon), is the sister group of all remaining sigmodontines.

Although extensive, past sigmodontine diversity is not as great as the present (Pardinas et al. in press). The oldest South American known sigmodontine comes from the latest Miocene sediments of the Argentinean province of La Pampa. Sigmodontines are rare in early Pliocene beds and become more abundant in late Pleistocene sediments. While, more than a half of extant genera are also recorded as fossils (many from Holocene beds), only seven known genera have become extinct during geologic times. In addition, three genera presumably became extinct in historic times. The sigmodontine nature of some North American fossils, such as Prosigmodon, Bensonomys, and Symmetrodontomys, is questionable and deserves further scrutiny.

The understanding of sigmodontine evolutionary history has proved to be a complex task. All students agree on the fact that in spite of most sigmodontines being endemic to South America, the immediate ancestor of the group did not originate in South America. In other words, at some point in history sigmodontines or their immediate ancestor invaded South America. The basis for this agreement lies on the fact that no potential ancestor has yet been discovered in South America. The fossil record indicates that this invasion was as late as Late Miocene. As of 2003, two main aspects of sigmodontine historical biogeography remain unresolved. These are: 1) whether the geographic origin of the stock that gave rise to the sigmodontines was in the Old World or in Central-North America; and 2) the geographic placement of the basal sigmodontine radiation; in other words, how many sigmodontine lines invaded South America? Evidence at hand tends to indicate that the sigmodontine ancestor inhabited Central-North America. However, the numbers of sigmod-ontine lines that invaded South America remains to be solved. Therefore, it is not clear if North American sigmodontine are direct descendents of the first sigmodontines or if they represent secondary invasions to North America of lines first differentiated in South America.

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