The peculiar New World hystricomorph rodents comprise about half of the mammal species of South America (Upham and Patterson, 2012) and have been evolving in this continent for over 40 Ma (Antoine et al., 2012). During this period, they developed an ecomorphological diversity much greater than that of other rodent clades, even when only the extant fauna is considered (Mares and Ojeda, 1982; Wilson and Sánchez-Villagra, 2010; Hautier et al., 2012). This results especially from the evolution of particular morphologies in three of the four suprafamilial clades, Erethizontoidea (New World porcupines), Chinchilloidea (viscachas), and Cavioidea (maras and cavies), a pattern that begins to be apparent in the Oligocene fossil record (Wood and Patterson, 1959; Bertrand et al., 2012). In contrast, the superfamily Octodontoidea has remained morphologically conservative for much longer, the rodents of this group being unique among South American hystricomorphs in retaining a rat-like appearance (e.g. Redford and Eisenberg, 1992: pl. 17; Eisenberg and Redford, 1999: pl. 13). Remarkably, when considered in combination with their apparently narrower range of morphological innovation, Octodontoidea is the most diverse clade of hystricomorph rodents. In particular, the families Echimyidae and Octodontidae (including the subfamily Ctenomyinae, considered by neontologists as a family in their own right; see Verzi et al. 2014) comprise more than 60% of the extant species of South American hystricomorphs, and have the richest fossil record of the suborder (McKenna and Bell, 1997; Woods and Kilpatrick, 2005; Upham and Patterson, 2012).
The sister families Echimyidae and Octodontidae are two living clades with very different characteristics in terms of geographical distribution and diversity patterns. Echimyidae encompasses a high diversity (i.e. species richness) of small- to middle-sized rodents, with arboreal (spiny tree-rats, tree rats, bamboo rats), or terrestrial to fossorial (spiny rats) lifestyles, which occupy Amazonian, coastal and Andean tropical forests in northern South America, and occasionally more open, xeric habitats in the Cerrado and Caatinga (Eisenberg and Redford, 1999; Emmons and Feer, 1999).