To examine the evolution of burrowing specializations in the sister families Octodontidae and Ctenomyidae (Rodentia: Caviomorpha), we produced a synthetic phylogeny (supertree), combining both molecular and morphological phylogenies, and including both fossil and extant genera. We mapped morphological specializations of the digging apparatus onto our phylogenetic hypothesis and attempted to match morphological diversity with information on the ecology and behaviour of octodontoid taxa. Burrowing for sheltering and rearing is the rule among octodontids and ctenomyids, and adaptations for digging have been known from the Early Pliocene onward. However, only a few taxa have evolved fully subterranean habits. Scratch-digging is widespread among both semifossorial and fully subterranean lineages, and morphological changes associated with scratch-digging are not restricted to subterranean lineages. By contrast, various adaptations for chisel-tooth digging are restricted to some subterranean lineages and are combined differently in the octodontid Spalacopus, the fossil ctenomyid Eucelophorus, and some living Ctenomys. Some octodontid taxa are able to dig complex burrows in spite of having no substantial changes in musculoskeletal attributes. Hence, we suggest that, during the early evolution of those branches giving rise to fully subterranean ctenomyids and octodontids, a change in behaviour probably preceded the origin of structural adaptations.

Differentiation of genera of the modern (Late Miocene to Recent) South American rodent family Ctenomyidae would have been linked to the acquisition of disparate adaptations to digging and life underground. In accordance with this hypothesis, the delimitation of lineages and genera in the ctenomyid fossil record is evaluated here following an adaptation-rooted criterion that involves both an assessment of the monophyly and of the adaptive profiles of recognized clades. The application of such a criterion, including morphofunctional information, delimited four cohesive lineages among crown ctenomyids (i.e. euhypsodont species of the Late Miocene to Recent): Eucelophorus (Early Pliocene-Middle Pleistocene), Xenodontomys-Actenomys (Late Miocene-Pliocene), Praectenomys (Pliocene) and Ctenomys (including Paractenomys; Pliocene-Recent); in addition, the results supported the status of Xenodontomys as a paraphyletic ancestor of Actenomys. The cladogenesis that gave rise to the crown group would have occurred immediately after the acquisition of euhypsodonty in a Xenodontomys simpsonilike ancestor during the Late Miocene. This putative ancestor would have had fossorial habits and moderate digging specializations, an adaptive profile maintained in Xenodontomys-Actenomys. Eucelophorus and Ctenomys would have independently evolved subterranean habits at least since the Pliocene. Although the earliest history of the only living representative, Ctenomys, is known only fragmentarily, remains from Esquina Blanca (Uqu´ıa Formation), in north-western Argentina, suggest a minimum age of around 3.5 Ma (Early-Late Pliocene) for the differentiation of the genus. This date agrees with recent molecular estimates.

A quali-quantitative morphofunctional analysis of the craniomandibular joint in subterranean rodents of the family Ctenomyidae showed that specializations of this joint are coupled with adaptations to digging. The presence of a postglenoid articular region in the skull of Eucelophorus and Ctenomys implies a new position of the mandible in digging, different from those involved in gnawing and chewing. In this third position of the mandible, the mandibular joint is stabilized when the deeply inserted incisors attack the soil or an obstacle, preventing dislocation. The proposed new mandibular function imposes a mechanical constraint on size and shape of the auditory bullae in tooth-digger ctenomyids, because inflated bullae preclude a satisfactory opening of the mandible when it articulates in the postglenoid region. The configuration of the craniomandibular joint and other specializations for digging of Eucelophorus are unique among all South American rodents. The presence of non-homologous, and even more specialized, postglenoid cavities in burrowing rodents of other continents suggests a common requirement for stabilizing the mandibular joint when strong forces with incisors are developed. The less specialized postglenoid region of Eucelophorus and Ctenomys, with respect to that of other rodent clades, may be related to the more recent differentiation of ctenomyids.

Octodontoidea is the most species-rich clade among hystricomorph rodents. Based on a combined parsimony analysis of morphological and molecular data of extinct and extant species, we analyze the history of South American octodontoids and propose ages of divergence older than interpreted so far. Early Abrocomidae are recognized for the first time, and a new definition of the family is provided. Traditionally accepted fossil-based times of origin for the southern clades are reinterpreted as later stages of differentiation markedly uncoupled from the origin, differentiation implying specializations for open environments as shown in a morphospace of skull variation. Origin of crown groups is also strongly uncoupled from origin of clades as a consequence of extinction of deep lineages. In the resulting diversity pattern of modern southern clades of octodontoids, the combination of greater disparity, less content of evolutionary history, and lower taxonomic diversity, compared to their northern counterparts, appears at first counterintuitive. We propose that primary components of diversity derived from evolutionary transformation or anagenesis, on the one hand, and from cladogenesis and extinction, on the other, should not be considered associated, or at least not necessarily. Certain patterns of relationships between these distinct components could be driven by environmental dynamics. Like environments, octodontoid diversity would have been more stable in northern South America, whereas in the south, both strong adaptive change and extinction would have been triggered by emerging derived environments.

The relationship between masticatory morphology and chewing modes in all living genera of South American rodents in the family Octodontidae was analysed. Chewing directions and the patterns of molar occlusion were assessed. Factor and regression analyses of skull and jaw characters, and attributes of the adductor musculature, especially the line of action of masseters and pterygoids, were performed to check their relations with the chewing modes. Two basic chewing strategies are present in octodontids: oblique unilateral (associated with anterolingual jaw displacement, and alternate occlusion of left and right molar series), and propalinal bilateral (associated with mostly posteroanterior jaw displacement, and simultaneous occlusion). The skull and jaw characters examined are related only partly to these chewing strategies. The temporal pattern of muscle contraction provides a possible explanation for such a functional versatility. Propalinal grinding in octodontids could be achieved through simultaneous muscle contraction, with limited reliance on the lines of action of the involved muscles. Therefore, simultaneous contraction explains a similar propalinal masticatory mode in morphologically disparate genera. In accordance with phylogenetic information, oblique unilateral chewing is primitive in octodontids, and the derived propalinal mode has been developed independently at least twice in the evolution of the family.