Morphological analysis of the skull of the subterranean rodent Ctenomys , a highly speciose genus which uses both claws and teeth when digging, shows that for a broad range of species size, scaling was associated with both variation and maintenance of shape. Our results show that the angle of incisor procumbency (AIP), a character largely viewed as an adaptation to digging with teeth, is highly variable. We found a non‐significant relationship between AIP and basicranial axis (basioccipital + basisphenoid) length, a measure of overall skull size. Accordingly, both small and large Ctenomys species possess either high or low AIP. A significant relationship between AIP and diastema length, given the rostral allometry seen in Ctenomys , suggests that hypermorphosis to a certain extent influences AIP. However, the roots of the incisor are lateral to those of the cheek teeth and their position may thus shift freely. This observation supports the notion that skull structural design, and to a certain extent rostral allometry, underlies variation in AIP. On the other hand, the positive allometry of incisor width and thickness indicates that, in larger species, proportionately powerful incisors are able to resist greater bending forces. We found that the out‐lever arm of the jaw adductor muscles scales with positive allometry against basicranial axis length. However, we found an isometric relationship between in‐ and out‐lever arms. In this case, conservation of skull proportions, regardless of variation in size, is a feature possibly related to the maintenance of an effective tooth digging capability. Functional and ecological data are discussed when assessing the implications of size and shape variation in the skull of Ctenomys . © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 78, 85−96.

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.

We analyse the taxonomic status and diversity of the late Miocene Octodontoidea (Hystricognathi) Cercomys primitiva and related samples, a taxon previously known only from the holotype specimen. New findings associated with an extensive review of late Miocene and early Pliocene rodents have allowed us to recognize the occurrence of this and other related species in several localities of central and western Argentina, and in south‐central Bolivia. We discuss the invalidity of Cercomys and propose the new genus Metacaremys, which includes the type species Metacaremys primitiva comb. nov. and two new species, Metacaremys calfucalel sp. nov. and Metacaremys dimi sp. nov. Osteological, brain and dental morphology show that the new genus is not related to Brazilian Echimyidae, as previously considered, but to the southern family Octodontidae. Although the molar morphology of this genus is quite conservative, comparison of the samples shows a variation in size. We discuss the plausible evolutionary pattern explaining this variation and the implied biochronological and biostratigraphical information. It is recognized as an anagenetic lineage in which an increase in size occurs from the oldest species, M. primitiva comb. nov. (early late Miocene, c. 9.23 Ma), to the youngest species, M. dimi sp. nov. (Miocene–Pliocene boundary, c. 5.28 Ma). Metacaremys calfucalel is intermediate in size and age between these two species. The polarity of this pattern of change is consistent with that shown by other partially synchronous independent lineages of octodontoids, thus providing new evidence for the biochronological and biostratigraphic scheme of the late Miocene and early Pliocene of southern South America.