Craniomandibular joint in South American burrowing rodents (Ctenomyidae): adaptations and constraints related to a specialized mandibular position in digging

Abstract

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.

The oldest South American tuco-tuco (late Pliocene, northwestern Argentina) and the boundaries of the genus Ctenomys (Rodentia, Ctenomyidae)

“…The articular surface of the condyle is narrow and proportionally long. The base of a wide ventrolateral apophysis of the postcondyloid process (Verzi and Olivares 2006) (Fig. 3a) is preserved; it is more posterior than in modern Ctenomys, as in C. chapalmalensis.…”

“…Aside from this, Ctenomys is considered to be a very cohesive genus both morphologically and adaptively (Reig et al 1990; Lessa and Cook 1998). The results from morphofunctional and allometric analyses of craniomandibular features of living species have supported this notion of morphological identity (Lessa 1993;Mora et al 2003;Schleich and Vassallo 2003;Verzi and Olivares 2006;Vassallo and Mora 2007). However, depending on the conceptual framework applied, the boundaries of the genus can be less evident in the fossil record, and this situation may hinder our understanding of its evolutionary history.…”

“…The list of taxa and specimens examined can be found in Verzi et al (2004a, b) and Verzi (2008: Appendix S1). Nomenclature of osteological and dental characters follows Verzi (2001Verzi ( , 2008 and Verzi and Olivares (2006). The phylogenetic analysis was performed using the program NONA 2.0 (Goloboff 1993) to find the most parsimonious trees and to assess branch support in the cladogram.…”

Middle ear structure and bone conduction in Spalax, Eospalax, and Tachyoryctes mole‐rats (Rodentia: Spalacidae)

“…The function of the pseudoglenoid fossa in molerats, and other tooth-digging rodents in which a similar cranial morphology is present, is generally believed to relate to jaw stabilization, perhaps preventing excessive caudal displacement of the condyle; opinions differ regarding whether the condyle occupies the fossa when the animal is burrowing, chewing or inactive (Topachevskii, 1976;Zubtsova, 1986;Li et al, 2004;Verzi and Olivares, 2006). A stabilizing function does not preclude an additional role in the transmission of vibrations from jaw to ear, if the juxtaposition of temporomandibular joint and petrosquamosal sinus within the fossa were to facilitate this process.…”

“…The first of these was the ''pseudoglenoid fossa,'' a specialized articulation for the mandibular condyle posterior to the glenoid fossa, whereby vibrations of the mandible could be coupled directly to the auditory bulla. Found in a variety of fossorial rodents, the pseudoglenoid fossa is alternatively referred to as the ''postglenoid fossa'' (Verzi and Olivares, 2006). The second adaptation identified by Rado et al was a unique incudo-periotic articulation, whereby the short process of the incus is firmly anchored within a bony cup, the cup being attached to the skull via a thin periotic lamina.…”