Rodents, together with bats, are among the ecologically most diverse and most speciose groups of mammals (Wilson & Reeder, 2005). Moreover, rodents show one of the most extreme specializations of the feeding apparatus, with a single pair of highly specialized upper and lower incisors used for gnawing, and a small number of cheek teeth used for chewing. These specializations are associated with a specialized musculature (Schumacher, 1961) facilitating the antero-posterior jaw movements typical of rodents (propaliny; see Becht, 1953). The exceptional diversity of rodents is, moreover, associated with a diverse array of specializations of the jaw musculature (Weijs, 1973; Woods & Hermanson, 1985; Offermans & De Vree, 1989; Druzinsky, 2010a,b; Cox & Jeffery, 2011; Hautier et al. 2011a). Variations in the masseter complex, and the associated modifications of the skull, have traditionally been used as diagnostic characters to classify rodents (Hautier et al. 2011a).
The Laotian rock rat Laonastes aenigmamus was recently discovered in the Lao People’s Democratic Republic (Jenkins et al. 2005). Although this species was originally thought to belong to a new family, a reexamination of the specimen (Dawson et al. 2006) showed that it belongs to the otherwise extinct family Diatomyidae. Moreover, recent molecular analyses confirmed the placement of L. aenigmamus as a sister taxon to the Ctenodactylidae (Huchon et al. 2007). The difficulties in classifying L. aenigmamus using morphological characters stem from the fact that it presents a mixture of sciurognathous and hystricognathous characteristics (Jenkins et al. 2005; Dawson et al. 2006; Huchon et al. 2007). Indeed, a recent analysis of the anatomy of adult Laonastes demonstrated that the pars reflexa of the masseter complex in Laonastes has evolved independently in this species (Hautier & Saksiri, 2009). This suggests that the masseter complexity contains a functional rather than a phylogenetic signal, and that variation in the jaw adductors may be tuned to functional demands. However, the origin of the unusual muscular organization in this species remains unclear given that all of its closest relatives are extinct. Here, we propose to investigate the development of the masticatory system in Laonastes to better understand the origin of its derived morphology and potential functional specializations relative to other rodents.
Although developmental data may provide profound insights into the origin and homology of derived anatomical patterns, relatively few studies have investigated the ontogeny of the cranial system in mammals in general, and rodents in particular. Indeed, despite a recent revival of developmental studies investigating the early development and patterning of the cranial musculature in mammals (Smith, 1994; Goswami, 2007; Sánchez-Villagra et al. 2008; Wilson & Sanchez-Villagra, 2009; Hautier et al. 2011b), almost nothing is known about the late development and postnatal growth of the cranial system (but see, e.g. Strong, 1926; De Beer, 1937; Hughes et al. 1978; Maier et al. 2003; Flores et al. 2010), and even less about the development of the associated musculature. From a functional perspective, however, such studies can provide profound insights into the selective patterns operating during early ontogeny, which ultimately determine the adult form of an organism (e.g. Herrel et al. 2005; Wyckmans et al. 2007; Herrel & Holanova, 2008; Genbrugge et al. 2011) and may help understand the systematic position of taxa characterized by highly derived anatomical features (Maier et al. 2003). Here, we study ontogenetic changes in the shape of the cranium and mandible in the Laotian rock rat. Moreover, we explore whether the observed ontogenetic changes in skull shape are associated with ontogenetic changes in the jaw adductor musculature. Finally, functionally relevant aspects of the jaw adductor anatomy are compared with other rodents and interpreted in the context of what is known about the ecology of Laonastes.