Patterns of covariation in the masticatory apparatus of hystricognathous rodents: Implications for evolution and diversification

Abstract

The mammalian masticatory apparatus is a highly plastic region of the skull. In this study, a quantification of shape variation, the separation of phylogeny from ecology in the genesis of shape brings new insights on the relationships between morphological changes in the cranium, mandible, and muscle architecture. Our study focuses on the Ctenohystrica, a clade that is remarkably diverse and exemplifies a rich evolutionary history in the Old and New World. Current and past rodent diversity brings out the limitations of the qualitative descriptive approach and highlights the need for using integrative quantitative methods. We present here the first descriptive comparison of the whole masticatory apparatus within the Ctenohystrica, by combining geometric morphometric approaches with a noninvasive method of dissection in 3D, iodine-enhanced microcomputed tomography. We used these methods to explore the patterns of covariation between the cranium and the mandible, and the interspecific morphological variation of the skull with regard to several factors such as phylogeny, activity period, type of habitat, and diet. Our study revealed strong phylogenetic and ecological imprints on the morphological traits associated with masticatory mechanics. We showed that, despite a high diversification of lineages, the evolutionary history of Ctenohystrica comprises only a small number of morphotypes for the skull and mandible. The position of the eye was suggested as a key factor determining morphological evolution of the masticatory apparatus by limiting the number of possible pathways and promoting convergent evolution toward new habitats and diets between different clades.

First Approach to the Paleobiology of Extinct Prospaniomys (Rodentia, Hystricognathi, Octodontoidea) Through Head Muscle Reconstruction and the Study of Craniomandibular Shape Variation

“…Tooth-digging requires greater development of the masticatory muscles, especially of the masseter group (see Becerra et al 2014), as well as procumbent incisors suitable to deal with the substrate when excavating their burrows (Stein 2000;Becerra et al 2012). On the other hand, the relative size of the auditory bullae was demonstrated to have a clear association with environmental variation (in terms of vegetal cover and humidity) among caviomorphs (Hautier et al 2012;Álvarez et al 2013). In particular, caviomorphs with enlarged auditory bullae are adapted to semi-arid/arid environments (Ebensperger et al 2006(Ebensperger et al , 2008Traba et al 2010).…”

Studying morphological integration and modularity at multiple levels: concepts and analysis

“…Most empirical studies have focused on integration within populations, investigating how the variation among individuals is structured. This is only one aspect of the problem, however, because the concept of integration also applies at different levels [4], including genetic and environmental integration [8,9], integration of fluctuating asymmetry within individuals [9][10][11][12][13][14][15] and evolutionary integration across taxa in a clade [16][17][18][19][20][21]. Most plants, because of their modular body plans, offer additional opportunities to study integration among structures such as leaves or flowers within and among individuals in a population [22,23].…”

Diversity and evolution of femoral variation in Ctenohystrica

“…The adaptive diversity that characterizes the evolution of Ctenohystrica, and particularly the Caviomorpha, a group that dispersed from Africa to colonise South America (Poux et al, 2006;Rowe et al, 2010) and evolved on that continent during a period of splendid isolation in the Cenozoic, has been the subject of numerous morphofunctional and evolutionary studies (e.g. Verzi et al, 2010;Á lvarez et al, 2011a, b;Hautier et al, 2011Hautier et al, , 2012Cox et al, 2012;Geiger et al, 2013;.…”