Evolution Towards Fossoriality and Morphological Convergence in the Skull of Spalacidae and Bathyergidae (Rodentia)

Abstract

Rodents show a wide range of anatomical, physiological, and behavioral adaptations to life underground. Cranial and postcranial bone morphologies are deeply impacted by the modes of digging, which can involve either incisors or claws. However, the morphological variation of these elements still needs to be accurately quantified to assess the degree of specializations of the fossorial rodent families in regards to their respective evolution. Here, we focus on the morpho-functional characteristics of the masticatory apparatus in two families of subterranean rodents, the Spalacidae and Bathyergidae. We quantify the skull shape in five spalacid genera using geometric morphometric methods, as well as biomechanical estimates for adductor muscles, which are compared with data previously published on bathyergids. We show that the skull shape of spalacids has a greater disparity and lower biomechanical estimates than bathyergids, in which the fossorial activity, notably chisel-tooth digging, more significantly impacted the evolution of the skull. Among spalacids, Spalax shows the most extreme specializations to life underground and displays the highest number of morphological convergences with chisel-tooth digging bathyergids, especially regarding its cranial shape and high biomechanical estimate for the temporalis muscle. Fewer morphological convergences were observed between other spalacids and bathyergids. Different evolutionary histories can potentially explain discrepancies observed between the two families, the first bathyergid morphological adaptations to fossorial life being much older than those of spalacids.

Scaling patterns of cerebellar petrosal lobules in Euarchontoglires: Impacts of ecology and phylogeny

“…The absence of significant differences in relative lobule size between the fossorial rodents and other groups when analyzed in the context of endocranial volume in this analysis may relate to the evolution of digging and fossoriality within Rodentia. Fossoriality has evolved independently several times in different rodent groups (here represented by Bathyergidae, Spalacidae, Aplodontidae) and has produced disparate adaptations in skeletal bone structure (Amson & Bibi, 2021) and skull morphology (Fournier et al, 2021). This disparity is the product of a variety of factors including evolutionary history, soil type, time spent above ground, and significantly, digging behavior (Stein, 2000).…”

“…In some species, as in Ellobius, the head itself is used in a shoveling motion to excavate tunnels. The diversity of digging behaviors and disparity in fossorial adaptations (Amson & Bibi, 2021;Fournier et al, 2021) may have influenced subarcuate fossa size and therefore lobule size, especially if modifications have been made to the braincase. While it is most likely that the small lobules of these fossorial species relate to their reduced reliance on vision, a feature that characterizes fossorial groups (Stein, 2000), this result, together with previous analyses (Bertrand et al, 2018(Bertrand et al, , 2021 highlights the need for clade specific, fossil informed analyses of fossorial adaptations.…”

Incipient morphological specializations associated with fossorial life in the skull of ground squirrels (Sciuridae, Rodentia)

“…Mandibular and cranial forms were quantified using 11 and 62 anatomical landmarks, respectively (Figure 2 ; Supporting Information Online Material 2 ). This landmark data set was based on previous studies (Fournier et al, 2021 ; Gomes Rodrigues et al, 2016 ; Hautier et al, 2012 ) and was adapted to our sample. Landmarks #37 and #55 were not used in the analyses, because pterygoid processes are broken in some specimens, as was the tip of incisors.…”

“…More than 10 families of extant rodents show convergent adaptations to fossorial life, impacting their long bones and girdles, as well as their skull to various extents (e.g., Stein, 2000 ; Gomes Rodrigues et al in press). Subterranean rodents generally show the most extreme cranial specializations due to their more intense activity of chisel‐tooth digging (e.g., procumbent incisors and massive masticatory muscles; Fournier et al, 2021 ; Gomes Rodrigues et al, 2016 ; Landry, 1957 ; Lessa & Patton, 1989 ; Marcy et al, 2016 ; McIntosh & Cox, 2016 ) and to the constraints related to life underground (e.g., reduced eyes and pinnae; Nevo, 1979 ; Scarpitti and Calede, 2022 ; Stein, 2000 ). The morphology of subterranean rodents has been studied, but a focus on less specialized fossorial taxa would have allowed a better comprehension of the evolutionary mechanisms underlying fossorial adaptations in rodents.…”

Functional anatomy and disparity of the postcranial skeleton of African mole-rats (Bathyergidae)

“…Several studies of extant bathyergids have focused on the cranial and dental anatomy of a few species (e.g., Berkovitz and Faulkes, 2001;Hart et al, 2007;Barčiová et al, 2009;Van Daele et al, 2009;Gomes Rodrigues et al, 2011;McIntosh and Cox, 2016a;Caspar et al, 2021). Only more recent assessments have incorporated a comparative approach including a larger number of species (Gomes Rodrigues and Šumbera, 2015;Gomes Rodrigues et al, 2016;Mason et al, 2016;McIntosh and Cox, 2016b;Fournier et al, 2021). In general, these studies have found clear differences between chisel-tooth diggers and scratchdiggers, with the former having a more specialized dental and craniomandibular morphology, including more procumbent incisors, shorter snout, relatively wider and taller skulls with enlarged zygomatic arches, strongly hystricognathous mandible, and increased jaw and condyle lengths relative to their size, all features that facilitates higher bite forces and wider gapes to maximize breaking up soils (Gomes Rodrigues et al, 2016;McIntosh and Cox, 2016a,b).…”