Craniofacial suture complexity and digging mode importance in rodents

The morphology of cranial sutures (i.e., joints that connect the bones of the skull, are sites of growth, and absorb biomechanical stresses) is highly variable, especially in mammals. Exposure to different strain regimes as a result of development, function, and ecology can drive disparity in suture complexity at both the individual and taxonomic levels. Although the relationships between diet, muscle mass, static loading, and certain behaviors (e.g., headbutting) and suture morphology are well studied, the effects of fossoriality, particularly the use of the craniodental apparatus during substrate excavation, remain largely unknown. We synthesize data using an ordinal ranking scheme for digging mode importance in 107 rodent species; premaxillofrontal and nasofrontal suture complexity as measured by sinuosity index, power spectrum density, and spectral entropy, the latter of which is a novel suture complexity metric; and Bayesian multilevel models to examine the fossoriality-suture phenotype relationship at the suborder level and above and predict digging mode usage in 11 cryptic extant species. Most fossorial rodents utilize multiple digging modes, often at varying levels of importance. Chisel-tooth digging importance is associated with reduced nasofrontal suture complexity in multiple suborders, whereas scratch digging importance is correlated with elevated complexity in the premaxillofrontal suture. Suture complexity poorly reflects head-lift digging importance. We demonstrate that fossorial function is a complex, multivariate trait and reveal new insights into the functional morphology of cranial sutures across the most diverse mammalian order.