AbstractSuture shape and complexity are thought to influence skull function in mammals, supporting the evolution of ecological and morphological diversity. These aspects of suture morphology are seldom studied in a comparative context, especially relative to the multitude of comparative studies of cranial shape. Using a three‐dimensional comparative ontogenetic dataset spanning 22 species across the phylogenetic breadth of Mammalia and sampling from foetal to adult stages, we applied 3D geometric morphometrics and 2D complexity metrics to track the evolutionary and developmental morphology of three cranial sutures (interfrontal, sagittal and coronal). Shape and complexity vary across the three sutures, with complexity decreasing through ontogenetic stages for antero‐posterior sutures (interfrontal and sagittal) but showing a postnatal increase for transversal sutures (the coronal). This suggests that aging is the strongest influence on longitudinal suture complexity because of simplification and obliteration for sutures subject to tensile stresses. This adulthood trend can be explained by a necessity to consolidate the skull through fusion, coupled with the disappearing need to accommodate further brain growth. Transversally positioned sutures oppose the trend as they are subject to the compressive stresses of cranial mechanics. Additionally, our findings refute the hypothesis that placental mammals have more complex and variable sutures than marsupials reflecting their more disparate ecologies. Rather, developmental history was found to be the greatest influence on suture complexity and variability. As a result, the extreme altriciality of marsupials, and its related longer postnatal brain growth, allows them to match and surpass the suture variability found in most placentals, reaching levels otherwise found mainly in primates.