# A suture in time: The ontogeny of cranial suture morphology in mammals

## TL;DR

This study explores how cranial sutures in mammals develop and change shape, revealing how biomechanical forces and developmental strategies influence skull structure across species.

## Contribution

The study provides a comparative ontogenetic analysis of cranial suture morphology across 22 mammal species using 3D geometric morphometrics.

## Key findings

- Antero-posterior sutures simplify with age, while transverse sutures become more complex in adulthood.
- Marsupials and placentals show similar suture variability despite differing developmental strategies.
- Suture complexity trends reflect biomechanical stresses and developmental history rather than ecology.

## Abstract

Suture 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.

Mammal cranial sutures are important indicators of the biomechanical and developmental pressures acting upon the skull. Across three prominent sutures dividing the vault of the mammalian skull, divergent patterns emerge both taxonomically and developmentally. Despite their different developmental strategies and evolutionary histories, marsupials and placentals display comparable disparity in suture shape. The developmental trajectories of suture complexity indicate a consistent increase until adulthood, where paired and unpaired sutures display different trends. The complexity of unpaired sutures like the sagittal and interfrontal plateaus in adulthood, while the paired ones like the coronal keep increasing their complexity, pointing towards differential reactions to biomechanical stresses in the skull.