Divergent patterns of cranial suture fusion in marsupial and placental mammals

Cranial sutures, both open and closed, support a myriad of skull functions, including redistributing strain, accommodating brain expansion, supporting cranial bone growth, and protecting the brain. Thus, variation in the degree, timing, and pattern of suture fusion has functional implications. Using a comparative ontogenetic framework across Mammalia, we quantified degree and pattern of suture fusion through ontogeny for 22 mammalian species (N = 165). Suture closure was scored on a discrete scale for 31 cranial sutures and used to calculate closure scores for individual sutures and specimens. Ancestral state estimations found the degree of ancestral marsupial fusion to be more derived, differing from both the ancestral placental and ancestral therian. The average placental pattern followed the Krogman pattern of suture fusion (cranial vault, cranial base, circum-meatal, palatal, facial, and cranio-facial), whereas marsupials showed a distinct pattern. We propose a new pattern of suture fusion for marsupials: vault, cranio-facial, facial, circum-meatal, palate, cranial base. Delayed fusion of the marsupial cranial base is hypothesized here to support prolonged postnatal growth of the marsupial brain. Collectively, our study has identified a clear marsupial-placental dichotomy in the degree, timing, and pattern of suture fusion, with implications for understanding skull function and ontogeny.