A quantitative comparative analysis of the size of the frontoparietal sinuses and brain in vombatiform marsupials

Cranial sinuses result from the resorption and deposition of bone in response to biomechanical stress during a process known as pneumatisation. The morphology of a pneumatic bone represents an optimisation between strength and being light weight. The presence of very large sinuses has been described in a number of extinct marsupial megafauna, the size of which no longer exist in extant marsupials. With advances in digital visualisation, and the discovery of a number of exceptionally preserved fossil crania, a unique opportunity exists to investigate hypotheses regarding the structure and evolution of the atypically voluminous sinuses. Sinus function is difficult to test without first obtaining data on sinus variation within and between species. Therefore, the crania of seven species of extinct and extant vombatiform marsupials were studied using CT scans to provide a volumetric assessment of the endocast and cranial sinuses. Sinus volume strongly correlates with skull size and brain size. In the extinct, large bodied palorchestids and diprotodontids the sinuses expand around the dorsal and lateral parts of the braincase. Brain size scales negatively with skull size in vombatiform marsupials. In large species the brain typically fills less than one quarter of the total volume of the endocranial space, and in very large species, it can be less than 10%. Sinus expansion may have developed in order to increase the surface area for attachment of the temporalis muscle and to lighten the skull. The braincase itself would have provided insufficient surface area for the predicted muscle masses.