Cranial form and function of the largest ever marsupial, Diprotodon optatum Owen, 1838 (Marsupialia: Diprotodontinae)

Abstract

The extinct Diprotodon optatum was the largest ever marsupial, reaching over two tonnes. However, despite its large size, the cranium of Diprotodon is remarkably light, composed of thin bone and extensive endocranial sinuses. Cranial sinuses are air-filled cavities resulting from the resorption and deposition of bone through pneumatisation in response to biomechanical stress. Sinus morphology represents an optimisation between strength and weight reduction. The extraordinary preservation of the Diprotodon skulls found at Bacchus Marsh, southern Australia (37°40′S, 144°26′E), provides a unique opportunity to investigate hypotheses regarding the size and function 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 Diprotodon and six species of extinct and extant vombatiform marsupials were studied using CT scans to provide a volumetric assessment of the brain endocast and cranial sinuses. Sinus volume scales positively and brain size scales negatively with skull size, so that larger species have relatively smaller brains and larger sinuses than those of smaller species. In the large, extinct palorchestid and diprotodontid marsupials the fronto-parietal sinuses expand around the dorsal and lateral regions of the braincase. The sinuses may have expanded in order to increase the surface area for attachment of the temporalis muscle, as the braincase itself would have provided insufficient surface area for the predicted muscle masses. Detailed three-dimensional reconstructions of the cranium, mandible and jaw adductor muscles were produced for Diprotodon and four extant marsupials (Vombatus ursinus, Phascolarctos cinereus, Macropus rufus and Wallabia bicolor) to investigate functional hypotheses using reverse engineering. Finite element analysis was used to identify areas of high and low stress, mechanical efficiency and bite performance in the crania of each species to investigate the relationship between biomechanical performance and diet. In addition, manipulations to the Diprotodon cranial model were performed to investigate changes in skull, and sinus structure (e.g. the normal model with fronto-parietal sinuses, a filled-sinus model, and a midsagittal crest model). Despite the apparent fragile nature of the cranium of Diprotodon, the model performed well and is relatively strong, indicated by low median stress levels through the model. The mechanical efficiency of the skull was also relatively high. The large cranial sinuses do not seem to disadvantage Diprotodon in terms of performance, and may in fact help to dissipate stress over the cranium. When examining the results from the manipulated models of Diprotodon, further evidence suggests that the normal model with sinuses may help to move stress away from the nasal and orbital regions of the cranium. The sinuses also allow the skull to be significantly lighter than if the frontal and parietal bones were not pneumatised. This study points to the significance of large sinuses in Diprotodon, and perhaps all marsupial megafauna, to reduce the weight of the skull while maintaining biomechanical performance.