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

Abstract

Sharp, A.C. 2016. A quantitative comparative analysis of the size of the frontoparietal sinuses and brain in vombatiform marsupials. Memoirs of Museum Victoria 74: 331-342.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.

Description of new cranial material of Propalorchestes (Marsupialia: Palorchestidae) from the Middle Miocene Camfield Beds, Northern Territory, Australia

“…This is similar to the lateral squamosal sinus elaboration above the glenoid with the widening (horizontal expansion) of the posterior zygoma (see p. 14). The degree of sinus development shown in Propalorchestes is strong and significant in relation to the allometric trends and specializations of later and larger Vombatomorphia (Murray, 1992;Sharp, 2016). An indication of the evolutionary development of the sinus system and corresponding increase in skull size can be gained by the comparative outline featuring P. azael ( fig.…”

Cranial biomechanics, bite force and function of the endocranial sinuses in Diprotodon optatum, the largest known marsupial

“…Large sinuses have also been recorded in other large extinct diprotodontian marsupials (Murray, ; Black et al. ; Sharp, in press). In juveniles, the outer surface of the skull and the inner surface of the braincase are separated by a thick layer of cancellous bone called the diploe.…”

“…; Curtis et al. ; Sharp, in press). In very large mammals like Diprotodon , the braincase is often separated from the outside surface of the skull by frontal sinuses, epitympanic sinuses, squamosal sinuses and parietal sinuses.…”

“…In most cases, large sinuses are criss‐crossed by many thin‐walled trabeculae, which form a convoluted network of air cavities (Farke, ; Badlangana et al. ; Sharp, in press). It is unclear whether these struts provide support as the outer bone that forms the dorsal surface of the skull separates from the braincase, or if they are simply a byproduct of rapid pneumatisation (Farke, ; Tanner et al.…”

“…During ontogeny, the two surfaces separate and air cavities form as the diploe expands, increasing the size of the cavities as the separation continues. In a recent study comparing the variation in volume of the cranial sinuses and brain cavity in extinct and extant vombatiform marsupials, Sharp (in press) concluded that the cranial sinuses expand to accommodate the temporalis muscle because the relatively small brain in these species does not allow sufficient surface area for their attachment. It was identified that as skull size gets larger, the sinuses display positive allometric growth, and the brain negative allometric growth, so that in very large animals the sinuses are relatively larger and the brain is relatively smaller than in smaller animals.…”