Comparative finite element analysis involves standardising aspects of models to test equivalent loading scenarios across species. However, regarding feeding biomechanics of the vertebrate skull, what is considered “equivalent” can depend on the hypothesis. Using 13 diversely-shaped skulls of marsupial bettongs and potoroos (Potoroidae), we demonstrate that scaling muscle forces to standardise specific aspects of biting mechanics can produce clearly opposing comparisons of stress or strain that are differentially suited to address specific kinds of hypotheses. We therefore propose three categories of hypotheses for skull biting mechanics, each involving a unique method of muscle scaling to produce meaningful results: those comparing (1) the skull's efficiency in distributing muscle forces to the biting teeth, via standardising input muscle force to skull size, (2) structural biting adaptation through standardising mechanical advantage to simulate size-independent, equivalent bites, and (3) feeding ecology affected by size, such as niche partitioning, via standardising bite reaction force.

Comparative finite element analysis often involves standardising aspects of the models to test equivalent loading scenarios across species. However, in the context of feeding biomechanics of the vertebrate skull, what is considered “equivalent” can depend on the hypothesis. We use 13 skulls from diverse group of marsupial bettongs and potoroos (Potoroidae) to demonstrate that that scaling muscle forces to standardise unique aspects of biting mechanics can produce contrasting results of comparative stress or strain that are differentially suited to test specific kinds of hypotheses. We propose three categories of hypotheses for skull biting mechanics which each involve a unique method of muscle scaling: those comparing (1) the skull’s efficiency in distributing input muscle forces, via standardising input muscle force to size, (2) morphological biting adaptation through standardising mechanical advantage to simulate size-independent, equivalent bites, and (3) feeding ecology affected by size, such as niche partitioning, via standardising bite reaction force. SUMMARY STATEMENT Common approaches for scaling muscle forces in skull finite element models might not always offer reliable results for all hypotheses. We provide a framework for selecting the appropriate method.

The mammalian cranium (skull without lower jaw) is representative of mammalian diversity and is thus of particular interest to mammalian biologists across disciplines. One widely retrieved pattern accompanying mammalian cranial diversification is referred to as ‘craniofacial evolutionary allometry’ (CREA). This posits that adults of larger species, in a group of closely related mammals, tend to have relatively longer faces and smaller braincases. However, no process has been officially suggested to explain this pattern, there are many apparent exceptions, and its predictions potentially conflict with well‐established biomechanical principles. Understanding the mechanisms behind CREA and causes for deviations from the pattern therefore has tremendous potential to explain allometry and diversification of the mammalian cranium. Here, we propose an amended framework to characterise the CREA pattern more clearly, in that ‘longer faces’ can arise through several kinds of evolutionary change, including elongation of the rostrum, retraction of the jaw muscles, or a more narrow or shallow skull, which all result in a generalised gracilisation of the facial skeleton with increased size. We define a standardised workflow to test for the presence of the pattern, using allometric shape predictions derived from geometric morphometrics analysis, and apply this to 22 mammalian families including marsupials, rabbits, rodents, bats, carnivores, antelopes, and whales. Our results show that increasing facial gracility with size is common, but not necessarily as ubiquitous as previously suggested. To address the mechanistic basis for this variation, we then review cranial adaptations for harder biting. These dictate that a more gracile cranium in larger species must represent a structural sacrifice in the ability to produce or withstand harder bites, relative to size. This leads us to propose that facial gracilisation in larger species is often a product of bite force allometry and phylogenetic niche conservatism, where more closely related species tend to exhibit more similar feeding ecology and biting behaviours and, therefore, absolute (size‐independent) bite force requirements. Since larger species can produce the same absolute bite forces as smaller species with less effort, we propose that relaxed bite force demands can permit facial gracility in response to bone optimisation and alternative selection pressures. Thus, mammalian facial scaling represents an adaptive by‐product of the shifting importance of selective pressures occurring with increased size. A reverse pattern of facial ‘shortening’ can accordingly also be found, and is retrieved in several cases here, where larger species incorporate novel feeding behaviours involving greater bite forces. We discuss multiple exceptions to a bite force‐mediated influence on facial proportions across mammals which lead us to argue that ecomorphological specialisation of the cranium is likely to be the primary driver of facial scaling patterns, with some developmental constraints as possible secondary factors. A potential for larger species to have a wider range of cranial functions when less constrained by bite force demands might also explain why selection for larger sizes seems to be prevalent in some mammalian clades. The interplay between adaptation and constraint across size ranges thus presents an interesting consideration for a mechanistically grounded investigation of mammalian cranial allometry.

Decommissioning the dingo barrier fence has been suggested to reduce destructive dingo control and encourage a free transfer of biota between environments in Australia. Yet the potential impacts that over a century of predator exclusion might have had on the population dynamics and developmental biology of prey populations has not been assessed. We here combine demographic data and both linear and geometric morphometrics to assess differences in populations among 166 red kangaroos (Osphranter rufus)—a primary prey species of the dingo—from two isolated populations on either side of the fence. We also quantified the differences in aboveground biomass for the last 10 years on either side of the fence. We found that the age structure and growth patterns, but not cranial shape, differed between the two kangaroo populations. In the population living with a higher density of dingoes, there were relatively fewer females and juveniles. These individuals were larger for a given age, despite what seems to be lower vegetation biomass. However, how much of this biomass represented kangaroo forage is uncertain and requires further on-site assessments. We also identified unexpected differences in the ontogenetic trajectories in relative pes length between the sexes for the whole sample, possibly associated with male competition or differential weight-bearing mechanics. We discuss potential mechanisms behind our findings and suggest that the impacts of contrasting predation pressures across the fence, for red kangaroos and other species, merit further investigation.

The evolution of marsupial postcranial diversity and adaptation has long been conceptually tied to the ability of the otherwise highly immature neonates to actively move to the mother’s pouch after birth. This requirement is reflected in an unusually well-developed forelimb and anterior postcranial skeleton, which gave rise to the long-standing contention that marsupial postcranial evolution is under a developmental diversity constraint. In this chapter, the knowledge about early developmental processes and heterochrony behind marsupial postcranial development is summarized. This is followed by a discussion of recent finds that do not support the constraints hypothesis, arguing that these shine a new light on the usefulness of marsupial postcranial development in the study of vertebrate postcranial evo-devo. Australian marsupial postcranial diversity in particular is an excellent opportunity to study adaptations to the most common (and in the case of kangaroos, most specialized) locomotor modes in an old and isolated radiation of mammals. This topic is reviewed by providing an overview over the state of knowledge on the function and adaptation of the musculoskeletal system within the main locomotor categories of Australian marsupial mammals: generalized terrestrial quadrupeds, arboreal/scansorial species, gliders, fossorial species, and saltators (hoppers).