Closing in the skull: Drivers of ontogenetic changes in emu skulls, Dromaius novaehollandiae (Palaeognathae: Casuariidae)

Abstract

In modern birds (Palaeognathae and Neognathae), the adult skull combines tightly sutured and co‐ossified units with kinetic flexion zones, serving as a multitasking ‘second hand’. How the skull osteology changes postnatally in different bird lineages is underexplored, especially regarding the morphological and physiological changes across the altricial‐precocial spectrum. For example, it is unknown to what extent internal (i.e. functional biomechanics) or external (i.e. environmental) drivers influence the morphological changes observed through ontogeny and whether those same drivers persist across different lineages. We hypothesize that different regions of the skull (i.e. braincase, mandible, and face) develop skeletally mature features (e.g. suture closure, braincase shape change, robusticity) through ontogeny at different rates due to varying effects from various drivers. In this study, we qualitatively describe and quantitatively analyze, through 3D geometric‐morphometric (GMM) data, an ontogenetic series of 19 emus (Dromaius novaehollandiae), to examine the timing and rate of morphological changes in cranial elements through growth. We observed a mediolateral widening of the posterior portion of the braincase accompanied by an increase in robusticity of the posterior part of the mandible through development. The frontal‐parietal fontanelle closure time varies, with some midsize juvenile specimens having an almost completely or completely closed skull roof. This is in contrast with the feeding apparatus, where the articular and angular region of the mandible remain largely unfused in midsize specimens. Our GMM analysis showed juvenile, subadult, and adult specimens occupying distinct regions of morphospace, showing clear intraspecies variation during ontogeny. We plan on further using GMM to assess when morphological changes start to accumulate in the different regions of the skull and to compare it to our whole skull analysis for the morphological rate changes. With respect to the drivers of the ontogenetic changes, the effects of external drivers (e.g. food type, food availability) are likely small due to husbrandry consistency across emu farms. Internal drivers relating to functional biomechanics, however, may have a larger influence on the ontogenetic differences seen here in that the muscles of the feeding apparatus anchor onto the cranium, accounting for the observed differences in suturing and fontanelle closure and development of the braincase. The morphological complexity seen by including juveniles and subadults in our study highlights gaps in previous postnatal developmental studies. The universal drivers of postnatal ontogeny can be addressed further by applying this same method to other bird lineages on the altricial‐precocial spectrum, allowing us to investigate the degree to which feeding influences the morphological changes and timing to reach the adult condition.