Postorbital septation is a character that unites haplorhine primates. As a result, evidence of postorbital septation is expected for fossil primates that may be ancestral to both modern tarsiers and modern anthropoids. Our previous work on newborn primates has indicated that postorbital septation in tarsiers is secondary to growth of their large eyes. If this is the case, postorbital septation is convergent in tarsiers and anthropoids, and therefore should not be expected in their fossil ancestors. Here, we used morphometric analysis of the cranium and endocast of newborn and adult primates to show that postorbital septation in the order Primates may be explained by shape of the brain. We used a sample of over thirty species of modern primates representing all major clades, with individuals representing both newborn and adult stages. The newborn stage is most informative for understanding spatial relationships of soft tissues during formation of the skull. MicroCT image data were used to create virtual reconstructions of the cranium and endocast as an estimate of brain shape. Fixed landmarks were used to capture the shape of the cranium, and sliding semilandmarks represented the contour of frontal and temporal lobes on the endocast. As expected, morphometric analysis showed a significant effect of phylogeny on cranial and endocast shape in both newborns and adults. Notably, the phylogenetic effect was apparent in only the contours of the frontal and temporal lobes, independent of cranial shape. We present 3D models of the endocast and cranium of newborn primates to illustrate the spatial relationship of brain and facial structures as a mechanism for postorbital septation in anthropoid primates.

Primate auditory structures are morphologically indicative of taxonomy and phylogeny. The tympanic cavity houses the three articulating ear ossicles: malleus, incus, and stapes. The ossicles amplify sound waves from mechanical vibrations to the cochlea, a sensory organ, where it can then be interpreted by the organism. All primates are characterized by a petrous bulla, but the extent of the tympanic cavity varies across taxa. Our sample of newborn and adult individuals representing strepsirrhines (e.g., Galago moholi and Nycticebus pygmaeus) and haplorhines (e.g., Aotus nancymaae and Cebuella pygmaea) allowed us to study the comparative growth of these structures. Using a combination of conventional CT data and diffusion iodine contrast‐enhanced CT (diceCT) data, we created 3D models of auditory structures using 3DSlicer. Metric data included angles between ear ossicles, surface area of the tympanic membrane, surface area of the stapes footplate, and overall tympanic cavity volume. Many differences among major clades are already evident at the newborn stages in this sample. The pneumatization of accessory cavities in newborns has been initiated. For example, in the newborn Galago moholi, pneumatization of accessory cavities is most pronounced posteriorly. In contrast, in the newborn Aotus nancymaae, we see early evidence of the anterior accessory cavity. Mesh‐to‐mesh comparisons of endocasts of the cavity indicate structural changes through ontogeny. Additionally, the surface area of the tympanic membrane remains consistent through growth. This research allows for a better understanding of the relationship between the functional morphology of the auditory system and audible frequency levels. Haplorhines are adapted to lower frequencies than strepsirrhines. This adaptation is discernable at birth, and key spatial relationships of ossicles are maintained through ontogeny even as spatial configuration of the cranium as a whole undergoes profound change due to pneumatic expansion of sinus cavities and growth of soft tissue structures.

Most primates are characterized by orbital convergence, the displacement of the orbits toward the rostral midline. In platyrrhine primates and tarsiers, this convergence is so extreme that orbits meet in the midline at the apical interorbital septum. This produces a spatial separation of the anterior cranial fossa and the nasal cavity. The olfactory bulbs of the brain pass dorsal and then rostral to the orbits to reach the nasal cavity via a bony tube, the olfactory duct. Nerves are expected to constrain growth of musculoskeletal structures. We hypothesized that the network of olfactory nerves effectively tethers the olfactory bulbs to the mucosa of the nasal cavity, and the growth of facial structures in these primates results in rostral displacement of the olfactory bulbs through ontogeny. In addition, growth of the midface may be constrained by this functional relationship. This comparative analysis was based on 3D reconstructions of cranial structures in newborns and adults from representative platyrrhine species (e.g., Aotus nancymaae and Cebuella pygmaea) and tarsiers (Tarsius syrichta). Spatial relationships of the olfactory duct were estimated using endocasts based on conventional microCT scans. In addition, diffusion iodine contrast‐enhanced CT (diceCT) images were used to study Aotus nancymaae at multiple stages of ontogeny. DiceCT allowed us to reconstruct the actual position of olfactory bulbs relative to the olfactory duct and the distribution of olfactory nerves in the nasal cavity. 3DSlicer and Blender software were used to render 3D models. These data confirm our hypothesis that the rostral projection of the olfactory bulbs relative to the frontal lobes of the brain increases through ontogeny. In addition, the paths of olfactory nerves are diverted around the convergent eyes. The ontogenetic relationships occurring in platyrrhine and tarsier olfactory ducts have evolutionary implications for biological variation in the primate skull.