In this report we provide data on dental eruption and tooth germ maturation at birth in a large sample constituting the broadest array of non-human primates studied to date. Over 100 perinatal primates, obtained from natural captive deaths, were screened for characteristics indicating premature birth, and were subsequently studied using a combination of histology and micro-CT. Results reveal one probable unifying characteristic of living primates: relatively advanced maturation of deciduous teeth and M1 at birth. Beyond this, there is great diversity in the status of tooth eruption and maturation (dental stage) in the newborn primate. Contrasting strategies in producing a masticatory battery are already apparent at birth in strepsirrhines and anthropoids. Results show that dental maturation and eruption schedules are potentially independently co-opted as different strategies for attaining feeding independence. The most common strategy in strepsirrhines is accelerating eruption and the maturation of the permanent dentition, including replacement teeth. Anthropoids, with only few exceptions, accelerate mineralization of the deciduous teeth, while delaying development of all permanent teeth except M1. These results also show that no living primate resembles the altricial tree shrew (Tupaia) in dental development. Our preliminary observations suggest that ecological explanations, such as diet, provide an explanation for certain morphological variations at birth. These results confirm previous work on perinatal indriids indicating that these and other primates telegraph their feeding adaptations well before masticatory anatomy is functional. Quantitative analyses are required to decipher specific dietary and other influences on dental size and maturation in the newborn primate.

Midfacial reduction in primates has been explained as a byproduct of other growth patterns, especially the convergent orbits. This is at once an evolutionary and developmental explanation for relatively short snouts in most modern primates. Here, we use histological sections of perinatal nonhuman primates (tamarin, tarsier, loris) to investigate how orbital morphology emerges during ontogeny in selected primates compared to another euarchontan (Tupaia glis). We annotated serial histological sections for location of osteoclasts or osteoblasts, and used these to create three-dimensional "modeling maps" showing perinatal growth patterns of the facial skeleton. In addition, in one specimen we transferred annotations from histological sections to CT slices, to create a rotatable 3D volume that shows orbital modeling. Our findings suggest that growth in the competing orbital and neurocranial functional matrices differs among species, influencing modeling patterns. Distinctions among species are observed in the frontal bone, at a shared interface between the endocranial fossa and the orbit. The medial orbital wall is extensively resorptive in primates, whereas the medial orbit is generally depositional in Tupaia. As hypothesized, the orbital soft tissues encroach on available interorbital space. However, eye size cannot, by itself, explain the extent of reduction of the olfactory recess. In Loris, the posterior portion of medial orbit differed from the other primates. It showed evidence of outward drift where the olfactory bulb increased in cross-sectional area. We suggest the olfactory bulbs are significant to orbit position in strepsirrhines, influencing an expanded interorbital breadth at early stages of development.

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Extant primates are notable for their relatively convergent, or forward‐facing orbits, although some other mammals exhibit similarities, such as a postorbital bar supporting the eye laterally. Here, we use a novel approach of co‐registering micro‐computed tomography (CT) scans of late fetal nonhuman primates with serial histological sections, to infer how orbital morphology emerges during ontogeny. We use three primates (loris, tarsier, tamarin) that have absolutely or allometrically large eyes, and draw a contrast to the tree shrew (Tupaia glis), which may more closely resemble ancestral primate cranial form. Serial slices were annotated to create three‐dimensional cranial reconstructions that reveal the distribution of osteoclasts and osteoblasts. Reconstructions revealed that the superomedial orbit is resorptive in all primate species, although the amount of medial resorption is greatest in the tarsier and tamarin. In contrast, the orbit of Tupaia is depositional except for small inferomedial foci adjacent to infraorbital nerves. Broadly, orbital growth remodeling patterns in primates may be explained by the degree of orbital convergence, which far exceeds that in Tupaia. However, the three primates vary in the degree of orbital convergence and frontation, suggesting that a different factor (e.g., proportional eye size) may influence the similarities in orbital growth patterns.