At birth, tarsiers lack a postorbital bar or septum

Among primates, partial or complete posterior closure of the orbit has been widely accepted as a shared derived characteristic justifying an exclusive tarsier-anthropoid clade, while some regard the tarsier lateral orbit as an elaborated postorbital bar (POB). To test these competing hypotheses while minimizing the confounding effect of tarsier orbital hypertrophy, we compared tarsiers and other primates at early (fetal and newborn) ages using dissection, micro-CT scans and soft tissue histology. Our findings demonstrate unanticipated variation in the anatomy and development of the zygomaticofrontal (ZFA) articulation, which forms the orbit's lateral framework. Tarsiers uniquely exhibit a combination of two features: absence of a pre- and peri-natal frontal spur to join with the zygomatic to form the ZFA; and, the spur's substitution by an elaborate ligament, which envelops the eye laterally as an expansive postorbital membrane (POM) that merges with the anterolateral fontanelle of the lateral cranial vault. In lacking a frontal spur, tarsiers are distinct from strepsirhines, while the ligamentous structure of the POM distinguishes its ZFA from that of anthropoids, which is a typical facial suture at and prior to birth. The POM of tarsiers may be thought of as an accessory fontanelle, a structural compromise that provides flexible stability and spatial separation of bones while anticipating rapid postnatal growth of an enormously enlarged eye. We regard the tarsier POM as part of a neomorphic eyeball hypertrophy complex, and reject the hypothesis of derived homology of the postorbital septa of adult tarsiers and anthropoids on histological, developmental and functional grounds.