Synchondroses of the anterior chondrocranium in primates

The sphenoethmoidal synchondrosis or prespheno‐septal synchondrosis (PSept) is a chondrocranial joint_that does not fuse in humans until age six or seven and is suggested to fuse near birth in non‐human primates. As such, the PSept may play a role in variation in basicranial angulation or midfacial protrusion. Here, we assess indicators of cartilage growth and modeling activity at birth at the anterior end of the presphenoid bone in a broad range of primate species. A serially sectioned sample of 36 infant non‐human primates were studied using light microscopy. The cranial base region was examined to verify whether a synchondrosis separates the sphenoid and septum as well as distribution of osteoclasts along the anterior aspect of the presphenoid. Contrary to previous statements, all primates possess a synchondrosis on the anterior face of the presphenoid bone. In long faced primates (lemurs and lorises), the nasal septal cartilage articulates with the presphenoid in a blunt ended PSept. In contrast, the presphenoid invades the nasal septum in monkeys. Modeling patterns of the presphenoid differed among monkey species. For example, there is resorption of the lateral side of the presphenoid in small species such as marmosets and squirrel monkeys, resulting in a narrow interorbital septum. These findings demonstrate that the PSept is not fused in all non‐human primates at birth, as previously suggested. Because the presphenoid projects anteriorly to a different extent in the primate species under study, we suggest that the nature of PSept growth may affect variation in the facial shape more than basicranial angulation. Support or Funding Information Funded by the National Science Foundation; Grant numbers: BCS‐1231350, BCS‐1231717, BCS‐0959438, BCS/IOS‐0924592