Foramen magnum

The foramen magnum is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes through the foramen magnum as it exits the cranial cavity. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the tectorial membranes and alar ligaments. It also transmits the accessory nerve into the skull.

Bipedal species typically display an anteriorly placed foramen magnum to balance the head atop the vertebral column. In hominins, this anterior shift is uniquely coupled with increased cranial base flexion. Studies of foramen magnum position have shown its relationship to posture and locomotion. This forward placement is apparent in bipedal hominins such as modern humans, Australopithecus africanus, and Paranthropus boisei. This common feature of bipedal hominins is the driving argument used by the paleontologist Michel Brunet to support that Sahelanthropus tchadensis was bipedal and may be the earliest known bipedal hominin. The recognition of this feature has given scientists another tool for identifying bipedal mammals.