Final answer:
The foramen magnum placement is the physical feature in the skull diagnostic of bipedalism, with its central position under the skull indicating an upright, bipedal stance.
Step-by-step explanation:
The term for the physical feature in the skull that is diagnostic of bipedalism is the foramen magnum placement. In bipedal organisms, the foramen magnum is positioned further underneath the skull, indicating the head is balanced atop a vertical spine, which is a characteristic of upright walking. While the provided references do not explicitly mention the foramen magnum, they discuss various anatomical changes associated with bipedalism such as the development of spinal curves and the repositioning of the big toe. These changes align with a foramen magnum placed more centrally under the skull, supporting the head over a bipedal stance.