Final answer:
The 'U-shaped' bone just below the mandible with only muscle attachments is the hyoid bone. The mandible itself is the moveable bone of the skull, and it forms the lower jaw with various significant landmarks, such as the mylohyoid line for muscle attachment.
Step-by-step explanation:
The 'U-shaped' bone located just below the mandible that has only muscle attachments is the hyoid bone. The mandible itself is the lower jaw and the only moveable bone of the skull, consisting of a left and right bone that fuse during the first year to form the adult skull's single U-shaped mandible. This U-shaped structure of the mandible is evident in the relationship between the body and the ramus of the mandible.
The mandible has various landmarks, such as the mylohyoid line, which extends along the inner aspect of the mandibular body where the muscle that forms the floor of the oral cavity attaches. Not part of the skull, the hyoid bone is considered a component of the axial skeleton and lies below the mandible in the front of the neck. It serves as a base for the tongue and is connected to muscles of the jaw, larynx, and tongue, assisting in movements essential for speech and swallowing.