Final answer:
The synapsid skull, characterized by a single opening behind the eye, is a unique bone structure that distinguishes synapsids, including mammals, from other vertebrates.
Step-by-step explanation:
A key characteristic that makes synapsids unique with respect to their bone structures is the presence of a single opening in the skull behind the eye. This characteristic is called a synapsid skull. In contrast, other vertebrates have either no openings (anapsids) or two openings (diapsids) in the skull.
This unique bone structure in the skull of synapsids, including mammals, is important because it allows for specific cranial adaptations that are not present in other vertebrates. For example, the presence of one opening in the synapsid skull is associated with the evolution of the jaw joint and the middle ear bones, which are important for chewing and hearing.
Overall, the synapsid skull is a defining characteristic of mammals, and it sets them apart from other vertebrates in terms of their bone structures.