Complex sentence: The skeleton is somewhat typical of tetrapods, although the skull and pelvis are specialized, as are the ribs to allow the animal to collapse its thorax during diving and expand to accommodate large masses of food.
Split into simpler sentences: The skeleton is typical of tetrapods, although the skull and pelvis are specialized. In addition, the ribs are specialized to allow the animal to collapse its thorax during diving and expand to accommodate large masses of food.