The smallest bone in mammals is the stapes. The stapes bone is located in the middle ear and plays a crucial role in transmitting sound vibrations from the eardrum to the inner ear. It is often referred to as the stirrup bone due to its shape.
Option 1: The septomaxillary bone is not the smallest bone in mammals. It is a bone found in the skull that separates the nasal and oral cavities.
Option 2: The dentary bone is the largest bone in the lower jaw of mammals. It is not the smallest bone.
Option 3: The femur is the longest bone in the mammalian body and is not the smallest bone.
Therefore, the correct answer is option 4 - stapes.