One of the structural adaptations of mammals for movement on land is the development of the

Analyze the question: It asks for a structural adaptation in mammals that helps with movement on land (locomotion).

Explain the concept: Land vertebrates, including mammals, have limbs built on the pentadactyl plan, meaning a limb with five digits. The limb has a humerus or femur, then a radius and ulna or tibia and fibula, then carpals or tarsals, metacarpals or metatarsals, and phalanges. This design can be modified for walking, running, climbing, digging, or swimming, which makes it the basic structural adaptation for terrestrial movement.

Step-by-step evaluation of the options:

• A. Hollow bones: These are a feature of birds. They reduce body weight for flight, not for movement on land in mammals. Incorrect.
• B. Pentadactyl limb: This is the typical mammalian limb. It is jointed and supports the body's weight, so it allows walking, running, and jumping on land. Correct.
• C. Fused thoracic bones: Fusion of bones is not a typical mammalian locomotion adaptation. Mammals have ribs attached to separate thoracic vertebrae and the sternum, which allows flexibility for breathing. Incorrect.
• D. Streamlined body: This is an adaptation for movement in water, as in fish and whales, because it reduces water resistance. Incorrect.

Answer: The pentadactyl limb is the correct choice.

Exam tip: Link each adaptation to its habitat:

• Hollow bones → flight (birds)
• Streamlined body → water (fish)
• Pentadactyl limb → land (mammals and other tetrapods)