Final answer:
The tetanurans evolved a fused sacrum as part of their adaptation for more efficient locomotion on land. This aligned their limbs and digits in the direction of motion, which is a key skeletal change as animals transitioned from aquatic to terrestrial habitats.
Step-by-step explanation:
Evolutionary novelties of the tetanurans include a fused sacrum. Throughout the evolution from aquatic to terrestrial habitats, changes in the skeletal system of animals include the development of limb placement underneath the body and modifications for more efficient land movement.
The appendicular skeleton adapted, with limbs positioned under the body, reducing force required for forward movement, and thereby making the locomotion more energy-efficient. A prominent characteristic is the rotation of the femur and humerus, aligning the limbs and digits in the direction of motion. Such adaptations led to a more advanced and specialized skeletal structure, including a fused sacrum which is one among the various evolutionary advancements seen in later tetrapods and their descendants.