Final answer:
Quadrupedal primates have a more rigid and straight spine compared to their aquatic ancestors, adapted for efficiency in terrestrial locomotion with limbs positioned under the body.
Step-by-step explanation:
Quadrupedal primates typically have a spine that reflects their mode of locomotion. Unlike aquatic animals, which exhibit a lateral undulating vertebral column, terrestrial animals, including quadrupedal primates, have undergone evolutionary changes such as having limbs placed underneath the body, decreasing the need for large adductor muscles. This skeletal adaptation allows for more efficient forward movement. Additionally, the rotation of the femur and humerus to point the limbs and digits forward also aids in efficient land movement in these primates.
In quadrupedal primates, these evolutionary changes have resulted in a vertebral column that is generally more rigid and straighter compared to the flexible, undulating spine of their aquatic ancestors. The spine in quadrupedal primates provides a stable support for the attachment of the muscles that are necessary for their form of locomotion - walking on all four limbs.