Why would it be pointless for reptiles to incubate their eggs by sitting on them like birds? Reptiles are ectotherms and cannot increase their body temperature above the surrounding environment. Moreover, reptile eggs are not amniotic and lack a shell, so sitting on them could crush them. Additionally, reptiles lack feathers to cushion the eggs and prevent them from bumping into each other. Lastly, reptiles have teeth to defend their eggs from predators, eliminating the need to hide them.

Reptiles generally do not incubate their eggs by sitting on them like birds do for several reasons, but some of the points mentioned need clarification. Firstly, while reptiles are ectotherms (meaning they rely on external heat sources to regulate their body temperature), this does not inherently make it pointless for them to incubate eggs. However, since they cannot generate their own heat like birds (which are endotherms), the way they would provide warmth to the eggs would be less effective.

Secondly, the statement that reptile eggs are not amniotic and lack a shell is incorrect. Reptile eggs are actually amniotic, featuring a protective membrane called the amnion, and most reptile eggs do have a shell, although it's generally softer (leathery) compared to bird eggs. This softer shell would indeed be more prone to damage if a heavy reptile was to sit on them.

Thirdly, the absence of feathers in reptiles is true, but not all birds use their feathers primarily to cushion eggs. Feathers do help birds keep the eggs warm by trapping warmth, which is something ectothermic reptiles wouldn't be able to do effectively.

Lastly, the argument regarding reptiles having teeth to defend their eggs from predators is not a central reason why they do not sit on their eggs. Many reptiles actually use camouflage, hiding their eggs in nests or burying them, to protect them from predators, rather than actively defending them with teeth.

Extra: Understanding the differences between reptiles and birds when it comes to incubating eggs provides insight into how different groups of animals have adapted to their environments. Birds, as endotherms, have a steady body temperature that they can use to their advantage when incubating eggs, whereas reptiles, as ectotherms, must rely on external heat sources like the sun to incubate their eggs.

Bird eggs generally have a hard, calcified shell to protect the developing embryo, whilst reptile eggs often have a leathery shell that is permeable to gases, allowing for gas exchange which is important for the embryo's development. Due to their softer texture, these eggs might indeed be damaged if a large reptile were to sit on them.

Reptile incubation strategies can be quite diverse. For example, some reptiles, like sea turtles, lay their eggs on warm sandy beaches and let the sun provide the warmth needed. Others, like certain species of python, may use a form of incubation called "brooding" where the mother wraps around the eggs, but instead of sitting on them, they may generate heat through muscle contractions.

Lastly, the natural history of egg protection in reptiles is rich and varies from species to species. Some reptiles may guard or hide their nests, but the techniques for defending their offspring will differ depending on their ecological niche and evolutionary adaptations.