What structural features allowed reptiles to switch to a terrestrial lifestyle?

1) Adaptation of the integument of the body to be outside the water space, that is, in the formation of keratinized scales, which made it possible to exclude the loss of moisture.
2) Improvement of the musculoskeletal system of the body.
3) Changes in the circulatory system, which manifested itself in the formation of an incomplete septum between the two ventricles, which helped to prevent strong mixing of venous and arterial blood.
4) Improvement of the respiratory system, which manifests itself in the complication of the structure of the lungs, an increase in their area, the lack of breathing through the skin.
5) Improvement of the reproductive system, which manifests itself in the appearance of internal fertilization, which does not require the presence of water, as well as the ability to lay eggs in the keratinous membrane