Here is a list of some of the differences between modern-day reptiles and mammals.

1. Reptiles have horny or scaly skin. Mammals have fur or hair.
2. Mammals have a single type of skin cell for colour. Reptiles have 3 types of skin cell for colour.
3. Reptiles have low metabolism and require less energy. Mammals have high metabolism.
4. The body temperature of reptiles varies according to their environment (they are ‘cold-blooded’). Mammals are able to maintain a constant temperature (they are ‘warm-blooded’).
5. Reptiles (but not tortoise/turtles, snakes or crocodilians) have a “third eye”, known as a parietal or pineal eye. It has a cornea, lens and photoreceptors. Mammals have only a parietal gland, used for endocrine production.
6. Reptiles have small, relatively even teeth with single roots; they are replaced often. Mammals have different types of teeth with multiple roots – incisors, canines, premolars and molars – and replace their teeth only once.
7. Reptiles have 3 bones in the lower jaw: the dentary (holding the teeth), the quadrate and the articular. Mammals have only one lower jaw bone.
8. Reptiles’ ears have only one bone, the stapes. Mammals have 3 bones, (stapes, malleus, incus), and the very complicated organ of Corti.
9. The reproductive system of male reptiles includes a hemi- penis. This consists of two penises, which are used singly and repeatedly to fertilise several of a female’s eggs.
10. Most reptiles lay eggs. Mammals do not.
11. Reptiles do not usually guard their eggs or care for their young. Mammals all have milk glands and suckle their young.
12. The sex of the unborn young of reptiles is determined by external temperature. Hot temperatures result in more females.
13. Mammals breathe by way of a diaphragm in their chest (the thorax). Reptiles have no diaphragm and breathe very differently, with their cheeks and mouth.
14. Reptiles have a 3-chambered heart, except crocodilians which, like mammals, have a 4-chambered heart. Since their hearts cannot pump blood far upwards, reptiles move close to the ground (they ‘creep’).
15. The legs of reptile are splayed out from the body. The legs of mammals are positioned under the body.