UNIQUE ANATOMICAL TRAITS OF REPTILES

Reptiles

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All reptiles have a protective layer of dry skin that has few glands and is keratinized to form either scales or scutes. A lipid layer beneath this keratin provides some resistance to water loss and this has helped them adapt to terrestrial existence (Lillywhite & Maderson 1982; Roberts & Lillywhite 1980).

■ Reptiles only have a single occipital condyle articulating with the atlas. In contrast to amphibians they have a well-developed neck, which enables them to scan the horizon and survive on land (Bellairs 1969a).

■ Many reptiles, like snakes and lizards, have a kinetic skull. This means that a large part of the reptile skull fails to ossify and elastic cartilage allows for movement between different regions of the skull. Consequently, reptiles are able to raise their upper jaw like a hinge to increase gape during feeding. The quadrate bone that articulates between the upper and lower jaw can also move freely.

■ The transition to land has been facilitated by the development of an amniotic egg, enabling reptiles to breed independently of water. The production of a large yolked egg supplies a protective amnion and allantois for respiration and storage of waste products. The protective parchment-like shell prevents desiccation, allowing the embryo to become sufficiently developed before hatching (King & Custance 1982).

■ Most reptiles excrete mainly insoluble uric acid instead of soluble ammonia and urea. This prevents waste

Table 2.1 Taxonomy and classification of reptiles (Uetz 2000)

products inside the impermeable egg becoming toxic to the developing embryo.

■ With the exception of crocodiles, reptiles have a 3-chambered heart with two atria and one common ventricle. This allows reptiles to shunt blood either away or toward the lungs, facilitating thermoregulation and diving.