Reptiles
1. Reptiles are ectothermic vertebrates.
2. Their skin has scales, but no hair or feathers.
3. They have three-chambered hearts (except for alligators and crocodiles, which have four-chambered hearts).
4. They have claws on their toes (except those which do not have legs, such as legless lizards).
5. They are the first animals, in evolution, to develop the amniotic egg. This allows reptiles to lay eggs on land.
What do all these reptiles have in common? They all are cold-blooded (the reason lizards and snakes sun themselves is to warm up, just like you on a cold morning, or after a wade in Austin Creek) and they have scales. Did you ever see a lizard with feathers, or a hairy snake? No! Scales make up their only covering. Reptiles also lay eggs on dry land. You have heard of turtle eggs, right? Nature shows often have programs on sea turtles and how they lay their eggs on the beach. You may have seen some nature programs on alligators, another kind of reptile but not one we have, laying eggs in piles of rotting vegetation. All reptiles lay eggs. We know now that birds evolved from reptiles, and one of their linkages is the fact that they both lay hard-shelled eggs on dry land. Amphibian eggs, in comparison, are soft and must be laid in water.
Snakes, turtles, and lizards are all reptiles because they are cold-blooded, they lay eggs on dry land, and are covered with scales, never feathers or fur.
Class Reptilia Review
1. Which is the vertebrate class that is considered the first entirely terrestrial?
The first entirely terrrestrial vertebrate class, totally independent from the aquatic habitat, is the class Reptilia.
Image Diversity: reptiles
2. Compared to amphibians what is an example of evolutionary novelty present in beings of the class Reptilia against the loss of water through the skin?
The reptile skin is keratinized and impermeable to water while the amphibian skin is permeable. The skin impermeability made impossible the cutaneous gas exchange performed by amphibians and respiration became dependent on internal organs like airways and lungs.
3. What are examples of a carnivorous and a herbivorous reptile?
Snakes are carnivorous. Iguanas are herbivorous.
4. Do beings of the class Reptilia perform gas exchange in the same way amphibians do?
These beings do not have permeable skin so they do not make cutaneous respiration like amphibians do. Just like in birds and in mammals, their respiration is pulmonary.
5. How is the circulatory system of reptiles characterized? What is the basic difference between the reptile and the amphibian heart?
The circulatory system of beings of the class Reptilia is similar to the amphibian, closed and incomplete. Although the heart presents three chambers (two atria and one ventricle) in reptiles there is a beginning of ventricular septation and the mixture of arterial with venous blood is lessened.
Image Diversity: reptilian heart
6. Which is the type of nitrogen waste eliminated by beings of the class Reptilia?
These beings excrete mainly uric acid. This substance is less toxic than ammonia and it can be kept stored for a longer time inside the individual, including within eggs. In addition uric acid is practically insoluble and it depends less on water to be eliminated.
Image Diversity: uric acid molecule
7. How has the importance of the brain evolved from fishes to reptiles?
From the least to the most complex brain structure, it is evident that the brain, from fishes to beings of the class Reptilia, became larger and predominant in the central nervous system.
Image Diversity: reptilian brain
8. How is reproduction done in beings of the class Reptilia?
These beings reproduce sexually through internal fecundation by means of copulation between male and female individuals. They lay eggs with shell and extraembryonic membranes. The embryo thus develops within the egg and outside the mother’s body (there are also ovoviviparous reptiles that retain the egg within the body until hatching).
9. Do beings of the class Reptilia have direct or indirect development?
In beings of the class Reptilia the embryonic development is direct. So there is no larval stage.
10. Compared to amphibians what are the two reproductive novelties of beings of the class Reptilia for the survival in dry environments?
Compared to amphibians the two main reproductive innovations of beings of the class Reptilia for the terrestrial habitat are internal fecundation and shelled eggs.
Image Diversity: reptilian egg
11. Concerning the maintenance of body temperature how do beings of the class Reptilia classify?
Like fishes and amphibians, beings of the class Reptilia are heterothermic animals (also known as poikilothermic, or ectothermic), i.e., they are not able to control by themselves their body temperature and thus they depend on external warm sources (mainly the sun).
12. What is an example of a hypothesis which may explain why there is not a big representation of the class Reptilia found in polar regions?
Beings of the class Reptilia are abundant and more diverse in hot climate regions and they are rare in intensely cold regions like close to the earth poles. This is explained because these animals are heterothermic, i.e., they have “cold blood” and they need an external heat source to warm their bodies.
The heterothermic feature also explains why reptiles are more active during the day, a period when they can use the sun heat to warm their bodies.
13. Class Reptilia identity card. How are they characterized according to examples of representing beings, basic morphology, skin, respiration, circulation, nitrogen waste, thermal control and types of reproduction?
Examples of representing beings: snakes, turtles, crocodiles, lizards, dinosaurs (extinct). Basic morphology: tetrapods, some with carapaces (like turtles). Skin: impermeable keratinized, corneous plates (known as scales). Respiration: pulmonary. Circulation: closed, incomplete, heart with three chambers and partial interventricular septation. Nitrogen waste: uric acid. Thermal control: heterothermic. Types of reproduction: sexual, internal fecundation, shelled eggs with extraembryonic membranes.