The Biology and Diversity of Extant Reptiles.

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The Biology and Diversity of Extant Reptiles.

The word reptile itself does not describe a monophyletic group of vertebrates, like the mammals or birds. It is used to classify a polyphyletic group of animals that are a subset of the larger group of Amniotes. The term is best described through a cladogram:

Dr. Paul M. Barrett 2001

The reptiles first appeared in the early Carboniferous, having evolved from labyrinthodont amphibians. They had several distinct advantages over the amphibians that allowed them to flourish during the cold, dry Permian period. These lead them to be the most successful group of vertebrates and this period was referred to as the "age of reptiles." They exploited all terrestrial niches, a few marine and even one group took to the air. No other group of vertebrates had ever displayed such diversity and mammals today are their only match. They were the first group of tetrapods to successfully overcome the major problems of terrestrial life, namely desiccation, thermoregulation and respiration.

Adaptations to withstand desiccation

Unlike amphibians reptiles are able to tolerate dryer conditions and avoid desiccation. This allowed them to exploit more niches and therefore achieve greater radiation. They did this in a number of ways:

Skin. Reptilian skin is impermeable to water due to its unique structure. It has a thick dermis and a thick epidermis. The relative thickness of these layers helps somewhat in water conservation but it is the components of the epidermis that make the skin waterproof. See diagram:

The ?-keratin makes up the lower layer and is softer and more pliable then the darker harder ß-keratin layer. This gives flexibility and strength as well as providing a waterproof cuticle. Because of this reptiles were able withstand osmotic water loss and re-invade the sea. The Chelonia (turtles) have become the most prolific marine reptile and they also have an interesting skin that is worth mentioning. The have a shell covering their body, made out of a horny layer attached to a denser bony layer. It comes in two pieces, the carapace, for dorsal protection and the plastrom for ventral protection.

2 Excretion. Reptiles have modified their excretory products to conserve water. Because they do not have a complex enough kidney to produce urea they create uric acid, a large molecule that has a very low water content. Although the water content is lower than urea the trade off is that the molecule is expensive to make. Reptiles also reduce the water content of their faeces through active removal of NaCl. This creates an osmotic gradient to draw water out and desiccate the excrement. The excess NaCl is then excreted through the nose.
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3 Creation of the cloaca. This is a cavity into which the urinary, alimentary and genital ducts open. It has three sections, the coprodaeum where faeces collect and enter, the urodeum where the ureter empties out and water is removed and finally the proctodaeum, the collection point for uric acid and faeces. This centralisation of all the excretory products allows for greater water retention as well as providing for the next point.

4 Internal Fertilisation. Since the air would dry out gametes produced by the reptiles having a site for internal fertilisation prevents this and opens up ...

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