Lizards typically have limbs and external ears, while snakes lack both these characteristics. However, because they are defined negatively as excluding snakes, lizards have no unique distinguishing characteristic as a group. Lizards and snakes share a movable quadrate bone, distinguishing them from the sphenodonts, which have a more primitive and solid diapsid skull. Many lizards can detach their tails to escape from predators, an act called autotomy, but this ability is not shared by all lizards. Vision, including color vision, is particularly well developed in most lizards, and most communicate with body language or bright colors on their bodies as well as with pheromones.
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpents that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and most have only one functional lung.
Turtles are reptiles of the order Testudines (the crown group of the superorder Chelonia), characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic.
Tortoises (Testudinidae) are a family of land-dwelling reptiles of the order of turtles (Testudines). Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise has both an endoskeleton and an exoskeleton. Tortoises can vary in size from a few centimeters to two meters. Tortoises are usually diurnal animals with tendencies to be crepuscular depending on the ambient temperatures. They are generally reclusive animals.
A crocodile is any species belonging to the family Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The term can also be used more loosely to include all extant members of the order Crocodilia: i.e. the true crocodiles, the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae), as well as the Crocodylomorpha which includes prehistoric crocodile relatives and ancestors.
When did the reptiles become seperated from the amphibians? Actually we don't know. But we have to take our best guess and that's what we have done. The most primitive order of reptiles that we can agree on so far is that of the cotylosaurs which were lizard-like, insect eating, heavily limbed reptiles with solid roofed skulls and labyrinthine teeth and were very similar to amphibians of the time. Yet they were different enough that we have mostly agreed to start from this root-stock.
Sometimes change was fairly quick in evolutionary terms and other times it might take millions of years for any really significant change to manifest itself. Of course we know that one major difference that appeared early on was that reptiles, with their ability to lay eggs out of water were able to better colonize the land. But several other behavioral and anatomical changes had to be made before these differences were profound.
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