Success Of Reptiles

Representatives of the four modern orders of reptiles—Chelonia (turtles and tortoises), Squamata (lizards and snakes), Rhynchocephalia (tuataras), and Crocodilia (crocodiles, alligators, caimans)—survived the mass extinction of the Cretaceous period. These four orders of reptiles have diversified to more than 6,000 species. Reptiles successfully occupy a variety of terrestrial and aquatic habitats on all continents except Antarctica. Figure 41-5 is a phylogenetic diagram that shows hypotheses for the relationship among reptiles, birds, and mammals.

Modern Reptiles

The turtles have the most ancient origins and have changed very little in structure since before the time of the dinosaurs. Tuataras belong to a small group of lizard-like reptiles. The vast majority of living reptiles belong to the group snakes and lizards. The fourth line of living reptiles includes the crocodiles and their relatives. Crocodilians have changed very little in more than 200 million years. Like dinosaurs, crocodilians are descendants of the thecodonts.

In some ways, such as the structure of their heart, crocodilians resemble birds far more than they resemble other living reptiles. And crocodilians are the only living reptiles that care for their young. What does this mean in terms of their relationships to other vertebrate species? Today, many biologists think that birds are direct descendants of the dinosaurs.

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