Observe the above organism. It has a bill like a duck. Does that make it a bird? It has fur like a dog or beaver, so is it a mammal? But it also has webbed feet. What is this creature? These are the questions scientists asked when they first discovered the duck-billed platypus. It is classified as a mammal, but it also has some bird and even reptile DNA. Surprisingly, it also lays eggs, while almost all other mammals give birth to live young.
The platypus shows how great the diversity of life is on Earth. Organisms are not always easily classifiable, but most warm-blooded vertebrates can be classified as either a bird or a mammal.
Image copyright Joel Bauchat Grant, 2014. www.shutterstock.com. Used under license from Shutterstock.com.