A donkey, a crocodile, a fish, a parrot, and a frog all have one thing in common. They all have a backbone, or vertebral column, connected to a supporting bony skeleton inside their bodies. They are all vertebrates.
FA/MILY TREE OF VERTEBRATES
Although vertebrates are the animals we know most about, they actually make up just a tiny part of the animal kingdom. Vertebrates with limbs—tetrapods—are all descended from fish. Vertebrates can be divided into five groups: mammals, birds, reptiles, amphibians, and fish.
Mammals can be divided into three groups, depending on their means of reproduction. Placental mammals give birth to well-developed young. Marsupials give birth to undeveloped young. Monotremes (there are just five living species) lay eggs.
There are almost 10,000 species of bird. They are the living descendants of dinosaurs, but they developed the ability to fly. Feathers help them to fly, but also keep them warm.
Along with some amphibians, reptiles were the first vertebrates to live entirely on land. Their skin is dry and covered in scales to help retain water, a necessary adaptation, since many reptiles live in warm areas with limited access to water.
Modern amphibians have moist, soft skin, and most amphibians can absorb oxygen through this skin in addition to having lungs. They largely live on land but require damp conditions. Most have to return to water to lay eggs.
POISON DART FROG
Earth’s first vertebrates—fish— now form more than half of all vertebrate species. Gills allow them to breathe underwater.
TAKE A LOOK—A PEEP INSIDE
Vertebrates have a backbone and internal bony skeleton. They also have a highly developed nervous system, and a larger brain for body size than invertebrates. Blood is pumped around the body by the heart, supplying the vertebrate’s body with food and oxygen and removing waste products. They breathe using lungs.
BONE is a lightweight, living organ, and it is found only in vertebrates. Because it is supplied with blood vessels, it can grow (unlike the hard casing of an invertebrate such as a crab, which has to be shed to allow growth).