Which of the following statements about vertebrates is true?

The statement that vertebrates possess a backbone is true because it is one of the defining characteristics of this group of animals. Vertebrates belong to the subphylum Vertebrata, which encompasses various classes, including mammals, birds, reptiles, amphibians, and fish. The backbone, or vertebral column, provides structural support, houses and protects the spinal cord, and plays a significant role in locomotion and flexibility. This feature differentiates vertebrates from invertebrates, which do not have a backbone.

In contrast, other statements are not accurate: while vertebrates are a diverse and important group, they do not make up the majority of animal species; in fact, most animal species are invertebrates. Additionally, not all vertebrates live in water, as many are terrestrial or capable of existing in various environments. Finally, vertebrates include more than just mammals; they comprise a wide range of animals with different classes, including reptiles and birds, making it incorrect to state that they are exclusively mammals.