Vertebrates are animals with backbones. Traditionally, they have been divided into five Classes: Pisces (fish), Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals). Modern studies would suggest that these groupings are simplistic - fish have more than one origin, for example, and, as most people know, birds are evolved from dinosaurs, so it is not possible to have a natural group (which scientists refer to as a 'clade') of traditional reptiles without including birds.
As Shetland has been a group of islands since the last ice age, the vertebrates in the islands can only have arrived by three means: they could fly, like birds or bats; they could swim, like cetaceans (whales and dolphins), seals or turtles; or they could be brought by man, like all the terrestrial mammals and amphibians.
For more on Shetland vertebrates, refer to the pages linked in the menu. For mammals see pages on land mammals (including bats), cetaceans (whales and dolphins), and seals. Amphibians and reptiles are covered on the herptiles page. There will be coverage of fish at a later date.