This philatelic issue features prehistoric sea creatures that lived in Australia around 500 million years ago, during the Cambrian Period of the Palaeozoic Era. This time in Earth's history, between 540 and 520 million years ago, witnessed a rapid increase in biodiversity known as the Cambrian "explosion". The animals that emerged were the ancestors of modern sponges (poriferans), bristle-worms (annelids). snails and cockles (molluscs), crustaceans (arthropods), sea-stars and sea-urchins (echinoderms), and the group to which we belong (chordates). Cambrian animals lived at a time when life on the planet was basically confined to aquatic environments: the continents were essentially barren, and no plants or animals would successfully inhabit them until many tens of millions of years later. One of the most important novelties of this animal radiation was the appearance of predation, which, in turn, drove the development of skeletons. shells and other hard structures for protection in a Cambrian "arms race: predators developed "weaponry" such as grasping claws for attack, and their prey had to evolve defensive features such as spines and other forms of armour, as well as the ability to burrow or swim quickly to avoid being eaten. While most fossils are the body parts of animals that do not decay easily, such as shells, others contain the remains of soft tissues, such as eyes, guts and muscles, which are rarely preserved. This is largely due to their rapid burial in a low oxygen environment. The fossils found in the world-renowned Emu Bay Shale of Kangaroo Island (South Australia) represent the only window in the Southern Hemisphere into these extraordinary Cambrian creatures.