Collectively, invertebrates are animals that lack vertebrae. Unlike vertebrates, invertebrates do not share one common ancestor, but have instead evolved from many different animal lineages. The term “invertebrates” is therefore not a biological term, but instead one that simply and informally groups together those animals that are not vertebrates. Perhaps the most well-known invertebrates include arthropods (insects, arachnids and kin), annelids (true worms), molluscs (octopus, snails and kin), cnidarians (jellyfish, corals and kin), echinoderms (sea urchins, starfish and kin), poriferans (sponges), and a host of other worm-like animals. The oldest body-fossils known of invertebrates comes from sponge, or sponge-like, animals from the late Precambrian period in South Australia, approximately 665 million years ago. The oldest trace fossils of invertebrates however, come in the form of burrows that were presumably made by worm-like creatures approximately 1 billion years ago. From there, individual lineages of invertebrates would develop and evolve. Some would be closely related, while others very distantly, and still others being more closely related to vertebrates than to other invertebrates. Despite how different they may be from each other, the one unifying factor of being spineless has made them the most populous varieties of animals on Earth (around 97% of the animal kingdom) when compared to those that do possess a spine.