muscles, which move parts of the animal by contracting, and nerve tissue, which transmits and processes signals. In November 2019, researchers reported the discovery of Caveasphaera, a multicellular organism found in 609-million-year-old rocks, that is not easily defined as an animal or non-animal, which may be related to one of the earliest instances of animal evolution. Fossil studies of Caveasphaera have suggested that animal-like embryonic development arose much earlier than the oldest clearly defined animal fossils. and may be consistent with studies suggesting that animal evolution may have begun about 750 million years ago. Nonetheless, the earliest widely accepted animal fossils are the rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and Hydra), possibly from around , although fossils from the Doushantuo Formation can only be dated approximately. Their presence implies that the cnidarian and bilaterian lineages had already diverged. The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian, were the first animals more than a very few centimetres long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate kingdom, Vendozoa. Others, however, have been interpreted as early molluscs (Kimberella), echinoderms (Arkarua), and arthropods (Spriggina, Parvancorina). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal, in other words, an animal significantly more complex than the cnidarians. The small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," Halkieria and Microdictyon, were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals. In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Precambrian animal fossils. A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit