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Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Ediacaran Period immediately preceding the Cambrian. Some of the odd fossils of the biota from the Ediacaran may also have been animals representative of living phyla, although this remains a somewhat controversial topic. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years. Trace fossils made by animals also show increased diversity in Cambrian rocks, showing that the animals of the Cambrian were developing new ecological niches and strategies — such as active hunting, burrowing deeply into sediment, and making complex branching burrows. Finally, the Cambrian saw the appearance and/or diversification of mineralized algae of various types, such as the coralline red algae and the dasyclad green algae.
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