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Bryozoan fossils found at last in deposits from the Cambrian period
Molecular evidence has long indicated that aquatic animals called bryozoans should be found among the fossils of the Cambrian period, around 541 million years ago. Yet they have been conspicuously absent, until now.
Multicellular animals called metazoans underwent a profound diversification of their forms during what is known as the Cambrian radiation, which began about 541 million years ago. This resulted in the formation of most of the main animal groupings (phyla) known today in a geologically relatively short time of less than 15 million years. Some phyla seem to have missed this biodiversification event, as judged from their absence in the fossil record for the Cambrian period (which ran from 541 million to 485 million years ago). However, genetic evidence, based on a ‘molecular clock’ to estimate when they emerged, suggests that they were present in this Cambrian ‘kitchen’. The most prominent animal phylum missing from the Cambrian record until now has been the Bryozoa, a group of aquatic animals that exist as individual organisms connected by tissue to form colonies. These animals are abundantly present later in the fossil record. Writing in Nature, Zhang et al.1 present a collection of fossils from early Cambrian deposits of China and Australia that are unequivocally bryozoans, and thus present evidence that solves one of the mysteries regarding the early diversification of animals.