Fossil Record
The oldest crown-group polychaete fossils come from the Sirius Passet Lagerstatte, a rich, sedimentary deposit in Greenland tentatively dated to the late Atdabanian (early Cambrian). Many of the more famous Burgess Shale organisms, such as Canadia and Wiwaxia, may also have polychaete affinities. An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.
Being soft bodied, the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts, and the mineralized tubes that some of them secrete. Most important biomineralising polychaetes are serpulids, sabellids and cirratulids. Polychaetes cuticle does have some preservation potential; it tends to survive for at least 30 days after a polychaete's death. Although biomineralisation is usually necessary to preserve soft tissue after this time, the presence of polychaete muscle in the non-mineralised Burgess shale shows that this need not always be the case. Their preservation potential is similar to that of jellyfish.
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