There is little consensus on the number of families. Some treatments accept as little as five,[2][3] while other authors list over a dozen. Here, the more inclusive view of the Terebellida is followed, based on a major review of polychaete systematics.[1]Cladistic studies have hitherto only analyzed a rather small proportion of polychaetes; hence it may be that some of the families today included in Terebellida by most authors will eventually be again recognized to belong elsewhere.
A notable terebellid is the Pompeii worm (Alvinella pompejana), an alvinellid. It is the most heat-tolerant complex organism known on Earth. Found near hydrothermal vents deep in the Pacific Ocean, it thrives at a temperature of 50 °C (122 °F). This is near the theoretical limit for eukaryotes, whose mitochondria disintegrate at about 55 °C (131 °F).
Polychaetes lack a firm skeleton, so they do not fossilize well. For this reason, only a small number of prehistoric bristle worms have been described. Several of these have been placed in the Terebellida suborder, though often only tentatively so, owing to the ongoing uncertainties about polychaete phylogeny. However, as the terebellid habit of constructing characteristic tubes in which to live seems quite ancient, such structures are occasionally found in conservation Lagerstätten. Polychaete remains associated with such ichnofossils can therefore be quite certainly be allocated to the Terebellida.[citation needed] Prehistoric polychaete genera historically presumed to be Terebellida include the following, though most are now recognized as trace fossils of uncertain makers:
^Fauchald K (1977). "The polychaete worms, definitions and keys to the orders, families and genera". Natural History Museum of Los Angeles County Science Series. 28: 1–188.
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