Annelida is an extremely diverse group of animals
commonly called segmented worms.
Comprising some 15,000 extant species, annelids are ubiquitous on
earth and are found in marine, fresh water and terrestrial habitats,
and have an enormous range of size from less than a mm to several
meters. Some species use sexual reproduction, and others use asexual
three predominant annelid classes,
Class Oligochaeta (earthworms) and Class Hirudinea (leeches) are
the most commonly known, but Class Polychaete that is represented
in essentially all marine environments contains the greatest diversity.
Because the creatures have soft bodies, fossilization is exceedingly
rare. Equivocal forms such as the Dickinsonia
are known from the Precambrian. The best-preserved
and oldest specimens come from Cambrian Lagerstätte such
as the Burgess Shale of Canada, and the Middle Cambrian strata
of the House
Range in Utah. The Annelids are also well represented
among the Pennsylvanian-age Mazon
Creek fauna of Illinois. The hard jaws of some polychaete
worms, such as Fossundecima
konecniorum from Mazon Creek, are sometimes common enough
utility for stratigraphic correlation.
Echiura (the distinct Phylum Echiura is recognized by some scientists)
were initially regarded as an annelid group, were then excluded,
but newer evidence suggests they are in fact annelids (Hessling
and Westheide, 2002), albeit this placement remains unresolved.
The Echiura fossilize poorly and the earliest known specimen is
ellongimus from Mazon Creek dating to the Pennsylvanian.
worms, diverse, mainly marine worms with parapodia that have
worms, also considered a separate phylum