Palaeoscolecida (Palaeoscolecids)
The palaeoscolecids are a group of ecdysozoan worms resembling armoured priapulids. They are known from the Lower Cambrian to the late Silurian; they are mainly found as disarticulated sclerites, but are also preserved in many of the Cambrian lagerstatten. They take their name from the typifying genus Palaeoscolex.
Palaeoscolecids bear an annulated trunk ornamented with circular patterns of phosphatic tesselating plates; a layered cuticle; and an armoured proboscis. They are usually a few centimetres in length. There is no one character that unites the palaeoscolecids as a clade (indeed they are likely paraphyletic), and few individual specimens contain all characteristic palaeosolecid traits.
They are considered to be a clade within the cycloneuralia, although their position within this group is unresolved; they may lie with the priapulids or nematomorpha. They have also been described as a sister-group to the ecdysozoa, although as more characters are described a position closer to the priapulids becomes most probable. A nematomorph affinity appears to be an artefact that results from under-sampling of the priapulid stem group.
Other genera include Cricocosmia from the Lower Cambrian Chengjiang biota. Their relationship with the archaeopriapulida is also unclear; it could be that both groups are a paraphyletic assemblage containing the priapulids.
In addition to the genera listed in the taxobox, other worms are commonly referred to the palaeoscolecids, even though they lack the cuticular structure that defines the group. These include Louisella, Maotianshania, Cricocosmia, Tabelliscolex, Tylotites and others.
Zdroj: http://en.wikipedia.org/wiki/Palaeoscolecid
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Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]