Palaeoscolecid
Encyclopedia
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.
.
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.
, Maotianshania, Cricocosmia, Tabelliscolex, Tylotites and others.
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
to the late Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
; 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.
Morphology
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.Taxonomic position
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 nematomorphaNematomorpha
Nematomorpha is a phylum of parasitic animals that are superficially morphologically similar to nematode worms, hence the name. They range in size in most species from long and can reach in extreme cases up to 2 metres, and in diameter...
.
They have also been described as a sister-group to the ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
, 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
Archaeopriapulida
The archaeopriapulida are a group of priapulid-like worms known from Cambrian lagerstatte. The group is ancestral but very similar to the modern Priapulids. It is unclear whether it is mono- or polyphyletic...
is also unclear; it could be that both groups are a paraphyletic assemblage containing the priapulids.
Taxonomy
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 LouisellaLouisella
Louisella is a genus of priapulid worm known from the Middle Cambrian Burgess Shale. It was originally described by Charles Walcott in 1911 as a holothurian enchinoderm.polychaete, and according to one website has also been interpreted as a polychaete...
, Maotianshania, Cricocosmia, Tabelliscolex, Tylotites and others.