Palaeoscolecida
| Palaeoscolecid Temporal range: Upper Early Cambrian–Silurian |
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| Scientific classification | |
| Kingdom: | Animalia |
| Clade: | Cycloneuralia |
| Class: |
†Palaeoscolecida Conway Morris & Robinson, 1986 |
| Families, genera and species (sensu Harvey et al. 2010) |
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The palaeoscolecids are a group of extinct 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 lagerstätten. 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 long and narrow, and can reach tens of centimetres in length. Their cuticle is annulated, typically in complete rings, but sometimes the rings split or only encircle part of the trunk. Each annulus is essentially identical to its neighbours; the only trunk differentiation is at the anterior and posterior. The anterior is radially symmetrical, typically comprising an introvert, whereas the trunk is bilaterally symmetrical. The posterior hsots the terminal anus and sometimes one or two hooks. 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.
Palaeoscolecids are a little tricky to define, and probably represent a paraphyletic grouping. Their most current systematic diagnosis references their annulated worm-like body form, the presence of rows (usually) of phosphatic plates, and a straight gut, with the anus at the end of the animal. The group contains a wide and continuous spectrum of morphological variety, making further division of the group difficult; moreover, non-palaeoscolecid taxa likely evolved from palaeoscolecid-like ancestors, and it is thus difficult to demarcate a single clade that corresponds to the palaeoscolecid concept.
They are considered to belong to 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. Their relationship with Archaeopriapulida is unclear, and either group may be paraphyletic to the other.
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