Palaeoscolecid

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 hosts 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. == Growth ==

Palaeoscolecids can grow by the continuous addition of plates, or by the continued growth of individual plates. == Taxonomic position ==

Palaeoscolecids are somewhat challenging 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. 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. This said, their pharynx has the sixfold symmetry that likely characterised the ancestral ecdysozoan, rather than the fivefold symmetry of priapulans. A nematomorph affinity appears to be an artefact that results from under-sampling of the priapulid stem group. Martin R. Smith and Alavya Dhungana suggested in a 2022 publication that palaeoscolecids are a grade including sister taxa to Panarthropoda, highlighting similarities between the dorsal plates of taxa such as cricocosmiids and those of lobopodians such as Microdictyon. This proposal was made in response to a 2021 paper that found in a phylogenetic analysis that paleoscolecids were stem-group priapulids. == Taxonomy ==

As palaeoscolecids may represent a grade rather than a clade, drawing up a formal taxonomy proves problematic. • Scathascolex minor (Burgess Shale, mid-Cambrian) • Sahascolex (early Cambrian of Siberia) • Gamascolex (early Ordovician, Czech Rep.) • Plasmuscolex (early Ordovician, Czech Rep.) • ?Guanduscolex minor (early Cambrian, China) • ? Family Chalazoscolecidae: • ?Chalazoscolex (Sirius Passet; fine structure unclear) • ?Xystoscolex (Sirius Passet; fine structure unclear) • Wronascolex? johanssoni (mid-Cambrian, Scandinavia) • Maotianshania? sp. (late early Cambrian, Scandinavia) • Corallioscolex Muller & Hinz 1993 • Kaloskolex Muller & Hinz 1993 • Houscolex Zhang & Pratt (Order and Family uncertain) • Hunanoscolex Duan & Dong (= Ornatoscolex Duan & Dong Palaeoscolecids sensu lato Other long and narrow Palaeozoic worms that exhibit an invariant body width are commonly referred to the palaeoscolecids, even though they lack the cuticular structure that defines the group; this 'Palaeoscolecid sensu lato' group includes Louisella, Cricocosmia, Tabelliscolex, Tylotites and others. Linnean taxonomy ; Order Uncertain • Family Chalazoscolecidae Conway Morris & Peel 2010 • : Defined by the presence of smooth, folded and sclerite-bearing cuticular regions • Chalazoscolex pharkus • Xystoscolex boreogyrus ; Order Cricocosmida Han et al. 2007 : Defined by the presence of an unarmoured neck between the proboscis and the trunk, and a single pair of posterior hooks. :**Palaeoscolex :**:Defined by presence of Milaculum-type plates, i.e. rectangular with parallel rows of nodes :** Ashetscolex Muir et al 2014 :** Sanduscolex Muir et al 2014 As such, P. ratcliffei and P. huainanensis should not be included in Palaeoscolex. has simple sclerites with a single (small but prominent) node in the middle, so can be separated from Palaeoscolex(unless this simplicity is taphonomic). Its introvert has a six-fold symmetry, whereas its proboscis has quincuncially arranged teeth that resemble those of other Cambrian ecdysozoan worms. Utahscolex Originally described from the Spence Shale of Utah, Utahscolex has four transverse rings of plates per annulus, arranged as two 'bands' of double rows of plates separated by a central naked zone. Occasionally, single row bifurcates into two rows (for up to 6 rows per annulus). The plates are circular, and unornamented. Platelets and microplates are absent. == References ==