The palaeoscolecids are a group of extinct ecdysozoan worms resembling armoured priapulids. They are known from the Lower Cambrian^[6] to the late Silurian;^[7] they are mainly found as disarticulated sclerites, but are also preserved in many of the Cambrian lagerstätten.^[8] They take their name from the typifying genus Palaeoscolex.^[7] Other genera include Cricocosmia from the Lower Cambrian Chengjiang biota.^[9] Their taxonomic affinities within Ecdysozoa have been the subject of debate.

Quick Facts Scientific classification, Families, genera and species (sensu Harvey et al. 2010) ...

Palaeoscolecida Temporal range: Upper Early Cambrian–Silurian[1] PreꞒ Ꞓ O S D C P T J K Pg N
The Burgess Shale palaeoscolecid Scathascolex minor. Image from Smith (2015)[2][3]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Clade:	Cycloneuralia
Class:	†Palaeoscolecida Conway Morris & Robinson, 1986
Families, genera and species (sensu Harvey et al. 2010[4])[5]
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Palaeoscolecids bear an annulated trunk ornamented with circular patterns of phosphatic tesselating plates; a layered cuticle; and an armoured proboscis.^[4] 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.^[4] The anterior is radially symmetrical, typically comprising an introvert, whereas the trunk is bilaterally symmetrical.^[4] The posterior hosts the terminal anus and sometimes one or two hooks.^[4] 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.^[4]

Palaeoscolecids can grow by the continuous addition of plates, or by the continued growth of individual plates.^[10]

Palaeoscolecids are somewhat challenging to define, and probably represent a paraphyletic grouping. Their most current systematic diagnosis^[11] 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.^[11]

They are considered by some to belong to the Cycloneuralia,^[12] although their position within this group is unresolved; they may lie with the priapulids or Nematomorpha.^[13] They have also been described as a sister-group to the Ecdysozoa,^[14] although as more characters are described a position closer to the priapulids becomes most probable.^[4] This said, their pharynx has the sixfold symmetry that likely characterised the ancestral ecdysozoan, rather than the fivefold symmetry of priapulans.^[15] A nematomorph affinity appears to be an artefact that results from under-sampling of the priapulid stem group.^[4] Their relationship with Archaeopriapulida is unclear, and either group may be paraphyletic to the other.^[5][1] Some authors choose to include paleoscolecids within Priapulida.^[16]

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.^[17] This proposal was made in response to a 2021 paper that found in a phylogenetic analysis that paleoscolecids were stem-group priapulids.^[18]

As palaeoscolecids may represent a grade rather than a clade, drawing up a formal taxonomy proves problematic.^[4] What is more, two parallel taxonomies exist: a form taxonomy for sclerites, and a true taxonomy for articulated fossils. The most recent holistic study of priapulids by Harvey et al. (2010) defines a core of palaeoscolecids characterized by a cuticle that is made up of interlocking plates of multiple sizes, and a looser assemblage (palaeoscolecids sensu lato) including other unconfirmed and palaeoscolecid-like forms: