Palaeoscolecidomorphs are vermiform fossils of debated affinity within Ecdysozoa known from the Cambrian to Silurian. Their body plan includes a spine-bearing introvert, an eversible, toothed pharynx, paired posterior hooks and a slender, annulated trunk usually bearing biomineralized sclerites. While at least superficially scalidophoran-like, they have been assigned to various positions within Ecdysozoa, including to the stem-groups of Panarthropoda and Nematoida. Here, we present a new palaeoscolecidomorph from the Sirius Passet Lagerstätte of North Greenland, Avannaascolex apalos gen. et sp. nov. Its morphology is typical of Palaeoscolecida, with a high length-to-width ratio, a small introvert with a small number of spines in each row and terminal paired hooks flanking the anus, but it is unusual in lacking any apparent trunk sclerites. We interpret this as a primary absence based on comparative preservation of other ecdysozoans from Sirius Passet. Introvert spines preserved in relief in Avannaascolex provide shape details (e.g., triangular collar spines) that potentially inform on palaeoscolecidomorph affinities. The holotype preserves an unpaired ventral nerve cord, lending support to the presence of an unpaired ventral nerve cord in the ecdysozoan last common ancestor.

Key words: Ecdysozoa, Cycloneuralia, Palaeoscolecidomorpha, Palaeoscolecida, Scalidophora, Cambrian Explosion, Sirius Passet, Greenland.

Thomas P. Farrell [thomas.farrell@bristol.ac.uk; ORCID: https://orcid.org/0000-0002-5005-3536] (corresponding author), Palaeobiology Research Group, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, BS8 1RJ, Bristol, UK; The Natural History Museum, Cromwell Road, London SW7 5BD, UK. Mirinae Lee [mirinae.lee@kopri.re.kr; ORCID: https://orcid.org/0000-0003-4415-2447], Division of Glacier and Earth Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon 21990, Republic of Korea. Morten Lunde Nielsen [mortenlunde@gmail.com; ORCID: https://orcid.org/0000-0002-4719-6317], Jakob Vinther [jakob.vinther@bristol.ac.uk; ORCID: https://orcid.org/0000-0002-3584-9616] (corresponding author), Palaeobiology Research Group, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, BS8 1RJ, Bristol, UK. Luke A. Parry [luke.parry@seh.ox.ac.uk; ORCID: https://orcid.org/0000-0002-3910-0346], Department of Earth Sciences, University of Oxford, S Parks Road, OX1 3AN, Oxford, UK. Arne T. Nielsen [arnet@ign.ku.dk; ORCID: https://orcid.org/0000-0001-8194-8546], Department of Geoscience and Natural Resource Management, University of Copenhagen, Rolighedsvej 23, 1958 Frederiksberg C, Copenhagen DK- 1350, Denmark. Tae-Yoon S. Park [typark@kopri.re.kr; ORCID: https://orcid.org/0000-0002-8985-930X], Division of Glacier and Earth Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon 21990, Republic of Korea; University of Science and Technology, 217 Gajeong-ro, Daejeon 34113, Republic of Korea. Gregory D. Edgecombe [g.edgecombe@nhm.ac.uk; ORCID: https://orcid.org/0000-0002-9591-8011], The Natural History Museum, Cromwell Road, London SW7 5BD, UK.