Hexactins in the ‘protomonaxonid’ sponge Choiaella and proposal of Ascospongiae (class nov.) asa formal replacement for the Protomonaxonida


Authors: Botting JP

Published in: Bulletin of Geosciences, volume 96, issue 3; pages: 265 - 277; Received 22 November 2020; Accepted in revised form 19 May 2021; Online 6 June 2021

Keywords: Porifera, Cambrian, evolution, protomonaxonid, Choiaella, sponge,

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The protomonaxonid sponges are a controversial early group with supposed ties to the demosponges, but the groupalso possesses features that imply a much earlier-branching position in sponge evolution. A new species, Choiaellahexactinophora sp. nov., shows a typical protomonaxonid body plan but also contains small hexactin-based spicules,a skeletal element today restricted to the class Hexactinellida, but which has been proposed to be plesiomorphicfor Porifera and lost in the other living classes. This finding from the Castle Bank fauna (Middle Ordovician of theBuilth Inlier, Wales, UK) confirms continuity of the protomonaxonid lineage, as hexactins are also known fromfossils interpreted to be among the earliest members of the group. The presence of hexactins effectively rules outa demosponge affinity, and supports previous interpretations requiring the protomonaxonids to be treated as an earlybranchingsponge clade rather than assigning them to an extant class. To facilitate this, this paper proposes a formalreplacement of the majority of the group as the Ascospongiae nov., an extinct class within Porifera.


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