The oldest palaeoloricate mollusc (Cambrian Series 2, Stage 4; North Greenland) and its bearing on aculiferan evolution

 

Authors: Peel JS

Published in: Bulletin of Geosciences, volume 95, issue 2; pages: 127 - 144; Received 4 January 2020; Accepted in revised form 11 May 2020; Online 30 May 2020

Keywords: Mollusca, Aculifera, palaeoloricate, Cambrian Series 2, North Greenland, Laurentia,

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Abstract

In the Aculifera-Conchifera model of molluscan evolution, spiculate aplacophorans and polyplacophorans with a dorsal series of shell plates are recognised as sister groups within the clade Aculifera, itself a sister group of the clade Conchifera that contains all other molluscs. While aculiferans and conchiferans had their common origin near the Precambrian-Cambrian boundary, the crown groups of most major molluscan classes are seen traditionally to have emerged in the latest Cambrian (Furongian)-Early Ordovician. Matthevia Walcott, 1885, from the latest Furongian-Early Ordovician of Laurentia, has been regarded almost universally as the oldest undoubted fossil chiton, a palaeoloricate. Palaeoloricates, however, are paraphyletic and Matthevia is variously interpreted as a time indicator for the origin of crown group Aculifera, within either the stem or crown groups of Aplacophora or Polyplacophora. The discovery of rare disarticulated plates from the early Cambrian (Cambrian Series 2, Stage 4) of North Greenland (Laurentia) extends the geological record of palaeoloricates by about 25 Ma. In bridging the gap between lower stem group aculiferans and the occurrence of Matthevia, the new finds offer the potential for a corresponding earlier origin of both aplacophorans and polyplacophorans within the aculiferan crown group. Head and intermediate plates are described from the Aftenstjernesø Formation and placed within a new taxon proposed as Qaleruaqia sodermanorum gen. et sp. nov.