Redescription of Macurdablastus and redefinition of Eublastoidea as a clade of Blastoidea (Echinodermata)

Eublastoids are a large clade of blastoids, stemmed blastozoan echinoderms diagnosed by their conservative body plan (three basals, four deltoid plates, and five radial plates), lancet plate supporting the ambulacra, and hydrospire respiratory structures. Although Eublastoidea is a highly successful clade in the middle and late Paleozoic it is absent from early echinoderm radiations during the Cambrian and Ordovician. Here we provide a reevaluation of Macurdablastus uniplicatus Broadhead from the Ordovician using detailed morphological assessment based on advanced synchrotron tomography and phylogenetic analysis. Macurdablastus uniplicatus falls outside Eublastoidea because of the morphological differences in lancet plate and respiratory structures. The oldest recorded eublastoid is thus middle Silurian in age. The re-evaluation of the morphology of Macurdablastus provides a basis for revising blastoid phylogeny and classification.