PHYLOGENY OF EARLY PALEOZOIC ECHINODERMS AND THE ORIGIN OF BLASTOIDS

Over the past century many attempts have been made to characterize the origin of blastoids. Most of these interpretations have attempted to derive blastoids from a known fossil taxon, e.g.: the edrioblastoid Astrocystites, the parablastoid Blastoidocrinus, the “eocrinoid” Bockia, and the glyptocystitoid rhombiferan Cystoblastus. Although many of these taxa bear a generalized morphological resemblance to blastoids, each possesses a number of unique derived characters, which argue against a position ancestral to blastoids on a phylogenetic tree. Here we examined the origins of blastoids using cladistic methodology and have made our interpretations based on sister taxon relationships.

To provide a more objective interpretation of the morphological transformations leading to blastoids and the recognition of derived character states, morphology of blastoids and closely related species allowing were compared. Thirty morphological characters (21 binary and nine multistate characters) were defined and scored for the edrioblastoid Astrocystites ottawaensis; the parablastoids Blastoidocrinus carchariaedens and Meristoschisma hudsoni; the “eocrinoids” Bockia neglecta and Lysocystites sculptus; the glyptocystitoid rhombiferans Glyptocystites elhersi and Cystoblastus leuchtenbergi; the hemicosmitid rhombiferans Hemicosmites extraneus and Caryocrinites ornatus; the coronoids Cupulocorona salopiae, Mespilocystites bohemicus, and Stephanocrinus angulatus; and all described Ordovician and Silurian blastoids: Macurdablastus uniplicatus, Decaschisma pulchellum, Polydeltoideus enodatus, and Troosticrinus reinwardti. Cladistic analysis results in two most parsimonious trees, 60 steps long differing only in the position of Astrocystites. Both trees show blastoids as a monophyletic group most closely related to Lysocystites. Coronoids form the sister group to the clades including Lysocystites and blastoids. The possession of hydrospires, recumbent ambulacra, and envaginated radial plates surrounding the lancet are blastoid synapomorphies and thus morphological developments, which occurred during the origin of the clade. Other features argued to be diagnostic for blastoids including narrow holomeric stem, presence of deltoid plates, bud-shaped theca, and blastoid thecal plate formula are plesiomorphic.