A MORPHOLOGICAL AND SYNCHROTRON-BASED TOMOGRAPHIC REEVALUATION OF MACURDABLASTUS AS THE OLDEST MEMBER OF BLASTOIDEA (ECHINODERMATA)

The origins of Blastoidea have been widely debated over the past several decades. Macurdablastus uniplicatus Broadhead, 1984 was described from the Benbolt Formation (Late Ordovician) of eastern Tennessee but was never formally assigned to a blastoid order. We utilized visual examination and tomographic microscopy to redescribe its external morphology and the nature of the hydrospire respiratory folds.

Reevaluation of the type specimens provides evidence that M. uniplicatus is lacking several distinct blastoid characteristics. Macurdablastus uniplicatus possesses the conservative peristomal plating configuration similar to other derived blastozoans (e.g., rhombiferans, parablastoids, blastoids, coronoids, Lysocystites). The lancet plate of M. uniplicatus is restricted to the adoral one-third of the ambulacral rays, with a long radial platform extending the remaining length of the ambulacra whereas the lancet plate of blastoids extends for the complete length of the ambulacra. The restricted lancet plate is reminiscent of the coronoid ‘brachiolar facet plate’, which was later confirmed via Universal Elemental Homology to be a lancet plate.

The gross morphology of the respiratory slits of M. uniplicatus suggests fissiculate affinities but the hydrospire slits do not cross plate boundaries. In fissiculate species, the slits consistently cross the deltoid-radial suture in varying proportion. Unfortunately, the thecae are lacking ambulacral floor plating (side plates) that could aid in better understanding the evolutionary placement. Phylogenetic analyses were performed via maximum parsimony and maximum likelihood with representative species from previously ascribed groupings across the clade. Results from both analyses suggest that Macurdablastus uniplicatus is a transitional form between blastoids and an undetermined ancestor.