IMPLICATIONS OF THE OCCURRENCE OF PAIRED ANTERIOR CHAETAE IN THE LATE EARLY CAMBRIAN MOLLUSC PELAGIELLA FROM THE KINZERS FORMATION OF PENNSYLVANIA FOR RELATIONSHIPS AMONG TAXA AND EARLY EVOLUTION OF THE MOLLUSCA

The mollusc Pelagiella is one of the best known and most widely distributed of ”small shelly” taxa of the dramatic Early Cambrian diversification of metazoans. From specimens of various species, it is known to be characterized by a rapidly expanding dextral, logarithmic spiral shell, with varied patterns of spiral, granular and ribbed external sculpture; microstructural fabrics comparable to the simpler patterns of among those of molluscs; muscle attachment scars; and even in one case what appears to be a preserved digestive gland. Different interpretations of the presumed muscle scars have given rise to reconstructions of Pelagiella as a partially-torted basal conchiferan or as a vetigastropod. The situation has been further complicated by the discovery of an assemblage of shells exhibiting varying states of preservation, from flattened ’ghosts’ to partially developed steinkerns, with impressions of two tight clusters of long chaetae extending from their apertures. This discovery is consistent with the affiliation of molluscs, annelids and brachiopods as each other’s closest relatives among the Trochozoa, recognized on the basis of molecular data. The inference that Pelagiella has retained chaetae as a plesiomorphic character from a common mollusc-annelid-brachiopod-ancestor is less easily reconciled with the fact that gastropods appear to have emerged as a derived taxon within Conchifera, in molecular phylogenies of molluscs and related taxa. However, it is striking that branch-lengths separating the molluscan classes in these molecular phylogenies are extremely short relative to those linking higher taxa within the classes. These observations are consistent with the hypothesis that the molluscan classes diverged extremely rapidly by elaboration and specialization of different anterior-posterior components of a serially constructed ancestor, involving independent evolution of skeletal elements (shells) employing a common shell-developmental process, and the loss in various lineages of prior shared, derived characters including the radula, chaetae, and even the head.