Authors: (Michael J. Vendrasco, Department of Biological Science, California State University, Fullerton, California, USA)
Abstract: By the early Cambrian (540 mya), the initial adaptive radiation of
molluscs was well underway, part of the ‘Cambrian Explosion’.Molluscan
diversification continued through the Great Ordovician Biodiversification
Event (GOBE; to 405 mya). During the earliest Palaeozoic, the arms race
between molluscs and their predators began and accelerated, culminating in
the wide range of mollusc forms we see today. Recent discoveries about early
molluscs help answer some questions about the relationships and early
diversity within this phylum, but much is still uncertain about the functional
morphology, phylogeny and true range of morphological diversity in the early
Mollusca. The class affinities of many Cambrian molluscs remain uncertain,
and numerous competing taxonomic schemes and evolutionary scenarios have
been presented. Shell microstructure and other fine-scale morphological data
allow testing of some phylogenetic hypotheses, indicating for example that
separate lineages of laterally compressed forms gave rise to rostroconchs and
bivalves. The shells of early molluscs are similar to those of brachiopods,
indicating homologous organic armour as a shell precursor in these two taxa.
However the precise nature of the ancestral proto-mollusc remains elusive.
