Size of the earliest mollusks: Did small helcionellids grow to become large adults?

The generally accepted view that early mollusks were millimeter-scaleanimals is partly based on paleontological data. Millimeter-scale,exquisitely preserved mollusks are important constituents ofmany small shelly fossil assemblages and have been the focusof most modern studies of Cambrian mollusks. Centimeter-sizedmollusks occur in the fossil record as early as the earliestCambrian but have been neglected for decades in favor of theirbetter-preserved, millimeter-scale counterparts. Here we presenta large, limpet-like mollusk from the Lower Cambrian of Spainthat preserves an apical shell indistinguishable from the millimeter-scalehelcionellids that have come to epitomize the ancestral "conchiferan."The Spanish fossils provide direct evidence that at least somemillimeter-scale helcionellids represent juvenile or larvalshells of large, limpet-like mollusks, suggesting that the presumedgeneralized small size of Cambrian mollusks may be a taphonomicartifact.