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

Abstract

The generally accepted view that early mollusks were millimeter-scale animals is partly based on paleontological data. Millimeter-scale, exquisitely preserved mollusks are important constituents of many small shelly fossil assemblages and have been the focus of most modern studies of Cambrian mollusks. Centimeter-sized mollusks occur in the fossil record as early as the earliest Cambrian but have been neglected for decades in favor of their better-preserved, millimeter-scale counterparts. Here we present a large, limpet-like mollusk from the Lower Cambrian of Spain that preserves an apical shell indistinguishable from the millimeter-scale helcionellids that have come to epitomize the ancestral “conchiferan.” The Spanish fossils provide direct evidence that at least some millimeter-scale helcionellids represent juvenile or larval shells of large, limpet-like mollusks, suggesting that the presumed generalized small size of Cambrian mollusks may be a taphonomic artifact.