EARLY MOLLUSCAN EVOLUTION: EVIDENCE FROM THE TRACE FOSSIL RECORD

Trace fossils extend the history of early molluscan evolution because they have a higher preservation potential in siliciclastic strata, where body fossils are rare or absent, and also because they better record the activities of soft-bodied animals. The earliest mollusks are recorded by fan-shaped scratch arrays associated with death masks of the Ediacaran animal Kimberella. Cambrian traces-Climactichnites, Musculopodus, Radulichnus-and fecal pellets record giant, but shell-less mollusks that intermittently left the water to graze the biofilms of intertidal sand flats. After the Cambrian, these traces become progressively restricted. Their subsequent disappearance results from closure of a taphonomic window, rather than from extinction of these animals or their grazing behavior. Other Paleozoic groups of shell-less mollusks may have responded to increasing predation pressures by becoming infaunal. Unlike surface trails, their backfilled burrows-Psammichnites, "Aulichnites-Olivellites," Dictyodora-are commonly preserved. These burrows reflect the evolution of sophisticated search programs in shallow-marine as well as deep-sea environments, but none of them continues into the Mesozoic.