Analyses of predatory damage on subfossil hard parts can be used to document the trophic role played by species that were abundant prior to human impact even when pre-impact surveys are lacking or when predators have not left skeletal remains. Before upstream dams and water diversions, the bivalve Mulinia coloradoensis was the most common mollusk inhabiting the Colorado River Estuary. Today, only a small population has survived the environmental changes caused by reduction in the river's flow. When abundant, this species was a major source of food for predatory gastropods and crabs, as shown by the characteristic damage inflicted by these predators. Boreholes made by predatory gastropods were found in 23% of the 600 individuals sampled from shell accumulations that date from the era prior to upstream dams and diversions. Marginal shell damage characteristic of predatory portunid crabs was found in 27% of the individuals. Thirty-four percent of the individuals had damage from earlier attacks. Shells were tumbled to provide criteria to distinguish ante-mortem biological damage to the shell margin from post-mortem physical damage. Decline in the population of this prey species likely caused a decline in the populations of its predators and species higher in the food chain, prey-switching, or both effects. Restoration of river flow would increase populations of M. coloradoensis and species that depend on it for food, including commercially important crabs.