Extinct freshwater rhizodont fishes, known from Scottish and other rocks of Carboniferous age (c. 300 Myr BP), fed on large animal prey. Their extraordinary development of multiple lateral lines has never been explained. Functional analysis suggests that because the fish were so big (up to 7 m long), their body length was comparable to the range over which the amplitude of nearfield sound decays significantly with distance from the source. Several possible mechanisms, one very simple, could allow the fish to fix the location of prey in three dimensions, the vertical dimension being provided by the vertical proliferation of the lateral lines, which also maximised the overall signal to noise ratio for initial detection of prey at longer ranges. Comments are sought on this analysis! This interpretation of rhizodont fishes as ambush predators, possibly analogous to crocodilians, freshwater electric fishes, and the coelacanth Latimeria, is consistent with rhizodont locomotor adaptations, notably the lobe-fins which allowed slow locomotion and manoeuvring while keeping the body rigid to simplify the dataprocessing problem, and with paleoenvlronmental data.