The term "reef" has been used loosely in the literature. Some authors define it as any submarine structure with relief, while others are more specific, describing reefs as organic, calcareous, ridge-like and wave resistant. Defining rudists as reef-builders depends on

According to Kauffman and Johnson (1988), rudists morphologies became more cylindrical and uncoiled through geologic time. The "primitive" coiled forms are interpreted as having lived in lagoonal environments, while the upright forms thrived in more high energy habitats.

Early rudists, which had coiled shells, did not produce reefs as we define them. Intermediate forms (i.e. recumbents and clingers with spirogyrate forms) were, at their densest, probably "reef-like". That is, assemblages of these rudists (called bouquets, clusters and thickets) could have provided superstratal relief and limited baffling. More advanced rudists, with close-packing potential, a stable, elevated cylindrical shape and a non-spiraled cap valve do meet reef-building criteria (Kauffman & Johnson, 1988). These true rudistid reefs were paucispecific, composed of dead and living rudists, created superstratal relief that baffled current flow, were cemented to the substrate, grew rapidly and possibly harbored photosynthetic algae. Evidence of epibionts on these reef-building rudists shows that binding algae, sponges and bryozoans also lived in these reefs (Kauffman & Sohl, 1974).

the criteria one is using and an understanding of the lifestyle of the rudists in question. In the figure to the right a reef framework has been formed of rudists. Though representative of different species, both rudistid forms are elevators and are closely packed.

In the figure below, rudists A through D are primtive non-reefbuilding forms; E-I are intermediate forms and J-N are reef/framework building rudist forms (Kauffman & Johnson, 1988). Notice that all of the reef/framework building forms are also the elevator morphotype.

It is important to note that not all paleontologists agree that elevator rudists lived epifaunally. Gili (1995) and Skelton et al. (1995) have interpreted gregarious, cylindrical rudist thickets and bouquets as infaunal. If these rudists lived mostly within the sediment, they could not be called reef-building.