The Cenomanian (Middle Cretaceous) rudist reef at Paso del Rio, Colima, is the only reef known to contain Immanitas, a recumbent, caprinid rudist. Within the reef, four units are discernible. At the base is a siliciclastic-poor wackestone with upright caprinids. This grades upward into a silty, caprinid-Immanitine-radiolitid wackestone/packstone with caprinid and radillitid zones at the top. Overlying this unit is an argillaceous, Immanitine packstone. The reef is capped by a silty packstone (debris bed) containing Immanitas.

This reef represents a single cycle of framework evolution with a constructive and a destructive phase. The constructive phase is represented by the caprinid wackestone and the silty, caprinid-Immanitine-radiolitid wackestone/packstone. The caprinid and radiolitid zones at the top of the latter unit comprise the climax communities. The destructive phase is initiated by the reestablishment of Immanitas in the argillaceous, Immanitine packstone. The termination of the reef is evidenced by the debris bed, a silty, Immanitas packstone.

This zonation is somewhat similar to the zonation of Cretaceous rudist frameworks in the Caribbean reported by others with the following exceptions: (1) upright rudists at Paso del Rio are predominantly caprinids, whereas those in the Caribbean are predominantly radiolitids; (2) recumbent rudists at Paso del Rio are the caprinid Immanitas, whereas those in the Caribbean are the caprinid Titanosarcolites and the radiolitid Biradiolites; and (3) the two separate types of framework evolution described in the Caribbean are combined at Palso del Rio.

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