RUDIST BIVALVES AND REEF CORALS IN THE LOWER CRETACEOUS OF SOUTHWESTERN MEXICO: TIERED COEXISTENCE, COMPETITION FOR THE SAME ECOLOGIC NICHE, AND FACIES SUPERPOSITION

Rudist bivalves are common in the Lower Cretaceous of the Huetamo area, southwestern Mexico, and reef corals often occur in these same strata. Outcrops of the lower Aptian Cumburindio Formation near Turitzio, Michoacán, and the upper Albian Mal Paso Formation near Ciudad Altamirano, Guerrero, exhibit rudists and corals preserved in living position. Three kinds of relationships were recognized, two of which are based on direct faunal interactions and the third the result of superposition of laterally adjacent biofacies: 1) tiered coexistence; 2) competition for niche space; and 3) facies superposition.

Exposures of the Cumburindio Formation in the Arroyo Los Hornos, just north of Turitzio, reveal limestone beds predominantly composed of caprinid rudist species and relatively few, large (>1m), ramose coral colonies. The orientations of the coral colonies indicate that they grew in a vertically upright position and, for the most part, existed in the ecologic tier just above the rudist-dominated, upper surface of the substrate. Thus, as used here, tiered coexistence is the shared occupation of essentially the same habitat, but at different levels and with a minimum of niche space overlap. Competition for niche space occurred in varying degrees within both formations, but particularly in the back-reef and coral reef zones of the Mal Paso Formation where species of attached, erect, solitary radiolitid and caprinid rudists existed among a diverse assemblage of reef corals and stromatoporoids. Faunal changes induced by facies superposition appeared to have occurred in both formations. The most notable example was found near the top of the Cumburindio Formation section at Loma de San Juan, south of Turitzio, where a tabular bed of Amphitriscoelus species is directly overlain by a well-developed coral reef horizon.

These examples demonstrate that the faunal associations were dependant mainly on the interactions and precise ecologic requirements of the involved species. Furthermore, the cases where observed vertical changes in faunal composition resulted from superposition of mutually exclusive, laterally adjacent facies, including from corals to rudists as well as from rudists to corals, indicate that the rudist-dominated facies were deposited in an environment different from that of the Early Cretaceous coral reefs.