Morphologic Variation in Upper Pleistocene Demosponge Biostromes, Miami, Florida: Paleoecologic Implications

Modern shallow-marine sponge reefs are not common and are substantially smaller than upper Pleistocene biostromes of the Miami Limestone (Marine Isotope Stage 5e) recently discovered in Miami, Florida. Thin section analyses of sponge walls produced morphologic clues for interpretation of biostrome paleoecology. These large, monospecific biostromes, up to 3.5 kilometers in length, developed between ooid shoals in high-energy tidal channels, an unusual setting for biostromes composed of large upright vase- to barrel-shaped demosponges (Miamiamplia vasiforma).

This study focused on the largest known biostrome along C-100C Canal. Here a 60-meter traverse, along a prolific part of the biostrome and parallel to a paleochannel, revealed an average of 9 individuals per 10 meters of outcrop. Mean sponge height and diameter were 59 and 33 centimeters respectively; maximum height and diameter were both 80 centimeters. A less common laminar form was found draping bedding paleotopography. Laminar forms likely resulted from tumbling over of sponges and splitting of their walls, with continued growth.

Thin section morphologic analyses of M. vasiforma specimens suggest two hypotheses for upright sponge growth: either sponges episodically grew up from the sediment substrate after burial events that reduced the exposed sponge wall protruding (~20 centimeters) above the sediment/water interface; and/or sponges were fully exposed throughout life as tall, vase- or barrel-shaped individuals. Observed morphologic features support both growth models: complex, vertically stacked, cup-shaped, internal sedimentary cycles filling the gastral cavity suggest episodic growth; and concentrations of spicules, quartz grains, and structured spongin material at the base of specimens suggest a well-anchored, fully exposed lifestyle. Both growth forms were viable in this high-energy environment. Sponges that grew episodically were short, extending just above the sea floor, aiding their stability. Sponges that were fully exposed as tall, upright forms used concentrations of spicules and quartz grains toward their bases, anchoring within substrate.