Abstract

A paleoenvironmental study of the dasyclad Mizzia Schubert, 1907 is potentially important because, unlike most of the other fossil reef-dwelling organisms found in the Guadalupe Mountains, the fossil Mizzia (restricted to the Permian) has a modern analog, Cymopolia Lamouroux, 1916 (Cretaceous to Holocene). The overall morphology of the two genera is similar: both are articulated, and observations on our specimens suggest that both were branched. The transition from the remarkably high-diversity sponge and algal assemblage in the reef into the remarkably low-diversity dasyclad-dominated assemblage in the back reef is abrupt, occurring over distances as short as 10 m. Mizzia is the dominant and usually the only component in grainstones, packstones, and wackestones found immediately shelfward of the Capitan reef. The low percentage of broken segments (average 10%) and the discovery of several intact sections of thallus are evidence against wave action as a possible limiting factor. Because the climate in the Guadalupe Mountains was extremely arid and because dasyclads are euryhaline, hypersalinity is considered the most likely limiting factor in this case. Our conclusion that prolific stands of dasyclads formed in shallow protected lagoons immediately behind an emergent barrier reef is not compatible with the currently accepted marginal mound hypothesis for deposition of the Capitan reef complex. We suggest that the marginal mound model must either be modified or abandoned in favor of the originally proposed barrier reef model of deposition.

Key index words: Chlorophyta; Cymopolia; Dasycladaceae; Guadalupe Mountains, Guadalupian; Mizza; paleobotany; paleoecology; Permian