Abstract

The Permian Reef Geology Trail in Guadalupe Mountains National Park of West Texas provides excellent exposures of a section through the middle Capitan reef and associated facies of Guadalupian age. Samples were collected from the Capitan Massive at intervals of about 5 m along the Permian Reef Geology Trail. The samples begin at the outer shelf/reef transition, the shallowest part of the reef, and continuing downward. These samples show a change from grain-rich to cement-rich sediments about 10-15 m below the outer shelf/reef transition. They also show a dramatic drop in sponge diversity at a point approximately 15-20 m below this transition. These changes represent inferred water depths of at least 20-25 m and 25-30 m, respectively. From the outer shelf/reef transition an elevation about 15 m below, samples exhibit a high sponge diversity, with at least eight genera being common. Samples collected from the reef between 15 and 50 m below the outer shelf/reef transition are overwhelmingly dominated by Lemonea, a sphinctozoan sponge that is also very common within the upper Capitan.

In modern open marine environments, the diversity of heterotrophic sponges is greatest at about 20 m; diversity in shallower waters is limited because of turbulence. Our data implies that ecological factors besides turbulence influenced biotic distribution within the Capitan reef. The most likely explanation is the presence of a slightly warmer, more saline, and less oxygenated water mass below the pycnocline.

It is significant that Lemonea inhabited the deeper parts of the reef and also survived into the upper Capitan. We suggest that the abundance of Lemonea in both situations indicates that it thrived in stressed conditions that other sponges could not tolerate, and may be the reason this genus dominates many Late Permian reef faunas.