ORDOVICIAN CAVE-DWELLERS? AN EXAMINATION OF POSSIBLE CRYPTIC FAUNA FROM MEIKLEJOHN PEAK MUDMOUND, SOUTHWESTERN NEVADA

Cryptic fauna, or organisms which live in cavities, are a well-known component of modern coral reefs. Recent work on ancient bioherm ecology has revealed that many organisms which were once interpreted as framebuilders (e.g., archeocyaths, sphinctozoan sponges, etc.; Wood, 1995) might have instead been cryptobionts. This has important implications for the study of mudmounds, as many of the organisms which have been interpreted as bafflers and binders of carbonate mud may have alternatively been living inside the mudmound. Complex systems of cavities are common in Paleozoic mudmounds, and it seems likely that those cavities should have contained cryptic fauna, provided these cavities were open to the surface of the mound. The mid-Ordovician (Llanvirnian) Antelope Valley Limestone of southwestern Nevada contains a large (300 m wide by 80 m high) carbonate mudmound at Meiklejohn Peak. The core of the mudmound is composed primarily of wackestone and mudstone (40-60%), localized lenses of packstone and grainstone, and a variety of features which suggest that a complex system of cavities existed while the mudmound was being formed, including stromatactis and the enigmatic zebra limestone found primarily near the base of the mudmound. These cavities are believed to have been open to the surface of the mudmound based on the presence of pelletoids and bioclasts. Initial results from the mudmound suggest that the red pigments which line the bottom of many stromatactis cavities may have been microbial in origin, as are pelletoids found throughout the cavity system. Current research is focusing on the origin of skeletal material within the cavity system to determine its origin: i.e., whether it was washed into the cavities, or if those organisms were exploiting this niche within the mudmound. If results from this study reveal the presence of a significant cryptic fauna within the mudmound framework, then the role of those organisms which have been interpreted as trappers and binders of lime mud in Paleozoic mudmounds should be reconsidered.