HIGH DENSITY POPULATION OF THE OYSTER PYCNODONTE NEWBERRYI (STANTON) IN THE TROPIC SHALE (CENOMANIAN-TURONIAN) OF THE KAIPAROWITS BASIN, SOUTHERN UTAH

The marine Tropic Shale of southern Utah spans the Cenomanian-Turonian boundary as a transgressive–regressive sequence of the Greenhorn Cyclothem. Recent exploration of the Tropic Shale in the Kaiparowits Basin in Glen Canyon National Recreation Area and Grand Staircase-Escalante National Monument has documented at least four species of plesiosaurs, numerous fish and sharks, and a variety of invertebrates including inoceramids and cephalopods. Invertebrates are rare throughout much of the Tropic Shale, but they are locally common within some of the bentonite beds. In particular, the Late Cretaceous oyster Pycnodonte newberryi (Stanton) occurs in high density, monospecific concentrations within some of the bentonite beds, in marked contrast to the otherwise scarcity of this species throughout the shale. Resting upon Bentonite A, the lowest bentonite in this region and typically about 10 meters below the Cenomanian-Turonian boundary, are local accumulations of Pycnodonte newberryi in high abundance. The fossil oysters occur only in the upper surface of the bentonite bed. From a one square meter area upon Bentonite A in one area, we counted all specimens with an intact umbo, and established a minimum number of 142 individuals per square meter. The outcrop area of the bentonite that contains the sample site is estimated at approximately 18,600 square meters. By extrapolation from our sample, we calculate 2.6 x 106 individuals in this bed. We established three working hypotheses regarding this high density population: (1) the oyster concentration is the result of time-averaged accumulation; (2) the ash beds provided nutrient enrichment in the otherwise nutrient poor sediments of the Tropic Shale; and (3) the ash provided a more suitable substrate than other marine sediments in this habitat. Additional field investigations will test these hypotheses in other areas.