TRIASSIC ALATOFORM BIVALVES OF TETHYS: WALLOWACONCHID MEGALODONTOIDS IN THE OMAN RANGE OF THE UNITED ARAB EMIRATES

Discovery of large numbers of wallowaconchid bivalves (alatoform megalodontoids) in Norian strata of the northern Oman Mountains in the United Arab Emirates has shown that these recliner bivalves are a normal component of the late Triassic biota in central Tethys. Until this discovery, only one individual had been found outside North America and the family was considered to be endemic to the Americas. Wallowaconchids in the Oman Mountains of the UAE commonly occur in monospecific assemblages, mostly in life position, containing millions of individuals within a single bed. The greatest development of these large bivalves occurs in Wadi Naqab, located 20 km southeast of Ras al Khaimah in the northeastern part of the UAE. Densely crowded monospecific assemblages extend for hundreds of metres within several beds, similar to occurrences of rudist bivalves. The UAE wallowaconchids occur on mud substrates deposited in shallow water environments and some individuals lived crowded among coral heads that grew only to 15-20 cm high, indicating a very shallow subtidal setting. The UAE occurrence extends the range of wallowaconchids from eastern Panthalassa to central Tethys, but they have not been found in western Tethys. Although typical globose megalodontids occur througout the late Triassic tropics, other types of megalodontoids (megalodontids, dicercocardiids) are not found with wallowaconchids in the UAE. Dicerocardiid megalodontoids, known only in western and central Tethys, appear to have an endemism in western Tethys comparable to wallowaconchid endemism in eastern and central Tethys. Wallowaconchid occurrence corresponds closely to times of maximum Triassic reef development, suggesting that the Norian was the warmest interval of the Triassic. These reefal biotas present evidence of warmer climate during a time of lower atmospheric CO2 levels, providing evidence of a short interval of warmer climate superimposed on the general climate of the early Mesozoic.