PROGRESSION OF THE EARLY MESOZOIC MARINE REVOLUTION IN TWO OCEANS: CASE STUDIES FROM TETHYS AND PANTHALASSA

The timing and nature of the early Mesozoic marine revolution (MMR) is not well understood. To determine the paleoecological characteristics and geographic origins of the event, two successions of bulk samples were collected from shallow marine carbonates of Late Triassic (Norian) age from Tethyan and Panthalassic sequences in the Lombardian Alps (Northern Italy) and west-central Nevada, respectively. Shelly prey taxa comprise the majority of the observed specimens, and demonstrate the gradual influence of the MMR through changes in dominance and paleoecological structure through this stage. The earliest phase of the MMR exhibited increasing pressure by demersal or benthic durophagous predators in Tethys during the mid-Norian (Late Triassic). At this time, shelly prey expanded the infaunal niche and/or became increasingly mobile. Panthalassic benthic communities also transitioned from predominantly stationary epifaunal assemblages to those dominated by mobile infauna, but the transition occurred later in the stage and to a lesser degree than Tethyan assemblages. Both regions also exhibited increased ecological importance of mobile epifauna and stationary recliners later in the Norian stage at the expense of stationary epifaunal taxa. Panthalassic assemblages tended to be more ecologically diverse than those of Tethys, but similar paleoecological trends to those observed in Tethyan sections were underway despite the different taxonomic assemblages. The spatial and temporal differences in MMR progression for the two collections may reflect regional radiations of predator taxa. These trends are largely coincident with known Late Triassic radiations of benthic or demersal taxa adapted for durophagy (e.g.: molariform teeth and crushing appendages) and prey taxa with a variety of morphological characteristics specialized for avoiding or deterring this strategy of predation (e.g.: cementing, boring, or deeply burrowing bivalves). Thus, the earliest stages of the MMR reflected the amplification of these particular types of interactions.