LATITUDINAL GRADIENTS OF BENTHIC PALEOECOLOGICAL CHANGE IN RESPONSE TO INCREASED DUROPHAGOUS PREDATION DURING THE LATE TRIASSIC

Diversity of benthic predator and prey taxa are thought to exhibit a negative relationship with latitude, such that predation pressures are greatest in tropical environments, lower in temperate regions, and lowest in polar areas. This hypothesis was evaluated for benthic assemblages from the Norian Stage (~224-204 Ma) of the Late Triassic, an interval in which durophagous predators are thought to have proliferated in shallow marine benthic environments. Shelly invertebrate assemblages were compared from level-bottom marine sedimentary successions in two low-latitude regions (northern Italy and west-central Nevada) and a high-latitude region (South Island, New Zealand). Between the three regions, over half the observed genera were present in two or more of the sedimentary successions but none shared ecological dominance at any point within the Norian Stage. Among shelly invertebrates in the low-latitude regions, diversity and paleoecological complexity increased during the Norian Stage, as infaunal, cementing, and mobile taxa became more abundant. Conversely, diversity decreased during the Norian Stage in the high-latitude region, although early Norian diversity was exceptionally high relative to the other localities. Furthermore, no increases in infaunal or cementing taxa were observed in the high-latitude region, but body size and shell thickness increased during Norian Stage. Like the low-latitude regions, however, ecological niche use was expanded in the high-latitude assemblages as the benthic taxa exploited a wider range of epifaunal niches. Local conditions were likely to have imposed constraints on both predators and prey that predictably varied along latitudinal gradients.