PROLIFERATION OF AVOIDANCE STRATEGIES IN THE LATE NORIAN (LATE TRIASSIC): NEW OBSERVATIONS OF A MAJOR PALEOECOLOGICAL TRANSITION IN LEVEL-BOTTOM BENTHOS FROM NEW ZEALAND

Shelly invertebrate assemblages from several shallow marine level-bottom deposits in Italy and Nevada exhibit a mid-Norian paleoecological transition from dominant stationary epifaunal benthic taxa to assemblages characterized by more infaunal and mobile taxa. Here, the results of systematic bulk sampling efforts from shallow marine deposits in the Southern Hemisphere are presented. Bulk samples from a sedimentary sequence of coarse siltstones representing deposition in a well-oxygenated shallow marine level-bottom environment were collected from the Taringatura and Hokonui Hills of South Island, New Zealand.

Early and Middle Norian marine assemblages from the studied sections in New Zealand were dominated by stationary epifaunal bivalves (e.g.: Halobia), in association with brachiopods and rare gastropods. Late Norian fossil assemblages were dominated by two unique Monotis subgenera: semi-infaunal Maorimonotis, and Inflatomonotis, a particularly inequivalve and robustly plicate taxon whose members lived attached to algae above the sediment-water interface. These subgenera in the Late Norian samples are notably different from other Monotis taxa which are typically thin-shelled and not inflated, and likely utilized a narrower range of epifaunal life habits.

The Early and Middle Norian faunal assemblages from New Zealand are paleoecologically similar to contemporaneous level-bottom deposits from the Northern Hemisphere, but the Late Norian assemblages are distinct from both the older Norian assemblages from the same depositional sequence and from contemporaneous Late Norian deposits in the Northern Hemisphere by the preponderance of taxa utilizing an unusual supra-seafloor attachment strategy and the distinct lack of burrowing bivalves. However, all the examined successions exhibited a mid-Norian decline in unattached stationary seafloor-dwellers, particularly for bivalves with thin shells. These transitions may be related to the Late Triassic taxonomic radiation of specialized demersal and benthic durophagous predators, and the mid-Norian proliferation of infaunal or algal-attached life habits may reflect increased interaction with shell-crushing predators roaming the seafloors at this time and selection for those taxa able to successfully avoid or escape the seafloor.