30 MILLION YEARS OF TAXONOMIC AND PALEOECOLOGIC DYNAMICS IN A HIGH-LATITUDE, LOW DIVERSITY SHALLOW MARINE SYSTEM: LATE TRIASSIC AOTEAROA (NEW ZEALAND)

Marine sediments in the Southland Syncline of New Zealand record a Late Triassic (Norian—Rhaetian, ~227-201 Ma) succession of fossil assemblages which exhibit gradual taxonomic shifts between primarily endemic taxa in relatively consistent depositional environments. Common taxa include bivalves and brachiopods, with rare echinoderms, gastropods, bryozoans, and scaphopods. Four successive faunal assemblages occurred during the early, middle, and late Norian Stage (generally comparable with New Zealand stages: Oretian, Otamitan, and Warepan).

Paper clams (Halobia) that dominate the oldest (Oretian) deposits gradually decline relative to other bivalve genera, Manticula and Hokonuia. Two species of Manticula and two species of Hokonuia are observed in the assemblages, but M. problematica and H. limaeformis are most prevalent in nearly all sampled Otamitan deposits, with more inflated species M. mirabilis and H. rotundata appearing in younger Otamitan deposits. Specimens of M. problematica and H. limaeformis also tend to be larger in these younger deposits. Brachiopods are present but rare in the Oretian and early Otamitan assemblages but are more diverse, abundant, and tending towards larger sizes in the later Otamitan. The Otamitan—Warepan faunal transition is very abrupt in this region, likely due to a substantial unconformity between the Otamitan and Warepan deposits, and Warepan assemblages are dominated by inflated, ribose specimens of the bivalve Monotis (in particular, subgenera Inflatomonotis and Entomonotis).

The observed faunal shifts reflect significant taxonomic and morphological changes in shallow marine environments of the region, without substantial behavioral differences (all dominant taxa are sessile epifauna) or environmental shifts. Potential forcing agents are explored and compared between regions in driving the observed changes, including latitudinal gradients of predation.