PALEOECOLOGICAL AND MORPHOLOGICAL TRENDS OF THE BIVALVE MONOTIS THOUGH THE WAREPAN (LATE NORIAN, LATE TRIASSIC) FROM SOUTH ISLAND, NEW ZEALAND

The Late Triassic bivalve genus Monotis is notable in New Zealand for its tendency to form paucispecific shell beds characterized by multiple cooccurring subgenera exhibiting a range of morphologies. Monotis includes a least five subgenera and eighteen species in New Zealand with significant morphological variation in size and inflation indicating the potential for different paleoecological strategies within the genus. Here we report on the paleoecology of Monotis-dominated deposits from the southern South Island of New Zealand and analyze morphological trends among Monotis subgenera during the Late Norian.

Four shallow marine bulk macrofossil samples from the southern South Island (Southland and Otago) display diverse assemblages of Monotis. Subgenera present in multiple samples exhibit morphological changes through the Late Norian including reduction in shell area and increased inflation. Sedimentological analysis of the strata containing samples indicates similar substrates for all of the Monotis bulk samples, suggesting that morphological changes in the Late Norian are not dependent on substrate.

The morphological trends of Monotis fit within a broader paleoecological turnover in the Late Triassic. Paleoecological patterns in New Zealand differ from those in other parts of the world during the Norian Stage, but the substantial change in Monotis size and inflation in shallow marine environments suggests an influence of increased predation pressure.