RAPID MORPHOLOGICAL CHANGE IN THE LATE TRAISSIC BIVALVE MONOTIS FROM AOTEAROA NEW ZEALAND: ENVIRONMENTAL OR BIOLOGICAL DRIVERS?

During the Late Triassic the biostratigraphically important bivalve genus, Monotis, exhibits extreme morphological variation in endemic subgenera from Aotearoa New Zealand. The morphological diversity of Monotis is critical to understanding Late Triassic paleoecology at high paleo-latitude settings with low-diversity benthic systems such as those from Aotearoa New Zealand. It is therefore necessary to understand relative influences of physical environment and biological interactions (e.g., predation) on the morphology of endemic Monotis. In this study we test for differences in sedimentary environments containing both endemic and cosmopolitan subgenera of Monotis and use geometric morphometrics to define morphological differences.

To quantify Monotis morphology and determine depositional environment, three bulk macrofossil samples were collected from upper Norian deposits in the Southland syncline. Traditional and landmark-based geometric morphometrics were applied to bulk sample specimens and museum collection type specimens of endemic Inflatomonotis and two cosmopolitan subgenera (Eomonotis and Entomonotis). To quantify inflation and shell whorl, a diagnostic but difficult to measure feature, a new technique digitizing semi landmarks along the demarcation line (central rib) was developed. Sediments were analyzed using point-counts from petrographic thin sections.

Results of both morphometric approaches indicate Inflatomonotis is significantly more inflated than Eomonotis or Entomonotis. Landmark-based analysis was also able to distinguish shape modifications to shell whorl that contribute to increased inflation. From petrographic analysis, sedimentary environment was not found to vary among the sites with and without Inflatomonotis, indicating that there is not a strong environmental gradient driving this morphological change. In the absence of clear environmental drivers, biological pressures, such as predation, that play a significant role in low paleo-latitude setting in the Late Triassic must be considered.