292-4 The role of environmental pressures on convergent morphologies of high-paleolatitude monotid bivalves from the Late Triassic

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part II

Presenting Author:

Authors:

Abstract:

Both environmental and biotic pressures contribute to morphological changes and, at times, convergent morphologies in bivalves. Monotid bivalves from the Late Triassic polar regions demonstrate apparently convergent morphological changes not seen in low paleo-latitude monotid taxa. In these high paleo-latitude systems, monotid bivalves make up the majority of biomass for preserved benthic invertebrates and biodiversity in bivalves; thus, making their morphological and potentially ecological changes significant. Convergent morphologies include a highly inequivalve and inflated form (inflated-type) and a form with greatly reduced frequency and amplitude of ribbing (smooth-type). These inflated-type and smooth-type morphologies are both found in the upper portions of Norian strata, and each taxon therein is endemic to their hemisphere. This work evaluates the preliminary evidence for environmental drivers of morphological change in monotid bivalve taxa from both northern and southern high paleo-latitude localities, Japan and New Zealand respectively. The depositional settings hosting monotid bivalves in Japan and New Zealand include shelf and shallow marine facies and are mineralogically similar. Sandstones from both localities contain abundant volcaniclastic grains and are characterized as lithic arkose to litharenite sandstones. In Japan, the appearance of both smooth-type and inflated-type morphologies in monotids were accompanied by a systematic increase in grain size associated with shallowing environments. Owing to the scarcity of outcrops showing stratigraphy perpendicular to the paleo-shoreline, it is unclear if more cosmopolitan morphologies remain in deeper settings. The association of the smooth-type monotid morphology and coarsening substrates differs from morphological patterns in other pectinid bivalves, such as scallops, where fewer but larger rib sculpture is often associated with shallow coarse-grained substrates. This may be an indication that biotic pressures, such as predation, may also be influencing shell morphology. In New Zealand, morphological changes are not clearly associated with any systematic changes in grain size or depositional environment, but do tend to appear in the upper-most Norian strata and often with other, more cosmopolitan varieties, of monotids. In both New Zealand and Japan, the inflated-type and smooth-type morphologies are rarely found together in the same deposit, indicating that these morphologies may be a response to distinct conditions. Given the similarities in monotid-bearing strata in New Zealand and Japan, it is likely that at least a component of their morphological convergence is due to environmental pressures.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6286

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.