Spatio-temporal trends in morphological disparity in molluscs and its interrelationship with different ecological and environmental factors

Sharma, Neha (2024) Spatio-temporal trends in morphological disparity in molluscs and its interrelationship with different ecological and environmental factors. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Neha Sharma (20RS098)) 20RS098.pdf - Submitted Version Restricted to Repository staff only Download (12MB)

Abstract

The Phanerozoic history of marine organisms is characterized by a secular increase in taxonomic and ecological diversities, coupled with subsequent morphological diversification (i.e., disparity). For marine invertebrates, a steep rise in disparity, associated with major morphological changes, happened in the early Paleozoic, culminating in the appearance of higher taxa (e.g., phylum and class); followed by subsequent minor morphological modifications at the later part of their evolutionary history (i.e., during the Mesozoic-Cenozoic), culminating in the origination and diversification of lower taxa (i.e., genera and species). These large-scale changes in disparity were mostly associated with the evolving ecological (i.e., feeding, tiering and motility) and environmental preferences of taxa through space and time under the influence of various macroecological and macroevolutionary phenomena like adaptive radiation, mass extinctions, to name a few. Therefore, analyzing spatio-temporal trends in morphological disparity can hint at underlying macroecological and macroevolutionary phenomena responsible for short- and long-term changes in disparity. Through this dissertation, using a geometric morphometrics-based approach, I investigate the patterns of evolution of morphological disparity in marine molluscs (gastropods and bivalves) across varying spatial (i.e., global/regional) and temporal (e.g., Phanerozoic/Recent) scales, in three chapters: (1) evaluating the temporal patterns in disparity of a gastropod family for ~200 Myr to understand how shell morphology of this family evolved despite undergoing any change in its ecologies (i.e., remained ecologically-conservative); (2) understanding how different biotic (ecologic preferences like tiering and motility) and abiotic parameters (substratum choice) influence community-level spatial patterns of disparity in ecologically diverse families of Recent marine bivalves; and, (3) understanding how these short-term spatial and temporal disparity dynamics scale up to the Phanerozoic when all marine bivalve genera are considered together, so that the major macroevolutionary and macroecological patterns of morphological evolution in Class Bivalvia could be understood. The results show that morphological disparity of marine molluscs can remain conservative over prolonged time periods, if the ecological and/or environmental (i.e., substratum) preferences of taxa remains largely unchanged, and, this conservatism holds regardless of taxonomic hierarchy (i.e., family/genus), and temporal-spatial (global/regional) scale. However, disparity did change for marine bivalves, associated with taxonomic and ecological radiations during the early Paleozoic and early Mesozoic, and the peak disparity occurred in the late Paleozoic, driven by the diversification of majorly infaunal families. From the Mesozoic-onwards, disparity is driven by pronounced diversification of infaunal families and genera, so much so that, by the Cenozoic, bivalve morphology becomes redundant, primarily because the newly-originating genera only occupy the already-existing morphospace, often appearing very close to one another (indicating morphological similarity). Major mass extinctions selectively remove some aberrant forms, shrinking morphospace occupancy, but newly appearing taxa eventually occupied many novel and existing parts of the morphospace. Therefore, although ecological and environmental preferences determine the spatio-temporal dynamics in disparity over ecological time scale, over macroevolutionary scales, processes like originations and extinction of higher and lower taxa appears to be more important factors in determining the long-term trends in the morphospace occupancy of a clade.

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