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The ichnogenus Teredolites and the Teredolites Ichnofacies is an accepted proxy for marine influence in paralic to open marine depositional environments. Actualistic approaches and the process-ichnologic framework allow independent analysis of both quantitative and semiquantitative data to interpret the physicochemical conditions when Teredolites are present. Measurements collected in modern environments and Jurassic–Eocene successions produce a spatially and temporally robust dataset of 14,137 borings from 17 tracemaking genera. Life history strategies of extant tracemakers are reflected in Mesozoic borings, which allow reconstruction of paleosalinity. Trends extrapolated indicate four ecotones in modern paralic to nearshore settings, which can be identified into the Late Cretaceous. These zones display variation in boring metrics (length [L], width [W], L:W ratio, size diversity index), densities, and relative composition of Teredolites. Zone 1 represents upper estuary to tidally modified fluvial successions with salinities from 0.5–10 ppt (oligohaline to alpha-mesohaline). Zone 2 represents upper to central estuary successions with salinities from 10–19 ppt (beta-mesohaline to lower polyhaline). Zone 3 represents central to lower estuary successions with salinities from 15–30 ppt (beta-mesohaline to euhaline). Zone 4 represents backshore to open marine successions with salinities from 20–30+ ppt (polyhaline to euhaline). Boring metrics compared against identified fossil genera suggest high species richness in the Western Interior Seaway coincided with physical adaptations to compensate for interspecific competition––differences in reproduction style and media preferences at the time of settlement––in the Late Cretaceous.
Tectonic changes have influenced the evolution of the marine community by changing the land and seaway configuration through time. Two such events during the Oligo-Miocene—the closure of the Tethyan seaway due to development of the Gomphotherium Landbridge leading to separation of the Arabian Sea from proto-Mediterranean Sea (∼ 19 Ma) and significant uplift of the Tibetan Plateau marking the initiation of the monsoon (∼ 16 Ma)—represent a classic case of tectonic shift influencing the regional environment of the Indian subcontinent. We investigated the taxonomic and body-size related response of the shallow marine fauna to this regional change using bivalves from 11 time-constrained shellbeds of the Kutch Basin (western India) from three formations—Maniyara Fort (Chattian), Khari Nadi (Aquitanian) and Chhasra (Burdigalian-Langian) representing a time span of ∼ 9 Ma (24.4–15 Ma).
Our collection of over 2000 individuals represents a total of 15 families and 61 morphospecies. The fossils are predominantly calcitic in nature and families of aragonitic composition are often preserved as molds indicating a potential negative effect of diagenesis. The taphonomic nature, however, does not vary substantially across shellbeds and hence, less likely produced a temporal pattern. The five most abundant species, Ostrea latimarginata, Ostrea angulata, Talochlamys articulata, Anomia primaeva, and Placuna lamellata occur in all the formations. The species composition of the Maniyara Fort Formation is substantially different from those of the younger formations, implying the possible effect of biogeographic separation. The absence of proto-Mediterranean taxa in Oligocene shellbeds supports limited faunal exchange between the Mediterranean-Iranian Province (MIP) and the western Indian Province (WIP) as early as ∼ 24.4 Ma (Chattian). Faunal exchange, however, continued between the WIP and the adjacent Eastern African-Arabian Province (EAAP). Formation-specific evenness shows a monotonic decrease from the Maniyara Fort to the Chhasra Formation. However, shellbed-specific analyses of diversity and body size do not show a strong directional trend through time and supports the stasis model. Although it is difficult to rule out the negative influence of taphonomy on the diversity of the studied fauna, the Oligo-Miocene marine bivalve fauna of the Kutch Basin demonstrates little or no influence of the Tethyan closure and Himalayan upliftment on the diversity through time.