Neoichnological activities of endobenthic invertebrates in downdrift coastal Ganges delta complex, India: Their significance in trace fossil interpretations and paleoshoreline reconstructions

Abstract

Neoichnological studies of the downdrift coastal Ganges deltaic region (Ganges Delta Complex) indicate the nature and environmental zonation of the lebensspuren of common endobenthic invertebrates dominated by the brachyuran amphibious crab Family Ocypodidae (Ocypode spp., Ilyaplax pusillus and Uca marionis) and are utilized to interpret analogous trace fossils and paleoshoreline environments. The polycha‐ete Diopatra cuprea, the gastropods Turritella spp., Tele‐scopium telescopium and Cerithidea obtusum, some bivalves and the boring crab Charybdis rostrata also produce diagnostic lebensspuren. The measured ichnoprofiles reveal the development of coast parallel Uca ‐Turritella (backswamps and salt‐marshes; 1 type burrows), Ocypode ‐ Ilyoplax (backshore to foreshore), Charybdis rostrata (foreshore relict woodground; boring structures), polychaete (middle‐lower foreshore; current ‐ oriented agglutinated burrows) and bivalve ‐ gastropod (lower foreshore; trails) ichnozones. Ocypode ‐ Ilyoplax, irrespective of ontogenic stages and sex, produce I, J, U, Y, and multibranched Y ‐ shaped burrows (juvenile ‐ old in backshore versus young ‐ adult in foreshore) in an orderly fashion. Their burrow density and diversity attain a maximum in backshore and upper foreshore respectively. Exceptionally, high burrow populations produce network burrow systems in the backshore. Juvenile pelletal designs (upper foreshore) and general landward burrow inclination are conspicuous. The described lebensspuren, having a wide range of ancient analogues, provide supportive evidence in the identification of lithified crab burrows, paleoshoreline environments and paleosealevel fluctuations.