Fossil ascidian spicules and their paleoenvironmental significance

Despite their relevance for paleoenvironmental reconstructions, ascidian spicules remain a poorly documented component of the marine micropalaeontological record. These calcareous structures, produced mainly by didemnid ascidians, have been reported in marine sediments since the Late Triassic period and are found throughout much of the Mesozoic and Cenozoic eras. However, their stratigraphic distribution is not fully understood due to the scarcity of specialised studies and their limited inclusion in routine micropalaeontological analyses. This review summarises the main occurrences of fossil ascidian spicules throughout geological time and considers their relevance to paleoenvironments. Available records suggest that the ascidian spicules can be used to indicate shallow-marine settings, salinity fluctuations, and marine incursions into continental and transitional systems. In some cases, intervals characterised by abundant spicules are associated with reduced planktonic microfossil abundance suggesting relative shallowing events and ecological restructuring within marine assemblages. While their biostratigraphic utility is underdeveloped, integrating them with sedimentological, geochemical, and micropalaeontological datasets greatly enhances their applicability in multiproxy studies for paleoenvironmental analysis. The available evidence highlights the importance of paying greater attention to this group in future micropalaeontological investigations, and reinforces the value of ascidian spicules as an underutilised paleoenvironmental proxy.