Within the Cretaceous period, at about 100 million years ago on the uniformitarian geochronological timescale, the earth experienced a strange event – that choked oxygen from the oceans and led to elevated marine extinction levels around the world. By studying planktonic microfossils and bulk sediment extracted from three sites around the world, direct evidence of ocean acidification was found. However, it also found evidence of massive volcanic eruptions at the same time, on the sea floor. Volcanic complexes and the Cretaceous seem to go together. However, the sediments are all thought to have been laid down in a successional manner.
The malformed sea shells are due to stress, it is said. The study authors hypthesize that ocean acidification was to blame [assuming the shells came into being after the aciidification of the environment]. Which might be true. Interesting read. Article was published in Nature Geoscience [January 2023] – see https://doi.org/10.1038/s43247-022-00641-0 …