Well, the question is not easy to answer. On the one hand, we need a data carrier that sediments reliably over several million years and thus transmits the isotope signatures of the water from the past. On the other hand, many comparative studies are needed to understand factors such as kinetic fractionation during the uptake of isotopes into the skeleton. Unfortunately, after some research I only stumbled across formainifers which fufill this criteria. In my opinion, it is also good that other animals/plants/planktons are not used so that science can concentrate on one area according to the motto simple and short.
Update:
Ok I now have a little more understanding of geostratigraphy using formainifers.
My findings that may be important:
Foraminifera have existed for about 560 million years
About 2% of all animals or animal-like organisms belong to foraminifera
In the North Atlantic, for example, foraminifera are responsible for 0.71 Gt yr-1 of calcite deposition
In short, the high biomass or the high mass of surviving biominerals as well as a long existence time are the reason why foraminifera are essential for biostratigraphy. How @Hexenbrei has also indicated, there is a solid data base on foraminifera.
