The concept of Fossil-Lagerstätten has evolved a lot since it was introduced by Dolf Seilacher in the 1970s and the discussion is still ongoing. I would suggest to have a look at Kimmig and Schiffbauer (2024) for a recent summary and definition.
Best,
Dieter
Kimmig, J. & Schiffbauer, J.D. (2024). A modern definition of Fossil-Lagerstätten. Trends in Ecology & Evolution 39(7):621-624; DOI: 10.1016/j.tree.2024.04.004