At the request of the OP, a little word on another group of organic-walled microfossil with a extensive, coherent Paleozoic fossil record: the Chitinozoans.

This enigmatic group was found by Eisenack (1931) and because of their resistance to acids & bases he thought they consisted of chitin, hence the name he gave them. However studies (Voss-Foucart & Jeuniaux 1972; Jacob et al. 2007) showed that they do not consist of chitin or any kind of chitin-like bioproduct but of a network of kerogen with aromatic compounds. The diagenetic path that lead to this chemical composition is however unknown, meaning that we know neither what was their chemical composition when those organisms (NB: we don't know as well what their biological affinities are, though metazoan eggs seem to be a popular hypothesis, see e.g. Paris & Nõlvak 1999) were alive nor how they preserved.

_{Additional sources:
Eisenack, A., 1931. Neue Mikrofossilien des baltischen Silurs I. Palaeontologische Zeitschrift, 13: 74–118.
Jacob, J., et al., 2007. New insights into the chemical composition of chitinozoans. Organic Geochemistry, 36(10): 1782–1788.
Paris, F. & Nõlvak, J., 1999. Biological interpretation and paleobiodiversity of a cryptic fossil group: The “chitinozoan animal”. Geobios, 32(2): 315-324.
Voss-Foucart, M.F. & Jeuniaux, C., 1972. Lack of chitin in a sample of Ordovician chitinozoa. Journal of Paleontology, 46: 769–770.}