Enigmatic microfossils from the early Silurian of Poland known as mazuelloids (= muellerisphaerids) have been identified as Baltisphaeridium-like acanthomorphic acritarchs that have been subject to post-mortem phosphatization. These acritarchs were the dominant components of large post-bloom macroaggregates that probably sank rapidly to the seafloor and were then overgrown by benthic coccoid cyanobacterial mats. Early diagenetic phosphatization of the remains probably occurred following shallow burial and in proximity to decomposing phosphate-rich macroaggregates. Depending on phosphate availability, the resulting phosphate layers were thin or thick and were deposited on the outside, inside, or enveloping the acritarch organic walls from both sides. The unusually large size of the mazuelloid-forming acritarchs was probably the result of periodically hypertrophic conditions driven by large blooms generated by upwelling currents.