Traces of an association of microscopic endoliths are described within the shell wall and internal cavity of phosphatic brachiopods with conjoined valves from the Holm Dal Formation (late Middle Cambrian) of Peary Land, central North Greenland. Ridged galleries which also penetrate early diagenetic, phosphatised, fibrous crusts on the shell interior are borings excavated by a multifilamentous euendolith. Meshworks of strands crossing the shell interiors are phosphatised encrustations seemingly originally deposited as radiating, acicular, aragonite crystals on cryptoendolithic filaments prior to their degradation. The Greenland fossils closely resemble a present day association of calcified green algae from recent Bermuda reefs, but were probably produced by Cyanobacteria.