Endolithic borings attributable to sponges, hitherto unknown before Devonian time, are common in trilobite fragments in Scandinavian Lower to Middle Ordovician limestones. Most borings consist of branching, anastomosing tubes, approximately 25 μm in diameter, and closed; end tubes, 50 to 150 μm long and 20 to 40 μm in diameter. The tube network may close and thereby form major chambers. The small borers were probably most successful in breaking down carbonate particles where rates of biogenic carbonate sedimentation were low.