Small anatomical details can significantly influence how marine animals organize themselves, yet such interactions are rarely visible in fossils. We report exceptionally preserved soft, bristle-like structures (“setae”) on 436-My-old brachiopods, enabling analysis of ancient behavior. Using advanced imaging and statistical analysis, we identified a regular, checkerboard-like spacing pattern among these organisms, closely corresponding to their setae length. This demonstrates that these delicate structures played an important role in maintaining spacing between individuals, thereby influencing population organization on the ancient seabed. Our findings point to a previously unrecognized mechanism shaping ecosystem structure in deep time, demonstrating how subtle anatomical features could exert notable ecological impacts in their ancient environments.