Setae-Mediated Population Dynamics in Early Silurian Brachiopods

Understanding the drivers of spatial patterns in ancient benthic communities is fundamental to paleoecology. While spatial arrangements often reflect key ecological interactions such as resource competition or cooperative behaviors, direct evidence of the biological mechanisms regulating interindividual spacing is uncommon in the fossil record. Soft structures like brachiopod setae, which are thought to function in sensing, feeding, or defense, are rarely found in post-Cambrian deposits due to taphonomic biases. Here, we report an early Silurian (~436 Ma) brachiopod assemblage from the Lower Red Beds of the Hanchiatien Formation in Guizhou Province, South China. This in situ fossil community is dominated by the rhynchonelliform brachiopod Nucleospira calypta, which features well-preserved marginal setae. Utilizing a multiproxy approach encompassing SEM, μ-XRF, and micro-CT, we investigated the ultrastructure and taphonomic pathways of these setae. Our analyses reveal that the setae, reaching up to 12 mm in length, were preserved primarily through early pyritization in a low-energy, quiet-water environment, followed by oxidation and calcareous coating. The undisturbed nature of this fossil aggregation provided an opportunity to conduct spatial point pattern analyses to evaluate population-level distributions. Nearest-Neighbor Analysis (NNA) and Thiessen polygon visualizations demonstrated a nonrandom, regular distribution among individuals. The NNA yielded an R-value of 1.2115, indicating spatial regularity, while Thiessen polygons exhibited consistency in shape and area. The measured average interindividual spacing between visceral cavities is approximately 11 mm, corresponding to 1.5 to 2 times the length of the preserved setae. We propose that these setae mediated spatial organization within this benthic community. Lacking a pedicle foramen, N. calypta likely adopted a free-lying lifestyle. As the brachiopods grew, the outward extension of their elongating setae, combined with the incremental sliding of their smooth, discoidal valves, may have allowed individuals to adjust their relative spacing. This setae-driven spacing mechanism would enable the population to achieve a more energetically optimal configuration, maintaining adequate distances to facilitate efficient feeding currents and minimize interference from neighbors. This study provides paleontological evidence demonstrating how morphological structures can influence spatial organization in deep time. It highlights a biological mechanism capable of shaping community structure, operating alongside passive environmental constraints and initial larval settlement preferences. Ultimately, this regular spatial distribution illustrates the ecological role of fine anatomical features in early Paleozoic marine ecosystems, offering insight into the behavioral ecology of extinct sessile communities.