Ancient seabed checkerboard: How setae shaped spatial distributions of Silurian brachiopods.

Abstract

Understanding the drivers of spatial patterns in fossil communities is fundamental to paleoecology, yet direct evidence for biological mechanisms regulating interindividual spacing remains elusive. Brachiopod setae, hypothesized to function in feeding or defense, are exceedingly rare in the fossil record, especially among post-Cambrian taxa. Here, we present the report of exquisitely preserved setae from an exceptional in situ fossil assemblage of the early Silurian rhynchonelliform brachiopod <i>Nucleospira calypta</i>. Multiproxy analyses (scanning electron microscopy, X-ray fluorescence, and microcomputed tomography) revealed intricate ultrastructural details and diverse taphonomic pathways, leading to a reinterpretation of apparent calcitic preservation as primarily iron oxides with subsequent coating. Critically, the undisturbed nature of this aggregation allowed rigorous spatial point pattern analysis (Nearest-Neighbor Analysis, Thiessen polygons). This revealed a statistically significant, nonrandom, checkerboard-like distribution among individuals within the studied fossil deposit, indicative of active spacing regulation. Strikingly, the measured average interindividual spacing quantitatively relates to the length of the preserved setae. This provides the direct paleontological evidence demonstrating that these subtle morphological structures could have actively mediated spatial organization within a dense benthic community. Our findings illustrate a biological mechanism capable of shaping community structure, operating beyond passive environmental constraints or initial larval settlement preferences, and highlight the potential for subtle anatomical features to exert significant ecological influence in deep time.