Microstructural biomechanics underpin Cambrian phosphatic brachiopod diversification

This study investigates the microstructure and biomechanics of the Wuliuan phosphatic brachiopods Iberotreta sampelayoi and Genetreta trilix from the Láncara Formation in northern Spain. Both taxa share the characteristic stacked columnar shell architecture of early linguliforms, yet quantitative analyses reveal marked differences in microcolumn geometry. Iberotreta possesses small, densely packed and highly regulated columns, whereas Genetreta develops larger, more heterogeneous columns with weaker geometric constraint. Statistical comparisons show that all column dimensions differ significantly between species, and allometric analyses indicate tightly coordinated growth in Iberotreta versus a less constrained pattern in Genetreta, consistent with greater sensitivity to local physiological or environmental conditions. Biomechanical modelling demonstrates that these disparities correspond to contrasting functional strategies: G. trilix withstands higher stresses, particularly in the lateral middle region, while I. sampelayoi maintains stiffness with reduced mineral investment, producing a lightweight and energetically efficient shell. Comparisons with extant brachiopods support a functional link between shell microstructure and biomechanical performance. Together, our results demonstrate that variation in column size, organization and allometry served as an adaptive axis in early phosphatic brachiopods, showing that micrometre-scale skeletal architecture played a central role in shaping the ecological complexity of Cambrian benthic communities during the early Cambrian.