Coupling and Decoupling of Morphological Disparity and Taxonomic Diversity through Fusulinid Evolution during the Late Paleozoic Ice Age

During the Late Paleozoic Ice Age, prolonged glaciation and subsequent climatic fluctuations exerted profound influences on marine ecosystems. Fusulinids, the first evolved group of larger benthic foraminifera, underwent a major radiation during this interval and reached peak species diversity in the late Carboniferous–early Permian. Although taxonomic diversity patterns of fusulinids have been well documented, their morphological disparity remains poorly understood. Here, we compile a species-level morphological character matrix based on 16 test characters for 840 fusulinid species spanning the Carboniferous to the Permian. Using this dataset, we reconstruct morphospace and quantify disparity by capturing the range, density, and relative position of morphospace using a 0.1 Myr time bin. We further perform quantitative analyses of disparity–diversity correlations and their lead–lag relationships through time. Our analyses reveal that fusulinid morphological disparity from the Carboniferous to the Permian is characterized by distinct stage-dependent restructuring of morphospace. During the Mississippian, disparity and morphospace occupation remained relatively limited. A major transition occurred in the Pennsylvanian, marked by changes in multiple characters and the rapid expansion of morphospace. In the Cisuralian and Guadalupian, morphospace continued to expand while its internal structure was reorganized. A further transition in the Lopingian is characterized by contraction of morphospace. Further analyses indicate that disparity and diversity were broadly coupled during the Carboniferous diversification phase, but became decoupled through much of the Permian, before converging again during the Guadalupian crisis. These patterns are closely linked to shifts in clade dominance: early morphospace expansion was driven by diversification of major clades, whereas later increases in disparity primarily resulted from clade replacement. We propose a three-stage macroevolutionary model for fusulinids: (1) a Carboniferous–early Permian phase of coupled expansion of diversity and morphospace; (2) a middle Permian phase characterized by clade turnover and disparity–diversity decoupling; and (3) a late Permian phase marked by environmental stress and synchronous decline of disparity and diversity.