Integration and Modularity in the Turtle Body Plan: Impacts on Disparity and Species Richness.

Evolution is shaped by development, natural selection, and physiological limitations that bias the range of variation observed in organisms, influencing patterns of diversification. This study investigates how patterns of morphological integration and modularity impact disparity and species richness across freshwater and terrestrial turtles. Modularity refers to the idea that biological systems, like organisms, organs, or traits, are organized into relatively independent, semi-autonomous units, called modules. Integration refers to how strongly different traits are interconnected or correlated with each other. We first hypothesize that the most diverse turtle suborder, Cryptodira, exhibit weaker integration and higher modularity than Pleurodira, leading to greater morphological disparity and species diversity. Second, we hypothesize that at the family level weaker integration and higher modularity promotes morphological disparity and species richness. To test these hypotheses we take linear measurements of limb, shell, and head characteristics of 1652 turtle specimens belonging to 270 species (70% of species level diversity). Covariation matrices were used to test hypotheses in a phylogenetic framework. Results partially support our hypotheses: Cryptodira show lower integration and higher modularity but unexpectedly lower disparity than Pleurodira. At the family level, higher modularity and weaker integration correlate with higher species richness, while integration is positively correlated with increased disparity. The most diverse families that have evolved terrestrial and aquatic lifestyles, Emydidae and Geoemydidae, exhibit high modularity, weak integration, low disparity, and higher species richness, whereas Kinosternidae and Trionychidae which have strictly aquatic species, exhibit moderate levels of modularity, high integratation, and high disparity. These findings highlight how patterns of trait covariation can shape organismal diversity, and the depth of our sampling provides key insight on how patterns of covariation can influence the diversification in a major order of vertebrates.