The mechanical function of a complex three-dimensional suture joining the bony elements in the shell of the red-eared slider turtle was studied. The study presented a concept of the structure-mechanics relationship that explains how the shell withstands minor loads by low-stiffness deformation and becomes much stiffer only when the load increases beyond a certain threshold. The red-eared samples were obtained from the Israel Nature and Parks Authority, and were oriented either anterior-posterior, or lateral. The imaging techniques used in the study were microcomputed tomography, scanning electron microscopy, and light microscopy, histology and mechanical testing to examine samples of the dorsal shell of the red-eared slider turtle. The results showed that the complex shape of the suture between adjacent bone segments provides easy deformation at small loads and a transition to a much stiffer shell due to the locking of the neighboring bone segments.