Mechanical function of a complex three-dimensional suture joining the bony elements in the shell of the red-eared slider turtle

Certain design strategies appear repeatedly in a variety of biological structures. One such motif consists of a soft and pliable interface joining much larger and stiffer elements. Examples include the craniofacial sutures between the bones of the skull, the sutures between the bony plates in shell of turtles and the periodontal ligament between teeth and their sockets. Yet the detailed mechanics of these systems are not fully understood. Turtles are believed to have existed already in the early Triassic, about 200 million years ago. They are thus one of the oldest non-extinct vertebrates. Their shell is therefore a particularly attractive subject for investigation since it has developed and conserved through such an extremely long evolutionary process and has achieved a highly optimized structure. The turtle shell has a 'sandwich' structure typical of flat bones like the skull of vertebrates. It consists of two external, relatively thin sheets of dense bone (internal endocortical and external exocortical bone plates) which contain very few voids, and between them a thick and very porous spongy bone layer. At the mid-distance between adjacent ribs the dermal bones are separated by soft sutures which have a unique and complex 3-D shape. The primary function of the shell is to protect the turtle from external trauma, and therefore it has to be stiff. However excessive stiffness may result in microdamage accumulation as a result of everyday activities like minor impact, and decrease the efficiency of respiration and locomotion. We speculate that the structure and architecture of the sutures allow easy deformation of the shell at small loads but cause it to become considerably more rigid at larger loads, reminiscent of composite materials with interlocking elements. We hypothesize that this mechanical property is related to the putative function of the suture in the turtle shell. In order to examine this hypothesis we studied samples obtained from shells of the red eared slider turtle (Chrysemys scripta elegans). We used several imaging techniques (micro-computed tomography, scanning electron microscopy and light microscopy), histology and mechanical testing. Based on these observations we present a concept of the structure-mechanics relationship of the shell, and present a simple mathematical model of the deformation pattern of the suture-containing samples in 3-point bending tests and compare its predictions to our experimental results.