Article: Biomechanics of trilobite exoskeletons

Most skeletal materials, such as bone and insect cuticle, are viscoelastic, but heavily mineralized structures such as mollusc shell, are linearly elastic. The type of microstructure used in making a skeleton is related to required mechanical strength and to the metabolic cost involved in construction. The effects of composition, microstructure, and architecture on mechanical properties are discussed, and then related to trilobite exoskeletons. Due to their composition and internal organization, trilobite cuticles can be regarded as ceramics that behaved in a linearly elastic manner. The small size of the calcite crystals and the presence of an organic framework reduced the risk of crack formation and slowed the progress of fractures. As a result of its crystal arrangement, the thin outer prismatic layer would have had good compressive strength, but only poor crack-stopping abilities, whereas the underlying principal layer added bulk to the cuticle and deflected cracks. Structurally, trilobite exoskeletons are analogous to monocoque shells, that is, they are strong 'thin shells' with the same composition throughout and behave as a ' stressed skin'. The overall architecture of the cephalon and pygidium is of a series of modified domes, strengthened by the presence of the doublure, whereas thoracic segments are compromise structures which allow articulation as well as conferring mechanical strength.