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A wrinkle formation mechanism with cutaneous aging is addressed through a mechanical calculation of linear buckling. Skin is divided into five mechanically distinct layers in this study. In general, the outer layer is stiffer than the inner layer, so buckling occurs in the outer layer against the uniform compression caused by muscle contraction. This buckling damages the skin and affects the formation of permanent wrinkles. We propose a multistage buckling theory for evaluation of the wrinkle property, namely, the specific wrinkle size and critical strain in three stages. The specific wrinkle size is derived as the wavelength of the minimum-buckling mode for infinite-length skin. A sensitivity analysis is carried out to investigate the effect of age-related changes of the mechanical parameters on the wrinkle property. We employ some aging hypotheses and prepare two sets of mechanical parameters, one for young and one for aged skin. The numerical results show that the buckling mode switch from Stage I to Stage II is the main reason why pronounced wrinkles suddenly appear in aged skin.

It has been suggested that spiriferide morphologies have evolved to adapt to a variety of environmental conditions. Through a computational fluid dynamics approach, we examined how the spiriferide original form was optimized for a lotic condition, specifically addressing the functionalization of the Devonian spiriferide brachiopod Paraspirifer bownockeri to generate passive feeding flows. The results using four models, each of which differed in the development of the spiriferide shell depression, i.e. sulcus, showed that a deeper sulcus functions to create strong spiral flows so as to align on the feeding organ inside the shell. Among the sulcus‐developed models, only the mimic of the natural form could generate comparative slow flows with a stable inflow area. The fossil record of spiriferides shows a morphological trade‐off between the development of the sulcus and wing form. We concluded that spiriferide shells with such a morphological combination evolved to produce various feeding strategies, resulting in diversification.

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