Mechanics of sculpture formation in Magadiceramus? rangatira rangatira (Inoceramidae, Bivalvia) from the Upper Cretaceous of New Zealand

The surface sculpture of the inoceramid bivalve Magadiceramus? rangatira rangatira consists of commarginal ribs and curious, transverse wrinkles. The wrinkles typically are at a high angle or orthogonal to the shell margin (‘antimarginal’) and thus differ from purely radial structures. They show features of distribution and morphology that reveal them to be products of margin-parallel compression of the shell-secreting mantle and its adjacent,  exible, uncalciŽ ed periostracum. The interaction of wrinkles with commarginal ribs indicates that the ribs also formed as folds of the mantle margin. During growth, commarginal folding caused withdrawal of the entire mantle margin towards the umbo, with a consequent reduction in perimeter length. Measurement of specimens indicates that fabrication of the commarginal ribs resulted in the magnitude of commarginal shortening that is required for the formation of transverse wrinkles. We infer that early in ontogeny, at the Ž rst development of these sculptures, the wrinkles resulted entirely from mantle contraction and resultant commarginal shortening. With subsequent growth, total wrinkling included a component of ‘pre-wrinkling’ inherited from the preceding growth stage; the contribution of pre-wrinkling to total wrinkling increased with shell size. The proposed mechanical model is two-phase. First, the transversely corrugated (pre-wrinkled) mantle and periostracum advanced and secreted a slightly concave growth increment. Secondly, the mantle subsequently contracted to create a commarginal rib and increase the number and amplitude of transverse wrinkles. This model is consistent with a homogeneous mantle lacking any differentiated and specialized rib-constructing segments. & Biomechanics, bivalves, constructional morphology, Inoceramidae, New Zealand, shell morphogenesis, surface ornament, Upper Cretaceous.