Estimation of allometric shell growth by fragmentary specimens of <i>Baculites tanakae</i> Matsumoto and Obata (a Late Cretaceous heteromorph ammonoid)

Abstract

We introduce a new biometric method to reconstruct ontogenetic shell development of Baculites species. In order to estimate original total shaft length from fragmentary specimens and to clarify their shell growth patterns, a large number of samples of Baculites tanakae Matsumoto and Obata, collected from the Upper Cretaceous deposits in Hokkaido, Japan, were examined. Biometric analysis revealed a characteristic allometric shell growth pattern of B. tanakae expressed by the formula L = 3.03H1.50, where L and H are original total shaft length and whorl height, respectively. The analysis gives a quantitative diagnosis of the morphology of this species and enables us to estimate L including the missing apical part. Reconstruction of the total shaft length reveals that the shell ornament of B. tanakae shifts ontogenetically from a smooth phase to a tuberculate phase via a ribbed phase. It also demonstrates wide intraspecific variation on switching timing of the shell ornament phases. The ontogenetic change and the intraspecific variation can be clearly discriminated from each other by our method.