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

Abstract

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 ...