Regular mode of increase, and constrained but variable growth, in the Silurian rugose coral Stauria favosa

Abstract

Stauria favosa exhibits a typical pattern of axial division (tripartite or quadripartite). The four protosepta first appear with a definite polarity in offset corallites, and metasepta are inserted serially, following Kunth's rule. There are no variations in essential modes of asexual reproduction. However, at the corallum periphery, up to four offset corallites appear to have been derived by discrete offsetting within the parent calices. Detailed observations of vertical morphological changes, however, indicate that this apparently unique mode of increase is derived from axial division at the sites of the four protosepta, as usual in this species. Just after the initiation of division, the contraction of polyps occurred in response to somewhat accidental, deteriorating habitat conditions. The combination of asexually divided corallites and rejuvenescence resulted in phaceloid daughter corallites, which subsequently underwent partial mortality at the corallum periphery. Stauria favosa always exhibits not only regular modes of asexual reproduction and septal insertion under the strict control of phylogenetic and developmental constraints, but also variable growth forms of corallites and coralla. This variability, though within limited options, is palaeoecologically significant for successfully coping with unpredictable habitats.