Article: Regular and flexible modes of division and hystero-ontogenetic growth in the Silurian rugose coral Stauria favosa
Y. Ezaki and Y. Yasuhara
New modules arise in colonial corals as the result of asexual reproduction. The Silurian rugosan Stauria favosa ordinarily exhibits cerioid coralla with a characteristic cross-shaped axial structure and a typical pattern of parricidal increase. Quadripartite increase at the sites of the four protosepta is most common, whereas cases of tripartite increase are rare. Parental protosepta are transformed into dividing walls, where the four protosepta first appear with a definite polarity in offset corallites. Daughter corallites inherit metasepta as metasepta, and catasepta as catasepta, within the same quadrants as those of the parent. Metasepta are inserted serially, following Kunth's rule, as is characteristic of rugosan protocorallites. As each daughter corallite derived immediately from the same parent is arranged with identical polarity, it grows equally and evenly both individually and as a group. Daughters thus form protosepta and metasepta under strict phylogenetic and developmental constraints. However, individual corallites grow and reproduce autonomously, by using all available skeleton and space of the parent. Although each module cannot modify essential modes of division, flexibility of the system was via changes in the density and arrangement of corallites, and regulating modes of growth, in tandem with adjacent corallites within the corallum. It is probable that regularity, due to constraints of several origins, as well as flexibility are typical of other rugosan colonies and played an important role in growth dynamics between corallites and corallum.