Article: Colony growth patttern and astogenetic gradients in the Cretaceous cheilostome bryozoan Herpetopora
The common Chalk anascan Herpetopora has runner-like encrusting colonies with uniserial branches. Following larval settlement, the ancestrula, described for the first time, budded two daughter zooids to initiate two first order colony branches which grew in opposite directions. Branches of higher orders were added by lateral zooidal budding from both sides of parent branches, usually at c. 80°. The 'mature' colony consisted of two conjugate sets of branches. Size/frequency distributions of zooidal length in H. laxata demonstrate the existence of two autozooidal polymorphs which differ in the length of their caudae. Independent astogenetic gradients of changing zooid size can be distinguished in each branch; in H. laxata these consist of an initial phase of progressively lengthening non-caudate polymorphs, followed by a threshold jump to a phase of caudate polymorphs, also of progressively increasing length. Each zooid in a colony could normally bud three potential daughters (one distal and two lateral). However, the frequency of buds actually formed declined with increasing branch order, and caudate autozooids generally budded more daughters than non-caudate autozooids in H. laxata. Intersections between branches had various possible outcomes; usually the growing branch terminated against the skeletal margin of the earlier branch, but sometimes growth deviated towards a pore window, probably by chemotropism, and occasionally branches were overgrown. Evidence of colony damage and repair includes 'intramural' budding, and normal and reverse polarity 'extramural' budding. Many colonies had complex histories involving mortality of zooids, fission, regrowth, and fusion. Functional interpretation of morphology suggests that growing colonies were proficient at exploring substratum space and seeking spatial refuges. They could withstand extensive damage and fragmentation, and had the capacity to repair damage and re-establish connections between the ramets formed by fragmentation.