Life histories of some Mesozoic encrusting cyclostome bryozoans

Single-layered, multiserial cyclostome bryozoans are almost ubiqitous as encrusters of Mesozoic hard substrata but little attention has been paid previously to the attributes of their life histories obtainable from their fossil skeletons. Colonies from 'populations' of one Triassic, five Jurassic and nine Cretaceous species from England and Slovakia are here studied using an image analyser to record colony size and shape, and the number, location and sizes of larval brood chambers. Survivorship curves relative to colony size demonstrate varying patterns of mortality for different species. None of the species shows evidence of a fixed maximum colony size. Some species were capable of producing frontal, or more commonly, peripheral subcolonies. These species typically have smaller colonies than species without subcolonies. Colony size at the onset of female sexual reproduction was found to be relatively constant in some species but variable in most, possibly indicating that an environmental cue triggered reproduction. Most colonies reproduced only once (semelparity) and apparently died shortly afterwards, but a few survived to reproduce a second time (iteroparity). No correlation among species was found between skeletal measures of reproductive effort and colony size. Flexibility in life history patterns predominate in the 15 studied species, the one notable exception being Actinopora disticha which was relatively deterministic.