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 Triass ic, five Jurassic and nine Cretaceous species from England and Slovaki a are here studied using an image analyser to record colony size and s hape, and the number, location and sizes of larval brood chambers. Sur vivorship 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 producin g frontal, or more commonly, peripheral subcolonies. These species typ ically have smaller colonies than species without subcolonies. Colony size at the onset of female sexual reproduction was found to be relati vely constant in some species but variable in most, possibly indicatin g that an environmental cue triggered reproduction. Most colonies repr oduced only once (semelparity) and apparently died shortly afterwards, but a few survived to reproduce a second time (iteroparity). No corre lation among species was found between skeletal measures of reproducti ve effort and colony size. Flexibility in life history patterns predom inate in the 15 studied species, the one notable exception being Actin opora disticha which was relatively deterministic.