GROWTH-RATES OF PTEROBRANCHS AND THE LIFE-SPAN OF GRAPTOLITES

Pterobranchs are the closest living relatives of graptolites. Their sk eleton is constructed from the same material, and in a homologous mann er. Growth rates of the pterobranch Cephalodiscus gracilis are describ ed for the first time and, along with rhabdopleuran growth rates, they are used to estimate the amount of time invested by a graptolite colo ny in growing its rhabdosome. This is a measure of minimum age. The si gnificance of age calculations is shown for individuals and large comm unities of graptoloids. Large individuals can be shown to be much olde r than the time it would have taken them to settle through seawater an d so it is shown that graptoloids moved up, as well as down, through t he water column. Life tables constructed for biserial graptoloids from the Utica shale in Quebec, Canada, suggest that these graptoloids die d from constant environmental stress. Graptoloid length can thus be a function of environment and should only cautiously be considered to be of taxonomic significance.