Assemblages of fossil brachiopods are often characterized by the absence af juvenile specimens. This absence may be due for instance to method of collection, transport by currents, mechanical destruction, diagenetic solution, predation, factors that all may work selectively on small specimens. In some instances, where it was assumed from a taxonomic analysis that these factors could be excluded, low or even absence of juvenile mortality was given as an explanation. These cases are reviewed and alternative explanations are proposed. So, the assemblage could be the result of mass-mortality (census-assemblage) of a population without juveniles due to the fact that this "census" was taken some time after the last successful recruitment. Another explanation is that adults and juveniles live spatially separated, the assemblage of shells accumulating at localities where the adults live then contains no juvenile specimens. Recent examples of spatial separation of adults and juveniles are given for different groups of marine organisms which shows this phenomenon to be widespread and not only occurring in vagile animals. As long as other explanations are available, low juvenile mortality should be used with great reservation to explain the absence of juveniles in fossil assemblages, as such mortalities are rarely observed in present-day marine animals. Although juvenile mortality is usually high in the planktonic larval stage it is by no means restricted to this stage as stated sometimes. Also just settled benthic organisms are highly vulnerable to predation and other causes of mortality. |
