The ultrastructure of the Crustoidea and the evolution of graptolite skeletal tissues

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Abstract
The Crustoidea are important in discussions of graptolite origins, because they represent a morphoecological type intermediate between the Rhabdopleurida and the Dendroidea. TEM [transmition electron microscopy] and SEM [scanning electron microscopy] studies of their rhabdosomes provide new data on the structural displayed by the graptolite periderm. Since the crustoid fusellar fabric does not differ markedly from the fuseller fabric of other graptolites and is dissimilar to that of pterobranchs, the fundamental graptolite fibril pattern was attained at the crustoid stage of graptolite evolution. The crustoid cortex is made of paracortex (the multiple deposition of sheets separated by an intersheet material in the form of condensed meshwork of fibrous material) and of psuedocortex (the accumulation of sheets and intersheet material devoid of fibrous character). The presence of sporadically genuine cortex is noted. Presumably the mechanisms of cortical fibrilogenesis were attained at the crustoid stage, but only in an incipient and incomplete form. Modes of periderm corticization are discussed. Sheets, common elements in secondary deposits of pterobranchs and graptolites, are compared, but some problems still remain unclear.