Abstract:
The ultrastructure of the organic periderm in Didymograptus sp. (Lower Ordovician), Pristiograptus dubius (Suess), and in the retiolitid Holoretiolites mancki (Münch) (both Upper Silurian), isolated by chemical treatment, has been studied with the transmission electron microscope. In the first two graptoloids, with continuous peridermal walls in the rhabdosome, the periderm is composed of the major fusellar and cortical components which are essentially the same as those recognized earlier in the dendroid graptolites (Urbanek and Towe, 1974). In addition, both graptoloids examined show important differences in the fabric ultrastructure of some homologous parts of the rhabdosome such as the nema in Didymograptus sp., the virgula in P. dubius, as well as the prosicula in both. These occurrences of sharp differences in the submicroscopic structure of homologous parts of rhabdosomes within the Graptoloidea are suggestive of rather substantial phylogenetic changes in the history of the group at the ultrastructural level.
In addition to the peridermal materials previously recognized in the dendroids, certain parts of the rhabdosomes in graptoloids are constructed of a peculiar fabric termed the virgular fabric. It has been recognized so far in the virgula of Pristiograptus dubius and in the lists of the peridemal framework (clathrium) of Holoretiolites mancki, where it is the only component of the skeleton. It is composed of layers made of lucent fibrils with a unique substructure, embedded in an electron dense and homogeneous matrix, and separated by thin layers formed by this matrix alone. The biochemical relationships of these fibrils with extant fibrous materials are uncertain but a correlation with certain collagens has been suggested.
The structural relationships of the outer cortical deposit of thecae examined in Didymograptus sp. and in Pristiograptus dubius are indicative of different modes of secretion of the cortex in both. In Didymograptus sp. the cortex is formed through an accumulation of the overlapping outer lamellae of fuselli, while in Pristiograptus dubius the layers of the cortex are laid down over the surface of the fuselli as independent units. Moreover, other observations seem to indicate that in Didymograptus sp. the cortex has been formed in a somewhat different way at certain places on the rhabdosome. There does not seem to be any single, uniform pattern of secretion of the cortex in the Graptoloidea.
