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| SCANNING ELECTRON MICROSCOPY OF SESSILE GRAPTOLITES Adam Urbanek, Grazyna Mierzejewska and Piotr Mierzejewski Acta Palaeontologica Polonica 1980, 25 (2): 197-212. |
ABSTRACT:
"The surfaces of rhabdosomes in dendroid and
tuboid graptolites and in Mastigograptus were
investigated with SEM. In general the surfaces
are remarkably smooth and essentially
featurless, even around the apertures. The only
exception are subparallel rollers recognized on
one specimen of Dictyonema sp., which pass at
places into accumulations producing a
reticulated network. The evidence available
indicates a foreign nature of these structure
which cannot be considered units of secretion of
graptolite cortex. The opinion is advanced that
the recent data are suggestive of collagen
fibrillogenesis occuring immediately adjacent to
the surface coat of epithelial cells. The lack of
bandages in sessile graptolites and the lack of
foreign agglutinated material on their
rhabdosomes, the intimate control of
fibrillogenesis by epithelial cells in extracellular
secretion of collagen are all presented as
evidence in favour of the membrane model of
graptolite periderm secretion."
Urbanek et al. 1980, p. 197.
"The surfaces of rhabdosomes in dendroid and
tuboid graptolites and in Mastigograptus were
investigated with SEM. In general the surfaces
are remarkably smooth and essentially
featurless, even around the apertures. The only
exception are subparallel rollers recognized on
one specimen of Dictyonema sp., which pass at
places into accumulations producing a
reticulated network. The evidence available
indicates a foreign nature of these structure
which cannot be considered units of secretion of
graptolite cortex. The opinion is advanced that
the recent data are suggestive of collagen
fibrillogenesis occuring immediately adjacent to
the surface coat of epithelial cells. The lack of
bandages in sessile graptolites and the lack of
foreign agglutinated material on their
rhabdosomes, the intimate control of
fibrillogenesis by epithelial cells in extracellular
secretion of collagen are all presented as
evidence in favour of the membrane model of
graptolite periderm secretion."
Urbanek et al. 1980, p. 197.
| Rollers on the periderm surface of the Ordovician Dictyonema sp. (erratic boulder, Poland). From Urbanek et al. 1980. |