|Cephalodiscus - type fibrils in the graptoblast fusellar tissue
Acta Palaeontologica Polonica 1984, 29, 3/4, 157-160
The aim of this paper is to present the first finding of a graptolite fusellar tissue composed of fibrils very similar to the
pterobranch fibrils. This tissue has been found in the wall of a Llanvirnian graptoblast classified as "Graptoblastoides" sp.,
which was etched from a glacial boulder of the Baltic origin.
The material of the fusellar fabric of the graptoblast under study is exclusively composed of straight, unbranched and
distinctly banded fibrils. Each fibril is provided with a large, distinct, lucent central core. The diameter of fibrils fluctuates
within the limits of 90 to 129 nm. The fibrils are irregularly displaced within the fuselli. In some parts they are loosely packed,
forming locally irregular aggregations.
Portal of Graptolites and Pterobranchs
The fusellar tissue described above
differs distinctly from the same tissue of
graptoblasts examined by Urbanek and
Rickards (1974). To the same degree
it also differs from the fusellar tissue in
different orders of the graptolites.
The ultrastructure of the graptoblast
fusellar tissue here described reveals a
fabric and pattern very similar to those
recognized in living cephalodiscids
pterobranchs. As it has been observed
the fusellar tissue of Cephalodiscus
solidus is also built of nothing but
straight unbranched fibrils provided
with a large, lucent central core.
However, cephalodiscid fibrils are
much thinner. Their diameter varies
from 19 to 22 nm.
One can say that the graptoblast
fusellar fibrils discussed above fill the
structural gap between the pterobranch
and the graptolite fusellar tissue. These
fibrils are of a cephalodiscid
ultrastructure and of graptolite
|General view of the graptoblast fusellar tissue (TEM micrograph).