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| Abstract of the paper published in Acta Palaeontologica Polonica 2003, 2, 99 - 111. |
Coenecia of extant hemichordates Rhabdopleura compacta and Rh.
normani were investigated using SEM techniques. Cortical fibrils
were detected in their fusellar tissue for the first time. The
densely packed cortical fibrils form a characteristic band-like
construction in fusellar collars, similar to some Ordovician
rhabdopleurids. No traces of external secondary deposits are
found in coenecia. Two types of internal secondary deposits in
tubes are recognized:
(1) membranous deposits, composed of numerous, tightly packed
sheets, similar to the crustoid paracortex and pseudocortex; and
(2) fibrilar deposits, devoid(?) of sheets and made of cortical
fibrils, arranged in parallel and interpreted as equivalent to
graptolite endocortex.
There is no significant difference in either the shape or
dimensions of cortical fibrils found in Rhabdopleura and
graptolites. The cortical fabric of both rhabdopleuran species
studied is composed of long, straight and more or less wavy,
unbranched fibrils arranged in parallel; their diameters vary from
220 to 570 micrometers.
The study shows that there is no significant difference between
extinct and extant Graptolithoidea (=Pterobranchea) in the
histological and ultrastructural pattern of their primary and
secondary deposits of the periderm. The nonfusellar periderm of
the prosicula is pitted by many depressions similar to pits in the
cortical tissue of graptolites.
To the right is an example of inner secondary deposits found in
zooidal tube of the Jurassic Rhabdopleura kozlowskii Kulicki, 1969
normani were investigated using SEM techniques. Cortical fibrils
were detected in their fusellar tissue for the first time. The
densely packed cortical fibrils form a characteristic band-like
construction in fusellar collars, similar to some Ordovician
rhabdopleurids. No traces of external secondary deposits are
found in coenecia. Two types of internal secondary deposits in
tubes are recognized:
(1) membranous deposits, composed of numerous, tightly packed
sheets, similar to the crustoid paracortex and pseudocortex; and
(2) fibrilar deposits, devoid(?) of sheets and made of cortical
fibrils, arranged in parallel and interpreted as equivalent to
graptolite endocortex.
There is no significant difference in either the shape or
dimensions of cortical fibrils found in Rhabdopleura and
graptolites. The cortical fabric of both rhabdopleuran species
studied is composed of long, straight and more or less wavy,
unbranched fibrils arranged in parallel; their diameters vary from
220 to 570 micrometers.
The study shows that there is no significant difference between
extinct and extant Graptolithoidea (=Pterobranchea) in the
histological and ultrastructural pattern of their primary and
secondary deposits of the periderm. The nonfusellar periderm of
the prosicula is pitted by many depressions similar to pits in the
cortical tissue of graptolites.
To the right is an example of inner secondary deposits found in
zooidal tube of the Jurassic Rhabdopleura kozlowskii Kulicki, 1969
| Edited by Piotr Mierzejewski, 27th Count of Calmont and Countess Maja Anna Korwin-Kossakowska 2003-2004 |
| Cortical fibrils and secondary deposits in periderm of the hemichordate Rhabdopleura (Graptolithoidea) |