Graptolite Net
edited by
Piotr Mierzejewski, the Count of Calmont and Countess Maja Anna Korwin-Kossakowska
The phenomenon of diapause is poorly understood
in graptolites. Only a few authors have dealt with
this problem, and they focused almost exclusively
on special dormant bodies called graptoblasts.
However, thecal occlusion seems to be the most
common diapause adaptation among fossil
graptolites and extant cephalodiscids.
Bulman defined occlusion as a "sealing of thecal
aperture by sclerotized film". These structures
have been recognized in the Graptoloidea,
Dendroidea, Tuboidea, but most often in the
Camaroidea. The abundant occlusion of autothecae
is a striking morphological feature of some
camaroid graptolite colonies.
  It is commonly believed that the sealing of the
autothecae in the Dendroidea and in the
Graptoloidea is related to the degeneration,
atrophy or necrosis of zooids. On the other hand,
Kozłowski compared the occluded autothecae of
camaroid graptolites to the gonozooids or ovicells
of extant cyclostomatous bryozoans. According to
him, the camaroid autothecae were occupied by
normal active zooids before their occlusion; after
sealing their apertures with diaphragms, they
degenerated in order to make sppace for their own
eggs or embryos. He described and illustrated two
types of occlusion in camaroid autothecae,
dependent on their shape: (1) autothecae with a
collum were occluded by irregular lamellae near
the base of the collum, and (2) autothecae devoid
of a collum were occluded by diaphragms
deposited directly over the apertures.
  Additional forms of occlusion structures have
since been observed (
Mierzejewski 2003); for
example, in an Ordovician dendroid-like camaroid
(Mierzejewski, Maletz and Sudbury in preparation)
the distinctly differentiated collum is occluded  by
a thick, distinctly thimble-shaped "stoppers"
inserted in the collum just beneath its aperture
(Fig. B).The lack of continuity between the
autothecal periderm and the "stoper" is
remarkable. On the other hand, autothecae in the
Tuboidea (closest relatives of the Camaroidea) are
occluded by a thick diaphragm made of cortical
tissue which merges directly into the thecal cortex
(Fig. C).
  Autothecal occlusion in the camaroid graptolite
Xenotheka klinostoma Eisenack, 1937 differs
sharply from other known camaroid and other
graptolites: it is made of a unique material, the
verrucose fabric, and it spreads across the entire
outer surface of the autotheca (Fig. A).
Sealing of the thecal aperture
(occlusion) in some sessile graptolites.
SEM micrographs.
A. Xenotheka klinostoma Eisenack,
1937, Llandeilo of Poland. Distal part
of autotheca.
B. Camaroid gen. et sp. nov.
Ordovician erratic boulder. Distal part
of autotheca.
C. Epigraptus kozlowskii
Mierzejewski, 1978. Lower
Ordovician of Estonia. Bitheca on the
thecorhiza surface.

Abbreviations: c, camara; co, collum;
o, occlusion; p, apertural processus; t,
thecorhiza, v, verruca of
Based on:
Mierzejewski, P. 2003. Autothecal
morphs and dormancy in the camaroid
graptolite Xenotheka. - Acta
Palaeontologica Polonica 48