Autothecal morphs and dormancy in the camaroid graptolite  Xenotheka
Mierzejewski, P. 2003. Autothecal morphs and dormancy in the camaroid  graptolite Xenotheka. Acta Palaeontologica Polonica
48 (1):  93-98.
The camaroid graptolite Xenotheka klinostoma Eisenack, 1937 is  described from the lower Llanvirn limestones of
Gilbergabrottet, northern Ă–land,  Sweden. Two distinct autothecal morphs are recognized: (1)
normal morph  (described for the
first time), i.e. an autotheca with an  unsculptured outer surface, devoid of both an outer lining and autothecal  occlusion, and
inhabitated by an active zooid; and (2)
sealed morph, i.e.  an autotheca coated and occluded, provided with a sculptured outer
lining made  of a unique verrucose fabric, and inhabited by an inactive or dormant zooid. In  addition, the existence of a
hypothetical (3)
unsealed morph or  re-opened autotheca, devoid of an autothecal occlusion but provided with  an outer lining,
and inhabited by a reactivated zooid, is predicted. The sealed  morphs may represent an adaptation which allowed their
inhabitants to survive  adverse conditions. The outer lining of
Xenotheka is compared with a  peculiar outer membrane found in
the modern hemichordate
Rhabdopleura,  from the intertidal zone of Fiji, and with camaroid extracamaral tissue.
Key words: Graptolithina, Camaroidea, microfossils, ultrastructure, dormancy,  Ordovician, Sweden.
Piotr Mierzejewski [mierzejewski@graptolite.net], Instytut Paleobiologii PAN,  ul. Twarda 51/55, PL-00-818 Warszawa, Poland.
Present address: ul. Filtrowa 83  m. 49, PL-02-032 Warszawa, Poland.
Introduction
At different times, the Ordovician benthonic microfossil Xenotheka  Eisenack, 1937 and its only species X.klinostoma Eisenack,  
1937,has been placed among the foraminiferans, chitinozoans,  graptoblasts, crustoid? siculae or problematica, and a
consensus has never  developed. Recently, this form has been assigned to the encrusting graptolite  order Camaroidea and, in
the absence of bithecae, within the family  Cysticamaridae (
Mierzejewski 2000a). The upper stratigraphic range of camaroid  
graptolites was thereby extended from the late Arenig to the Llandeilo. So far,  
Xenotheka has been reported from the Ordovician
of Germany, Estonia,  Sweden and Poland (see
Mierzejewski 2000a for references). Moreover, doubtful  remains were identified
by
Mierzejewski (1978a) as ?Xenotheka sp., from  an erratic boulder of Ludlow age found in northern Poland.
The periderm of studied Xenotheka autothecae is unique among  graptolites in having a thin ornamented, outer lining,
constructed of a curious  verrucose fabric (
Mierzejewski 2000a). Verrucose fabric is typically composed of  numerous tiny
'verrucae' or nipples connected to an irregular net of thread-like  elements of different thickness. Strikingly, the outer lining covers
not only  the periderm proper (comprising ectocortex, fusellar layer or fusellum,  endocortex, and an inner lining) but it also
occludes the autothecal aperture.  This outer lining is presumably an adaptation which aided survival through  periods of
unfavourable or wholly adverse conditions.
The primary aim of this paper is to describe the discovery and significance  of Xenotheka autothecae which lack an outer lining.
Some problems  concerning the morphology, dormancy and palaeoecology of
Xenotheka are  also discussed in comparison
with other camaroid graptolites and the modern  hemichordate
Rhabdopleura.
The material studied originates from grey glauconitic limestones of Valaste  age (lower Llanvirn) and was collected under the
guidence of Dr. Svend Stouge in  1985 at a locality known as Gillbergabrottet 1 in northern Ă–land, Sweden. For  detailed
information on the location, lithology, fauna and biostratigraphy of  the outcrop, see Bohlin (1949) and Grahn (1980). The
Xenotheka material,  along with numerous chitinozoans, scolecodonts, graptovermids and fragments of  sessile graptolites,
was extracted by dissolving the limestone in formic acid.  All specimens studied under the SEM were coated with platinum or
gold and  examined using a Philips XL 20 and Cambridge Stereoscan 180. Numerous specimens  of
X. klinostoma from the
Ordovician beds of Estonia, Ă–land,  erratic boulders of glacial origin from Poland, and the Krzy
Âże 4 borehole (northeastern
Poland, Llandeilo)  were used as comparative material. The material described was stored on a SEM  stub and deposited in the
Institute of Paleobiology, Polish Academy of Sciences,  Warsaw (abbreviated ZPAL).
Morphological observations
The material consists of only two isolated autothecae; specimens A (length 670  micrometers) and B (length 980 micrometers). Both
display the  typical retort-like silhouette typical of
Xenotheka (Fig.  1A1-2, B1). They are composed of two sharply  differentiated
parts: a bulb-shaped camara and a long, rather narrow collum,  oriented more or less in parallel to the substrate (specimen A)
or slightly bent  upwards (specimen B). The camara adhering to the substrate is composed of a flat  lower wall (also called the
basal membrane or sole) and a convex upper wall. The  lower wall appears structureless under the light microscope, but it is
very  rough and exhibits partially spongy or fibrous texture under the SEM. It extends  laterally to form a marginal membrane,
weak traces of which take the form of a  protruding edge (Fig. 1A1-2, B1, 3). The outline of the  lower wall is subcircular (in
specimen A) or oval and elongated (in specimen B,  Fig. 1B3). The proximal part of the camara is provided with a tiny  opening
for an autothecal stolon. There is a distinct difference between both  specimens in the position of the opening: in specimen A its
just above the  attachment surface (Fig. 1A), whereas in specimen B it is distinctly higher, on  the top of a subtrapezoidal
flattened wall of the most proximal part of the  autotheca (Fig. 1B). There is a strong similarity between this part of the  camara
and a blunt heel of a graptoblast, termed by Urbanek
et al. (1986:  p. 102, Figs. 2C, 5C) as the talus. The stolonal opening of the
autothecae  contains a well-preserved, thick-walled vesicular diaphragm of the autothecal  stolon. It is easy to see in specimen
B; the outer surface of this diaphragm is  remarkably smooth (Fig. 1B2, 4). The camara lateral walls bear no  adpression traces
of adjacent thecae.
The boundary between the camara and the collum is distinctly marked by a  radical change in diameter. The collum length and  
diameter vary between 200-300 micrometers and 80-105 micrometers respectively. The aperture of the  collum is open and devoid of any kind of
apertural processes.
The condition of the periderm is good, except where a few cracks and holes  betray its diagenetic history. In general, under the
light microscope the  periderm is black, very smooth and slightly lustrous. Only the distal part of  the collum is dark brown, with
indistinct traces of a fusellar structure. The  attachment surface is also black and seems to be rough. Under the SEM, the  
periderm surface remains very smooth and featureless; it is almost enamel-like  and covered only with a scattering of grains of
sediment (Fig. 1A). This  confirms that the outer surface of the periderm is a robust sheet fabric, which  completely masks the
underlying cortical fibril material. Thus, there is no  indication of the true shape and extent of cortical units. In spite of their  very
good state of the preservation, the specimens examined show no trace of  verrucose fabric or an outer lining. The proximal part
of the camara of the  specimen A displays parallel transverse lines, gently protruding over the  surface, which correspond to the
fusellar structure of the autotheca. No trace  of a regular zig-zag suture has been found. Only could one a characteristic  oblique
fusellar suture be observed (Fig. 1B2
). The height of fuselli width varies from 19 - 40  micrometers.
Discussion
The autothecae of X. klinostoma described above match closely those  from the Llandeilo of Poland (see Mierzejewski 2000a).
They are of the same  shape, dimensions and collum inclination, and equallly devoid of apertural  processses. The stolonal
vesicular diaphragm shown on SEM micrographs (Fig.  1A3, B2, B4) was previously recognized under  the light microscope and
TEM as a stolonal sheath or ring-like structure  (
Mierzejewski 2000a: Figs. 3A, 9). This structure closely resembles the  vesicular
diaphragms known in the camaroid
Bithecocamara KozĹ‚owski as well as the crustoid  Urbanekicrusta Mierzejewski, the tuboid
Kozlowskitubus Mierzejewski and the dendroids  Acanthograptus Spencer, and Koremagraptus Bulman. However, the  vesicular
diaphragms of these graptolites always constitute one component of a  heavily sclerotized stolon system. In contradistinction,
the vesicular diaphragm  of
Xenotheka is the only known sclerotized fragment of the stolon system.  The lack of stolonal
vestiges connected with the vesicular diaphragms, as well  as the absence of isolated stolon fragments from the residue of the
etching  process (cf.
Mierzejewski 2000a) may suggest that Xenotheka lacked a  well-sclerotized stolon system.
However, the autothecae of Xenotheka from Ă–land differ remarkably from  the Polish and Estonian examples in their surface
micromorphology: the periderm  is smooth and devoid of that peculiar, distinctly ornamented outer lining of  verrucose fabric,
and their apertures are not sealed. There can be no doubt that  the Ă–land specimens represent the expected 'normal' (i.e.
naked or uncoated)  autothecae of
Xenotheka.
Two clearly distinct morphs of the autothecae in X. klinostoma can  therefore be distinguished: (1) normal morph (i.e. a naked  
autotheca, with an unsculptured periderm and devoid of both an apertural  occlusion and an outer lining); and (2)
sealed morph
(i.e.an autotheca with an ornamented outer lining) and an apertural occlusion, both  made of the verrucose fabric. Clearly, the
naked autothecae were inhabited by  normal, active zooids, whereas the sealed autothecae were occupied by inactive  zooids.
According to the interpretation of the outer lining proposed by  
Mierzejewski (2000a), the coated autothecae should be regarded
as an anatomical  adaptation connected with diapause or dormancy (i.e. periods of arrested  ontogenetic development in
response to adverse environmental conditions). Such  an interpretation implies the existence of a third, so far unknown
post-dormant  morph an
unsealed morph (i.e. re-opened or 'uncorked' autotheca). This  hypothetical autotheca would be
covered with a continuous outer lining but lack  the membrane sealing its aperture. Such a re-opened autotheca must have
been  occupied by a reactivated zooid which was able to resume its normal life  functions as environmental conditions
improved. Obviously, if adverse conditions  were long lasting, the zooid closed in a sealed autotheca presumably became  
subject to biological processes connected with ageing and leading directly to  necrosis. It may be that the autotheca was sealed
and unsealed more than once.  There are ultrastructural observations that the outer autothecal surface was  coated with the
outer lining repeatedly within the lifespan of a colony. Some  TEM transverse sections through the outer lining show that it was
sometimes at  least double-layered (
Mierzejewski 2000a: Figs. 6A, 7A).
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 (e.g.
KozĹ‚owski 1962; Urbanek 1984; Urbanek et  al.
1986; Crowther et al. 1987; Mitchell
et al. 1993; Mierzejewski  2000b). Other forms of graptolite dormancy were described by KozĹ
owski (1949, 1962, 1971), Bulman (1970), Urbanek (1986),  and Mierzejewski (2001). Thecal occlusion seems to be the most common  
diapause adaptation among fossil graptolites and extant cephalodiscids. Bulman  (1970: p. V17) defined occlusion as a
"sealing of thecal aperture by sclerotized  film". These structures have been recognized in the Graptoloidea (e.g. Urbanek  1958:
p. 36), Dendroidea (e.g. Bulman 1933: p. 24; Kozł
owski 1949: pp. 43-44), Tuboidea (Mierzejewski 2001: p.  374, and herein Fig. 2A),
but most often in the Camaroidea (Kozł
owski  1949, 1971; Skevington 1963; Mierzejewski 2000a and herein; see also Urbanek  
1984, 1986 for discussion). 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 the Graptoloidea is related to the
degeneration, atrophy or necrosis of  zooids. On the
other hand, KozĹ‚owski (1949,  1971) 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 space  for their own eggs and 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 (KozĹ‚owski 1949: pl. 26, 8), and (2) autothecae  devoid of a collum were occluded by diaphragms deposited directly over
the  apertures (KozĹ‚owski 1949: pl. 26, 1; 1971: fig. 8). Additional forms of  occlusion structures have since been observed (Mier
zejewski,  
unpublished); for example, in an Ordovician dendroid-like camaroid (Gen. et sp.  nov. 1, to be described elsewhere by
Mierzejewski, Maletz and Sudbury, in prep.)  the distinctly differentiated collum is occlluded by a thick, distinctly  thimble-shaped
'stoppers' inserted in the collum just beneath its aperture (Fig.  2B). The lack of continuity between the autothecal periderm and
the 'stopper' is  remarkable. On the other hand, autothecae in the Tuboidea (closest relatives of  the Camaroidea, cf.
Kozłowski
1949; Skevington  1963;
Mierzejewski 2001), are occluded by a thick diaphragm made of cortical  tissue which merges directly into the thecal
cortex (Fig. 2C).
Autothecal occlusion in Xenotheka differs sharply from all 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. 2A  and
Mierzejewski 2000a:
figs. 1E, 2A, C, 5A). Previously, I had supposed that  the outer lining of
Xenotheka was secreted in "the form of an organic  
emulsion rising into the water, which subsided to the rhabdosome, covering the  surface, occluding the thecal aperture, and
then hardening"
(Mierzejewski 2000a:  p. 82). Crucial support for this interpretation would be provided by biological  models of
similar processes in fossil or modern hemichordates, and I think the  key to understanding the process can be found in Dilly
and Ryland's (1985)  investigations of modern
Rhabdopleura. This "living fossil", regarded by  Beklemishev (1951, 1970) and
Mierzejewski and Kulicki (
2001, 2002) as a member  of the hemichordate class Graptolithoidea, is the best biological model for  
studies on the skeletal tissues of fossil graptolites. According to Dilly and  Ryland (1985), the coenecium of their
Rhabdopleura
was distinctly linear,  forming a typical runner-type colony, overgrowing and penetrating the undersides  of coral boulders in the
intertidal zone of Fiji. The zooids of this form  exhibited a unique mode of tube building, with its horizontal tubes "contained  
within an irregular membrane that lined the cavity from which the horizontal  tube arose" (Dilly and Ryland 1985: p. 616).
Presumably, the enclosing 'bag' was  constructed by the zooids "plastering" a layer of secreted material across the  substrate
surrounding the fissure in which the tube was found. Remarkably, in  many cases the coenecium components were tightly
overgrown by the coral mass,  implying a rather close contact between the periderm proper and the peculiar  surrounding
membrane.
The morphology and fine structure of the rhabdopleuran skeleton are well  known (see Urbanek and Dilly 2000; Mierzejewski
and Kulicki 2001 for references)  but nothing similar to this outer membrane has ever been previously reported.  The unusual
anatomical structure of the colonies from Fiji is connected directly  with its unusual habitat:
Rhabdopleura is normally found so
far usually  at moderate depth, in relative cold waters, but has never before been found in  the intertidal zone of warm waters. In
this situation it seems clear that its  secretion of an outer membrane may be a physiological response to these peculiar  
environmental conditions. I regard the secretion of this peculiar structure as  very useful and instructive in the present
discussion.
It is possible that the outer membrane of Rhabdopleura is comparable  with the outer lining of Xenotheka. This in turn suggests
that  
Xenotheka may have inhabited a similar environment to Rhabdopleura from Fiji, i.e. rocky shorelines in the intertidal zone.
Such zone is commonly  bored by various bioeroders (e.g. bivalves, sponges, and various "worms"). Some  autothecae of
Xenotheka possess two distinct attachment surfaces or  curiously irregular soles which indicate that they encrusted with
irregular  sufaces (see
Mierzejewski 2000a: p. 72, fig. 1D). In such an environment the  most common adversity was a periodic
lack of water, connected with sea-level  change. It is of interest to note that crustoid graptolites could apparently  overgrew a firm
substrate in very shallow water and that some of were pioneering  encrusters (Mitchell
et al. 1993; cf. Mierzejewski 2000b).
Crustoids  routinely produced graptoblasts as a means of resisting environmental changes  which threatened the survival of the
colony. Secretion of an outer lining by the  camaroid
Xenotheka was probably another kind of diapause adaptation for  surviving
periods of unfavorable conditions. It seems possible that  
Xenotheka secreted a membrane of gel-like material which covered
the  surfaces of surrounding holes and fissures. As the tide turned, this material  settled down onto the rhabdosome and stuck
to the periderm forming the outer  lining and, simultaneously, occluding the autothecae. Presumably, this served to  keep the
organism alive and
Xenotheka was able to withstand unfavourable  influences when it would otherwise perish. The "droplet"
character of the lining  (
Mierzejewski 2000a), as well its indistinct and irregular layering, are  consistent with such an hypothesis.
The recognition of two (or even three?) autothecal morphs in Xenotheka  broadens our knowledge of graptolite polymorphy (the
frame of reference for  all such studies was established b
y KozĹ‚owski 1949,  and has been well reviewed by Urbanek 1986). The
autothecae of  
Xenotheka are, in my opinion, the first example of seasonal or  periodic morphs within a single species. This
phenomenon has not previously been  recognized in graptolites but is common among other invertebrates. The  dimorphism of
Xenotheka represents a new variety of the secondary  specialization of autothecal zooids which also manifests itself as tuboid  
microthecae and conothecae, crustoid graptoblasts, and the occluded autothecae  of camaroids and tuboids (see Urbanek
1986 for discussion).
The shape of a complete rhabdosome of X. klinostoma is unknown, as is  its early growth and mode of development. I have
studied nearly 100 specimens of  this graptolite from Poland, Estonia, Sweden and in glacial boulders, all of  which were
isolated autothecae. Yet in spite of their beautiful state of  preservation, two or more joined autothecae have never been found.
Moreover, no  other components of the
Xenotheka rhabdosome, such as stolons,  stolothecae or bithecae, are known. A short
process with an opening containing  the vesicular diaphragm (Fig. 1A3, B2, B4) is  the only trace of its stolon system. The
periderm of
Xenotheka is usually  well preserved and was evidently highly resistant to diagenetic influences. The  outer surface
of the periderm bears no traces of adjoining structures and is  uniformly coated with the outer lining. All these observations
suggest how the  
Xenotheka rhabdosome might be reconstructed. It was probably a rather  large, irregularly spreading
runner-type colony, attached to the substrate and  possibly penetrating it. The mode of branching is unknown, but was
presumably  dichotomous at irregular intervals. The rhabdosome was composed of autothecae  linked by naked stolons (i.e.
devoid of sclerotized stolonal sheaths). This may  be deduced from the lack of stolonal sheaths connected with autothecae, and
the  smooth surface of the vesicular diaphragm in the proximal part of each autotheca  (Fig. 1B2, B4). However, it may be that the
autothecae  were sometimes accompanied by vestigial stolothecae, as suggested by the  talus-like structure in specimen B (p.
94, Fig. 1B1, B3).
The lack of joined autothecae and their imprints on the periderm show that  the autothecae were very loosely dispersed. The
rhabdosome was devoid of  bithecae; its components were not embedded in the extracamaral tissue. Thus,  
Xenotheka
resembles a colony of the modern
Rhabdopleura from Fiji  (discussed above) in both the distribution of its thecae and its naked
stolons.  According to Dilly and Ryland's (1985) observations, their hemichordate formed a  very loose and linear (i.e.
runner-type colony), branching coenecium which  encrusted and penetrated the coral mass, with naked stolons (previously
unknown  in
Rhabdopleura) connecting the zooids together. This striking similarity  between the reconstructed rhabdosome of
Xenotheka and the  Rhabdopleura coenecium from Fiji reflects morphological plasticity under  varying ecological conditions
within the Hemichordata.
The outer irregular membrane of Rhabdopleura, the outer lining of  Xenotheka and the extracamaral tissue of some
Camaroidea likely represent  similar forms of hemichordate responses to unfavourable environmental  conditions, connected in
some degree with the phenomenon of diapause. These  structures may represent not only a strong functional analogy but also
an  interesting example of essential homology. It is possible that the secretion of  outer membranes or extracamaral tissue
should be interpreted as a colonial  response provoked by overgrowing encrusters.
Conclusions
1.  X. klinostoma is represented by at least two distinct autothecal  morphs, i.e. normal and sealed, corresponding to active
(pre-dormant) and  inactive (dormant) stages of the zooid life. The succeeding (hypothetical)  morph, containing a reactivated
zooid in the post-dormant stage, awaits  discovery.
2.  X. klinostoma may be categorized as a cryptic encruster, inhabiting  undersides and vacated borings in the intertidal zone,
forming runner-type  colonies with completely naked stolons.
3.  The outer lining of X. klinostoma, the outer irregular membrane of  some modern Rhabdopleura, and the extracamaral tissue
of the Camaroidea  may be equivalent structures, and can be interpreted as responses to similar  environmental signals.
Acknowledgements
Dr Denis Bates and Dr Peter R. Crowther (Belfast) kindly reviewed the  manuscript and provided helpful remarks. This study
was carried out at the  Institute of Palaeobiology, Polish Academy of Sciences, Warszawa. Some  observations were made at
the Institute of Historical Geology and Paleontology  of the University of Copenhagen. Professor Jerzy Dzik (Warsaw) supplied  
comparative material of
X. klinostoma from Ă–land. Dr. Svend Stouge  (Copenhagen) kindly provided help during my field work
on Ă–land, which was made  possible by a grant from the Danish Government (1985). Dr. Cyprian Kulicki  (Warsaw) provided
invaluable assistance in SEM studies.
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Fig. 1. Xenotheka klinostoma Eisenack, 1937; lower Llanvirn,  Gilbergabrottet 1, Ă–land. Isolated autothecae, normal morph. A.
Specimen  A (ZPAL G/x/1).
A1, lateral view; A2, upper  view; A3, proximal part of autotheca. B. Specimen B  (ZPAL G/x/2). B1;
lateral view;
B2, proximal  part of autotheca; B3, lower wall (attachment surface or  sole); B4, vesicular diaphragm. Abbreviations:
a, attachment  surface; d, vesicular diaphragm of autothecal stolon; a, autothecal aperture;  ca, camara; co, collum; g, grains of
sediment attached to periderm; m, remnants  of marginal (basal) membrane; o, basal opening in camara wall filled with  
vesicular diaphragm; s, mould of the terminal portion of the parental stolotheca  (?).
Fig. 2. Sealing of the thecal aperture (occlusion) in some sessile  graptolites. SEM micrographs. A. Xenotheka klinostoma
Eisenack,  1937; Llandeilo, borehole Krzy
Âże 4 (Poland), depth  473 m. Distal part of autotheca. B. Camaroid, Gen. et sp. nov. 1;  
Ordovician erratic boulder, northern Poland. Distal part of autotheca.
C. Epigraptus kozlowskii Mierzejewski, 1978 (Tuboidea);
Lower  Ordovician (Kunda Stage, Aluoja Sustage), Sukhrumägi in Tallinn, Estonia.  Bitheca on the thecorhiza surface.
Explanations: c, camara; co, collum; o,  occlusion; p, apertural process; t, thecorhiza, v, verruca of verrucose fabric.  Arrow shows
discontinuity between autothecal wall and occlusion.
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