Autothecal morphs and dormancy in the camaroid graptolite Xenotheka
The camaroid graptolite Xenotheka klinostoma Eisenack, 1937 is described from the lower Llanvirn limestones of
Gilbergabrottet, northern Öland, 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 Öland, 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, Öland, erratic boulders of glacial origin from Poland, and the Krzyże 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 Koz?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 Öland 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 Öland 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. Koz?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 Koz?
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; Koz?owski 1949: pp. 43-44), Tuboidea (Mierzejewski 2001: p. 374, and herein Fig. 2A), but
most often in the Camaroidea (Koz?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, Koz?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 (Koz?owski 1949: pl. 26, 8), and (2) autothecae devoid of a collum were occluded by diaphragms deposited directly over
the apertures (Koz?owski 1949: pl. 26, 1; 1971: fig. 8). Additional forms of occlusion structures have since been observed (Mierzejewski,
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. Koz?owski
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 by Koz?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 Öland. Dr. Svend Stouge (Copenhagen) kindly provided help during my field work on
Öland, 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, Öland. 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 Krzyże 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.
