Structure of protoconodont elements
HUBERT SZANIAWSKI
F O S S I LS A N D STRATA
ECOS III
A contribution to the Third European
Conodont Symposium, Lund, 1 982
Szaniawski, Hubert 1 983 1 2 1 5 : Structure of protoconodont elements . Fossils and Strata, No. 1 5,
pp. 2 1 -2 7 . Oslo. ISSN 0300--9 49 1 . ISBN 82-0006737-8.
T h e internal structure of t h e elements of 'Prooneotodus ' tenuis ( Miiller 1 959) a n d o f some other
protoconodont elements has been investigated under the optical microscope, scanning electron
microscope and transmission electron microscope. The elements are constructed of three layers :
thin outer organic cover, thick laminated organo-phosphatic middle layer and thin, faintly
laminated organic inner layer. The original structure of the organic matrix is not well preserved .
The outer layer is interpreted as a cuticle and the inner as a secreting layer of epidermis.
Comparison of structural details of protoconodont elements with grasping spines of Recent
C haetognatha shows their great similarity although there is no definite evidence oftheir identity.
I t is suggested that it is not the whole element of protoconodonts but only their inner or/and
middle layer that should be homologized with the basal plate of euconodonts. D Conodonta,
protoconodonts, Chaetognatha, structure, Upper Cambrian.
Hubert Szaniawski, Department ofPaleobiology, Polish Academy ofSciences, Al. Zwirki i Wigury 93, 02-089
Warszawa, Poland; 10th November, 1982.
Protoconodonts are an informal group of marine animals of
which only the organo-phosphatic, spinose elements are
known as microfossils, commonly occurring from the Upper
Precambrian to the Lower Ordovician. Protoconodonts are
usually assigned to Paraconodontida Muller but their
elements differ from typical elements of paraconodonts in the
mode of growth ( Bengtson 1 9 76) . Because the group is
distinguished on the base of structural features , only those
speeies can be regarded as protoconodonts in which such
features have been checked by means of special structural
investigations. Therefore at present on ly Amphigeisina daniea
( Poulsen 1 966) , Gapparodus bisulcatus ( Muller 1 959) ,
'Prooneotodus ' tenuis ( M uller 1 959) , Gapparodus heckeri
(Abaimova 1 9 7 8 ) , 'Prooneotodus ' savitzkyi (Abaimova 1 978) ,
Protohertzina anabarica Missarzhevsky 1 9 7 3 , P. unguliformis
Missarzhevsky 1 9 7 3 , and P. cultrata Missarzhevsky 1 9 7 7 can
be referred to the group. However, it is most probable that
many more species belong to it.
The probable relations hi P of protoconodonts to Recent
C haetognatha ( Szaniawski 1 980a, 1 982; Repetski &
Szaniawski 1 98 1 ; Bengtson 1 98 3 ) and also their possible
affinity with paraconodonts and euconodonts ( Bengtson
1 9 76, 1 97 7 ) makes this group of microfossils very interes-ting.
A better understanding of its systematic position may possi­
bly be reached by structural and comparative-anatomical
studies.
Published information on the s tructure of protoconodont
elements is based on optical microscope and SEM investiga­
tions ( Muller 1 959; Muller & Nogami 1 9 7 1 , 1 9 7 2 ; Bengtson
1 9 76, 1 9 7 7 , 1 983; Landing 1 9 7 7 ; Repetski 1 980; Szaniawski
1 980a, 1 982; Andres 1 98 1 ) . In the present study TEM has also
been applied in order to produce more information on the
organic matrix. TEM investigations of protoconodont ele­
ments are technically very difficult, but nevertheless easier
than in the case of conodont elements because they contain
much more organic material . The results ofthe new investiga­
tions have helped to provide for more detailed comparison of
protoconodont elements and the grasping spines of modem
chaetognaths.
Material and methods
The studies were made mostly on 'Prooneotodus ' tenuis ( M uller)
material obtained from the U pper C am brian in borehole cores
from northern Poland. Some speeimens of Gapparodus sp. from
the same locality and specimens of Gapparodus heckeri
Abaimova and 'Prooneotodus ' savitzkyi (Abaimova) from the
Upper C ambrian of C entral Kazakhstan ( material kindly
made available for study by Galina P. Abaimova, Novo­
sibirsk) were also examined.
Most of the specimens from Poland are black in colour.
They were processed from black or dark bituminous lime­
stones forrned in an anoxic environment. The specimens
possess well-preserved organic material and retain their shape
even after complete demineralization in mineral acids. In
lighter-coloured rocks one usually finds lighter-coloured
elements containing much less organic matter. Such elements
are much more easily destroyed in acid. The investigated
specimens are comparatively well preserved, although their
original structure is somewhat changed by recrystallization
and secondary mineralization .
The following preparation procedures have been applied :
For optical microscope observations : a - thin sections of
specimens embedded in original limes tone matrix, b - thin
sections of orientated specimens embedded in an epoxy resin .
For SEM studies : a - fractured untreated specimens, b fractured and etched specimens, c - etched sections of
specimens embedded in epoxy resin, d - etched sections made
22
Hubert Szaniawski
VII1,f---
FOSSILS AND STRATA 1 5 ( 1 983)
m I --A��IR'
A
B
Fig. 1. Schematic longitudinal (A) and cross (B) sections of an element of 'Prooneotodus ' tenuis (Miiller) with secondary mineralization as usually
preserved in the Upper Cambrian limestones of the Baltic region; il - inner layer, ism - intern al secondary mineralization, ml - midd le layer,
ol - outer layer, orz - organic-rich zone of the midd le layer, smz - secondarily mineralized zone of the middle layer.
with an ultramicrotome. For etching, different methods were
used depending on the state of preservation.
For TEM studies : oriented ultramicrotome sections of
demineralized specimens, prepared with a diamond knife and
not stained . Different methods of staining were tried but none
gave good results . Demineralization was done with strong
HN03 and H F .
The collection investigated is housed in t h e Institute of
Paleobiology of the Polish Academy of Sciences (abbreviated
as ZPAL ) .
Description o f the pres erved s tructure
Earlier investigations have shown that protoconodont ele­
ments were made of phosphate and organic material, possess a
three-Iayered structure, grew by basal-internal accretion and
have their main, midd le layer laminated ( Bengtson 1 976,
1 9 7 7 ; Szaniawski 1 982) . New s tudies make it possible to
present more detailed structural descriptions, especially of the
relationship between the organic and mineral components.
However, the original structure of the organic matrix is still
not well known because of diagenetic changes . The
generalized structure of 'Prooneotodus ' tenuis elements as pre­
served in the Upper Cam brian limestone of the Baltic region is
illustrated in Fig. l .
The outer layer is composed of compact structureless
organic matter. It is comparatively thin and its thickness
seems to be approximately constant along the entire length of
the element. The outer organic layer is rarely well preserved
because it is less resistant than the rest of the specimen and
thus detaches from the underlying layer easily ( Figs. 3A, 5 ) . In
some specimens the ou ter layer is secondarily mineralized
with phosphate or pyrite. Often it is covered or replaced by a
secondary phosphatic overgrowth ( Fig. 2 B ) .
The midd le layer is constructed of phosphatic crystallites
and organic matrix ( Figs . 5 , 6, 7) . The thickness of the layer
rapidly decreases in the basal part ofthe specimens . The layer
is faintly laminated . This lamination is clearly visible in thin
sections studied under the optical microscope and in etched
sections observed with SEM ( Fig. 4B) but it can hardly be
noticed in ultra-thin sections studied in TEM. This is because
the interlamellar spaces known in the elemen ts of euconodonts
do not exist here. Also the lamination is not caused by
alternation of pure organic and phosphatic laminae but rather
only by laminae more and less rich in organic matter ( Fig. 6B) .
The new laminae are added at the basal-internal surface
(Bengtson 1 976, 1 97 7 ) . Phosphatic crystallites are very fine,
usually irregular in shape, and irregularly distributed in the
organic matrix (Figs . 5, 7 ) . However, in some specimens the
crystallites are more regularly elongated ( Fig. 4A) and are
arranged sub-parallel to the layer surface ( Fig. 6C ) . The
organic matrix toa is more regular in some specimens and
forms thin sheets and enveloping individual crystallites . In
most of the studied elements from the Baltic region the midd le
layer can be divided into two zones ( Figs . 2C, 5A, 6A) . The
outer zone is usually the better preserved, contains much more
organic material, and is more clearly laminated. The inner
zone is strongly secondarily mineralized with calcite, phos­
phate and pyrite. In some specimens grains of amorphous
matter also occur there. C rystallites developed during the
mineralization are comparatively coarse ( Fig. 6A) . The
transition from the outer to the inner zone is usually gradual.
Development of the inner zone depends very much on the
preservation of the particular specimen involved .
The inner layer is very thin but nevertheless thicker than the
FOSS ILS AND STRATA 1 5 ( 1 983)
Fig. 2. Broken and etched elements of 'Prooneotodus' tenuis; explanations
as for Fig. I ; SEM. DA. Cross section of the element; ZPAL C . IV / 1 53 ;
X 480. D B . Fragment of a cross section o f a n element with secondary
phosphatic overgrowth; ZPAL C. IV / 1 593; X 1 000. DC. Fragment of
a cross section showing lamination of the inner layer and secondary
mineralization on both sides; ZPAL C . I 1 1 562; x 2000.
Structure ofprotoconodont elements
23
Fig. 3. Etched sections of 'Prooneotodus ' tenuis elements; explanations as
for Fig. I ; SEM. DA. Fragment of a longitudinal section of a broken
and etched element; ZPAL C . IV/ 1 59 7 . x 900. D B . Fragment of an
oblique section of a polished and etched element in its original
limes tone matrix; ZPAL C . IV/ 1 558; X 450. DC. Fragment of an
element embedded in epoxy resin, longitudinally sectioned and
etched; part of the inner layer removed to show structure of the middle
layer; ZPAL C . IV / 1 6 1 5; X 1 000.
24
Hubert Szaniawski
Fig. 4. Different preparations of 'Prooneolodus ' lenuis elements; explana­
tions as for Fig. l ; SEM. DA. Incomplete cross section through distal
part of an element embedded in epoxy resin. The element was
sectioned with an ultramicrotome and etched . Note elongate apatite
crystallites pulled out of the organic matrix; ZPAL C . IV/ l 607;
x 1 500. DB. Fragment ofan etched internal surface ofthe middle l �yer
showing its lamination and porosity. Inner layer removed; ZPAL C .
IV / 1 6 1 6; x 1 000. D C . Cross section o fa n element polished and etched
in its original limestone matrix. Middle layer dissolved; ZPAL C . IV /
1 560; x 300.
outer one ( Figs. 2 , 3A, 5 ) . I ts thickness decreases slightly
toward the base. This layer also is composed of organic matter
and phosphate crystallites, but organic matter predominates
( Figs . SA, 6A) and most probably the layer was originally
exclusively organic. The layer is laminated but has only a few
FOSSILS AND STRATA 1 5 ( 1 983)
Fig. 5. Ultramicrotome cross sections through distal part of a
demineralized element of 'Prooneolodus ' lenuis; white - empty space,
dark - undissolved amorphous material ( mainly organic) , black spots
- undissolved mineral crystals (pyrite?) ; explanations as for Fig. I ;
TEM. DA. Almost complete cross section of the element . Internal
cavity partly filled with amorphous material; X 1 200. DB. Detail of A,
X 7000. D C. Fragment of another cross section of the same element,
X 1 8,000.
laminae which are so faint that usually they cannot be
recognized under the optical microscope . They are best seen in
etched sections observed under SEM ( Figs . 2A, C, 3B) . On
both the inn er and the outer side the inner layer is usually
covered with secondary mineralization, which is often pyritic
( Fig. 2C ) . It is probably because ofthis mineralization that the
inner layer is the most resistant part of the whole element. It is
often the only remnant of a specimen.
The internal cavity extends almost all the way to the tip ofan
element. Usually it is filled with sediment and often with pyrite
and secondary phosphate.
FOSSILS AND STRATA I S ( 1 983)
Structure ofprotoconodont elements
25
Fig. 7. Ultramicrotome sections of 'Prooneotodus ' tenuis elements;
explanations as for Fig. I; TEM. DA. Fragment of a longitudinal
section showing irregular shape and arrangement of apati te crystal­
lites in the midd le layer. Outer layer not completely preserved;
X 1 3 ,SOO. D B. Fragment of the middle layer in cross section showing
distribution of apatite crystallites and their outline in seciion;
x 4S,000.
their midd le layer usually dissolves (Fig. 4C ) . At many
localities where 'P. ' tenuis elements occur, only the secondarily
phosphatized inner and outer layers, separated by a gap filled
with sediment, are preserved. Such a mode of preservation is
seen in the clusters described by Tipnis & C hatterton ( 1 9 79) .
Comparisons and interpretations
Fig. 6. Ultramicrotome sections of 'Prooneotodus ' tenuis elements;
explanations as for Fig. I ; TEM. DA. Fragment of a cross section.
Outer layer not preserved . Middle and inner layers secondarily
mineralized; x 4000. DB. Fragment ofa longitudinal section showing
lamination of the middle layer. Outer layer not completely preserved;
X 1 3,SOO. De. Fragment of a longitudinal section showing elongation
of apatite crystallites in an organic-rich zone of the middle layer and
undissolved mineral crystals (pyrite?) in the secondarily mineralized
zone of the midd le layer; x 1 4,000.
The described state of preservation of the structure of 'P. '
tenuis elements refers to speeimens preserved having black
colour. Lighter-coloured speeimens contain much less organic
matter. After brief etching of such specimens in 2-3 % HCI
I t is now possible to make a somewhat more detailed
comparison of protoconodont elements with on the one hand,
grasping spines of Recent Chaetognatha and, on the other,
elements of para- and euconodonts .
It has previously been thought that the main difference
between protoconodont elements and chaetognath spines is in
their chemical compositions, because the modem spines are
chitinous and the fossil forms are phosphatic. Although Clark
& Miller ( 1 969) established by means of electron-probe
analyses that elements of Gapparodus contain an admixture of
organic material, and Bengtson ( 1 976) suggested that the
admixture in primitive protoconodont elements is high, it has
not been demonstrated until now that the organic matter was
originally the main constituent of protoconodont elements .
However, the original relation.s hip of organic material to
phosphate is still not well known.
Chaetognath spines consist of three layers : a thin cuticular
26
Hubert Szaniawski
Fig. 8. Microtome cross section of a grasping spine of the Recent
chaetognath Sagitta maxima Conant; explanations as for Fig. I ; light
microscope photograph ( the same specimen was illustrated in
Szaniawski 1 982:809) ; ZPAL C . IV/ 1 700; x 1 000.
outer layer, a thick, fibrous, laminated middle layer and an
again thin epidermal layer (Fig. 8) . The chemical composi­
tions of the layers are most probably distinct because the outer
cuticle is comparatively easily damaged with KOH solution
whereas the fibrous layer is not. The cuticular layer, as far as it
is known, is structureless, as is the outer layer of protocono­
dont elements. Homology of the layers seems very probable.
The midd le layer of chaetognath grasping spines has fibrous
structure but the fibrils are arranged in fine lamellae. Definite
fibrils have not been found in 'P. ' tenuis elements, although
some preparations suggest their possible presence (Fig. 3e ) .
According to Bengtson ( 1 983) the elements of early pro­
toconodonts, Protohertzina unguliformis and P. anabarica, have a
fibrous structure, but the fibrils consist of acicular apatite
crystallites . It is possible that the middle layer of 'P. ' tenuis
elements originally possessed fibrous structure which has
be en damaged during the diagenesis. The secondarily
mineralized zone of the middle layer probably developed as a
result of contraction of the organic inner layer and filling of the
empty space with mineral solutions which later impregnated
also the inner part of the middle layer.
The inner layer of chaetognath grasping spines is formed of
a stratified secreting layer of epidermis . This layer is probably
homologous with the inner layer of protoconodont elements.
The pyritic mineralization of protoconodont elements and
especially of their inner layer is understandable when one
takes into account the fact that the pyrite usually forms as
organic material decays in anoxic conditions.
A possible structural difference between chaetognath
spines and protoconodont elements lies in the fact that the
middle layer in chaetognath spines is interrupted along their
edge (Fig. 8 ) , whereas in cross sections of the maj ority of the
'P. ' tenuis elements investigated the middle layer appears as a
complete ring. However, some sections of 'P. ' lenuis elements
suggest that their middle layer ori �inally had interruptions of
the same kind as seen in modem forms (Fig. 2A) . Some early
protoconodonts, e.g. Amphigeisina daniea, have all three layers
interrupted in the basal part of the element (Bengtson 1 976) .
Structural investigations completed at present . do not
provide definite proof of a protoconodont-chaetognath rela­
tionship, however, they do supply some arguments supporting
this hypothesis and, what is also important, they have
produced nothing that would contradict such a hypothesis .
A thorough comparison ofprotoconodont elements with the
FOSSILS AND STRATA 15 ( 1 983)
elements of para- and euconodonts was made by Bengtson
( 1 976) . Here only some questions of possible homologies will
be discussed. All euconodont elements are composed of two
parts, the element proper and a basal plate. Both parts grew by
buter accretion of continuously developed laminae (Miiller &
Nogami 1 97 1 ) . The basal plate, which contains much more
organic matter, usually has been considered as a probable
homologue of the complete protoconodont and paraconodont
elements (Bengtson 1 976; Szaniawski 1 980b: 1 1 6; Andres
1 98 1 ; Miiller 1 98 1 ) . Now, know'ing that the protoconodont
elements are constructed of three different layers, it seems
doubtful that the entire elements can be strictly homologous to
the one-Iayered basal plate of the euconodont elements. If any
homology between proto- para- and euconodont element
exists, it seems more probable that it is between euconodont
basal plates, paraconodont elements without their outer
organic cover, and the inner or/and middle layer of pro­
toconodont elements .
Acklnowledgements. - J would like to thank Galina P. Abaimova,
Novosibirsk, for providing some specimens for study, Stefan Bengt­
son, Uppsala, Anita Liifgren and Lennart Jeppsson, Lund, for
valuable comments to the manuscript, the late S. Crosbie Matthews,
U ppsala, and John Repetski, Washington, for linguistic corrections.
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Symposium on the Cambrian System
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