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New Data on the Morphology of Shell Muscles in Cambrian Helcionelloid Mollusks

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Pavel Parkhaev at Russian Academy of Sciences
Pavel Parkhaev
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Abstract

Scars of shell muscles have been discovered on internal molds of helcionelloid mollusks of the family Stenothecidae for the first time. The localization of the scars is similar to that in the family Coreospiridae. Since these two families evolved from the ancestral group independently, similar shell muscle systems of Cambrian helcionelloid mollusks may be explained by parallel development.
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254
Paleontological Journal, Vol. 38, No. 3, 2004, pp. 254–256. Translated from Paleontologicheskii Zhurnal, No. 3, 2004, pp. 27–29.
Original Russian Text Copyright © 2004 by Parkhaev.
English Translation Copyright © 2004 by
åÄIä “Nauka
/Interperiodica” (Russia).
INTRODUCTION
After a recent publication by this author in which
the muscle scars of the Cambrian univalved mollusks
were described for the first time (Parkhaev, 2002a), the
study of the internal molds of mollusks of this age using
SEM was continued in order to yield new data on the
morphology of the shell muscle system. In this way,
scars of shell muscles have been discovered in molds of
helcionelloid mollusks from the family Stenothecidae.
MATERIAL
The material being studied is housed at the Paleon-
tological Institute of the Russian Academy of Sciences
(collection nos. 2019, 4368, and 4664).
DISCUSSION
The internal molds of
Anabarella
sp. from the
Lower Cambrian of Transbaikal Region (Bystraya For-
mation, Georgievka Locality) contain a site with cellu-
lar microsculpture (Pl. 2, fig. 5) that is similar in
appearance to scars of shell muscles that have been
described earlier for Cambrian mollusks from the fam-
ilies Helcionellidae, Coreospiridae, and Onychochil-
idae (Parkhaev, 2002a). The scar is situated in the sub-
apical part of the mold at the boundary between its pos-
terior surface and the roof of the parietal train. The scar
consists of polygonal depressions that are 10–12
μ
m in
diameter and thin dividing balks approximately 3
μ
m
wide (Pl. 2, fig. 2). Such a type of microrelief of the
internal mold surface is a replica of the prismatic shell
layer, i.e., myostracum. The pallial myostracum shows
a similar prismatic structure in different groups of mol-
lusks and even in brachiopods (Taylor
et al.
, 1969;
1973; Williams and Wright, 1970; Popov, 1977;
Parkhaev, 2002a).
After the above-mentioned scars on the molds of
Transbaikalian anabarellas were discovered, I restudied
the vast collection of mollusks from the Lower Cam-
brian of South Australia, in which there are also mem-
bers of the genus
Anabarella
. The collection contains
several hundred differently preserved internal molds
and shells of
Anabarella australis
Runnegar, 1990.
Thorough investigation of the best preserved internal
molds revealed that some specimens bear muscle scars
on the subapical surface. The scars are not as distinct as
in Transbaikalian anabarellas (the polygonal depres-
sions and dividing balks are not as prominent in relief),
have a different shape, and slightly differ in position
New Data on the Morphology of Shell Muscles
in Cambrian Helcionelloid Mollusks
P. Yu. Parkhaev
Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia
e-mail: pparkh@paleo.ru
Received December 26, 2002
Abstract
—Scars of shell muscles have been discovered on internal molds of helcionelloid mollusks of the fam-
ily Stenothecidae for the first time. The localization of the scars is similar to that in the family Coreospiridae.
Since these two families evolved from the ancestral group independently, similar shell muscle systems of Cam-
brian helcionelloid mollusks may be explained by parallel development.
Key words
: morphology, Cambrian, univalved mollusks, shell muscles.
Explanation of Plate 2
Fig. 1.
Oelandiella korobkovi
Vostokova, 1962; specimen no. 4368/1008, internal mold; Lower Cambrian, lower part of the Tom-
motian Stage, Siberian Platform, southwestern Anabar Region, Kotu
œ
River: (1a) internal mold, right view,
×
26; (1b) umbilical area
with muscle scar,
×
250; (1c) same view, supposed margin of the scar is dotted,
×
250.
Fig. 2.
Anabarella
sp.; specimen no. 2019/1036, internal mold; Lower Cambrian, Botoman Stage; Transbaikalia, Chita Region, vil-
lage of Georgievka: (2a) internal mold, right view,
×
95; (2b) subapical area with muscle scar,
×
370; (2c) same view, supposed mar-
gin of the scar is dotted,
×
370.
Figs. 3 and 4.
Anabarella australis
Runnegar in Bengtson
et al.
, 1990; Lower Cambrian, Botoman Stage, South Australia, Yorke
Peninsula, Parara Limestone; (3) specimen no. 4664/1295, internal mold; Curramulka Quarry: (3a) internal mold, left view,
×
57;
(3b) subapical area with muscle scar,
×
110; (3c) same view, supposed margin of the scar is dotted,
×
110; (4) specimen
no. 4664/1648, internal mold; SYC-101 Borehole, depth 198.5 m: (4a) internal mold, left view,
×
60; (4b) subapical area with muscle
scar,
×
150; (4c) same view, supposed margin of the scar is dotted,
×
150.
PALEONTOLOGICAL JOURNAL
Vol. 38
No. 3
2004
NEW DATA ON THE MORPHOLOGY OF SHELL MUSCLES 255
1‡
Plate 2
300
μ
m1b 1c
30
μ
m
100
μ
m2‡ 2b 2c
3‡ 300
μ
m3b 3c
100
μ
m
4‡ 300
μ
m4b 4c
(Pl. 2, figs. 2, 3). A major part of the muscle scar in
A. australis
lies on the roof of the parietal train and
extends slightly to the posterior surface of the mold. In
addition, the scar goes as far as the lateral surfaces of
the train. The polygonal depressions are 10–15
μ
m in
diameter, and the width of the balks is about 3–5
μ
m.
Thus, muscle scars have been found in members of
the family Stenothecidae for the first time. It is note-
worthy that the position of these scars on the subapical
surface of the molds is also typical of another group of
Cambrian mollusks, i.e., the family Coreospiridae
(Pl. 2, fig. 1).
256
PALEONTOLOGICAL JOURNAL
Vol. 38
No. 3
2004
PARKHAEV
If the phylogenetic development of the Cambrian
univalved mollusks that was assumed by the author ear-
lier (Parkhaev, 2002b) is correct, i.e., the families
Coreospiridae and Stenothecidae evolved from the
ancestral Helcionellidae independently, this is an exam-
ple of parallel evolution of shell muscles in helcionel-
loid mollusks.
It is probable that the same localization of the mus-
cle attachment sites in coreospirids and stenothecids
was achieved by the general modification of the shell
shape in the ancestral helcionellids, which was accom-
panied by the displacement of the shell muscles from
the anterolateral areas to the subapical zone (figure). It
is obvious that, in the case of coreospirids, this dis-
placement is a result of spiral coiling of the shell. In
stenothecids, the shells of which are strongly laterally
compressed, the muscle displacement may be due to
this lateral compression. Today, unfortunately, we have
no data on the position of the muscle scars in the family
Trenellidae, which most probably evolved from Hel-
cionellidae and is ancestral to Stenothecidae (figure).
It is quite possible that trenellids possessed an inter-
mediate morphology in which muscles were located on
the lateral surface within the apical part of the shell.
It is noteworthy that the variability in shell structure
and the numerous cases of parallel development in shell
muscles are fairly common in gastropods. For instance,
a horseshoe-shaped shell muscle occurs among differ-
ent and relatively phylogenetically distant groups of
recent gastropods. In general, with slight modifications,
it is most typical of members of the primitive sub-
classes Scutibranchia and Cyclobranchia but also
occurs in some groups of Pectinibranchia (Capulidae
and Muricidae), Divasibranchia (Siphonariidae), and
even in Pulmonata (Acroloxidae and Planorbidae).
All these cases of independent development of the
horseshoe-shaped shell muscle are due to similar shell
morphogenesis (all the above-mentioned gastropods
have shells with a cap-shaped or similar form).
Obviously, the high plasticity of shell morphology
and, as a result, the plasticity in the structure of the shell
muscles were already present among Cambrian hel-
cionelloid mollusks. Thus, this supplies additional evi-
dence that helcionelloid mollusks belong to gastropods.
A further study of the microsculpture and microstruc-
ture of the shell of Cambrian univalved mollusks will
assist us in reconstructing the major evolutionary
changes in the shell muscular system, thus, in turn, pro-
viding additional material for the study of the evolution
of the earliest gastropods.
ACKNOWLEDGMENTS
I am grateful to A.Yu. Rozanov for critical com-
ments and valuable discussions, to A.V. Mazin for pho-
tographs, and to L.T. Protasevitch for technical assis-
tance in using SEM. The study was supported by the
Russian Foundation for Basic Research (project
nos. 02-04-06198, 00-04-48409, 00-15-97764, and
03-04-06065).
REFERENCES
1. P. Yu. Parkhaev, “Phylogeny and Systematics of the
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2. P. Yu. Parkhaev, “Muscle Scars of Cambrian Univalved
Mollusks and Their Systematic Importance,” Paleontol.
Zh., No. 5, 15–19 (2002b).
3. S. V. Popov, “Shell Microstructure and Systematics of
Cardiids” Tr. Paleontol. Inst. Akad. Nauk SSSR
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4. J. D. Taylor, W.J. Kennedy, and A. Hall, “The Shell
Structure and Mineralogy of the Bivalvia,” Bull. Brit.
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6. A. Williams and A.D. Wright,
Shell Structure of the Cra-
niacea and Other Calcareous Inarticulate Brachiopoda
(Palaeontol. Assoc., London, 1970).
Coreospiridae
Helcionellidae
Trenellidae
(
no data on the position
of shell muscles
)
Stenothecidae
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Full-text available
  • Sep 2002
The structure of the shell muscles among the Cambrian mollusks has been a stumbling block for malacologists studying the morphology and phylogeny of the most ancient members of the phylum Mollusca. Apart from several dubious finds of the muscle scars among very few forms of the helcionelloideans (Runnegar, 1981; Vassiljeva, 1990; 1998, Geyer, 1994), the reliable position of the muscles within their shells remains unknown, and this fact has been repeatedly noted in the literature (Runnegar and Pojeta, 1974; Pojeta and Run-negar, 1976; Yochelson, 1978; Peel, 1991). This cir-cumstance coincides with the fact that, to date, we have obtained extensive data on the helcionelloid mollusks: about 500 of nominal species are already described (personal estimation). It is really surprising that the shells or internal moulds of helcionelloideans have sometimes perfect preservation even with visible ele-ments of the wall microstructure (Runnegar, 1983; Bengtson et al. , 1990; Kouchinsky, 2000) but, at the same time, lack the muscle scars, which can be rather distinctly preserved among some other Cambrian mol-lusks (bivalves, bellerophontids, etc.). To explain such a riddle, I speculated (Parkhaev, 2000) that the absence of the muscle scars on the shell of helcionelloideans could have been caused by a spe-cific position of the muscle attachment zone. Probably, the muscles were attached to the shell on the parietal wall in the subapical area. Therefore, it is really difficult to find the scars in this place due to its minor area, not very common preservation, or overlapping by the sub-sequent whorl (in coiled shells). Recent investigations support this speculation, at least for the spirally coiled forms. Also, the scars of the shell muscles were found on the internal moulds of cap-shaped helcionellids. The result of this study is presented in the paper. MATERIAL The extensive collection (about 5000 specimens) from the Lower Cambrian of South Australia (housed at
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  • Jan 1970
  • A Williams
  • A D Wright
A. Williams and A.D. Wright, Shell Structure of the Craniacea and Other Calcareous Inarticulate Brachiopoda (Palaeontol. Assoc., London, 1970).