Soft-tissue preservation in the Middle Jurassic ammonite Cadoceras from Central Russia

Abstract

The findings of fossilized ammonite soft tissues are extremely rare, so each specimen may be important for understanding the anatomy of these cephalopods. This paper deals with soft tissue fragments and imprints preserved in the rear part of the body chamber of the Middle Jurassic ammonite Cadoceras stupachenkoi from Central Russia. At the base of the body chamber of this ammonite in front of the last septum, a mantle fragment with clearly visible longitudinal fibers and imprints of the palliovisceral ligament are preserved. In front and slightly to the side of the mantle fragment, a small area with branched structures is located; probably, these structures are fragments of gills. In general, the structure of the soft tissues in the rear part of the ammonite body looks very similar to that of modern nautilids, with one exception: mantle fibers are not directed forward as observed in Nautilus, but to the mid-ventral line, probably to the ventral muscle.

Full text

1 23
Swiss Journal of Palaeontology
ISSN 1664-2376
Volume 134
Number 2
Swiss J Palaeontol (2015) 134:281-287
DOI 10.1007/s13358-015-0082-1
Soft-tissue preservation in the Middle
Jurassic ammonite Cadoceras from Central
Russia
Aleksandr A.Mironenko

1 23
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Soft-tissue preservation in the Middle Jurassic ammonite
Cadoceras from Central Russia
Aleksandr A. Mironenko
1
Received: 19 February 2015 / Accepted: 19 May 2015 / Published online: 21 July 2015
ÓAkademie der Naturwissenschaften Schweiz (SCNAT) 2015
Abstract The findings of fossilized ammonite soft tissues
are extremely rare, so each specimen may be important for
understanding the anatomy of these cephalopods. This
paper deals with soft tissue fragments and imprints pre-
served in the rear part of the body chamber of the Middle
Jurassic ammonite Cadoceras stupachenkoi from Central
Russia. At the base of the body chamber of this ammonite
in front of the last septum, a mantle fragment with clearly
visible longitudinal fibers and imprints of the palliovisceral
ligament are preserved. In front and slightly to the side of
the mantle fragment, a small area with branched structures
is located; probably, these structures are fragments of gills.
In general, the structure of the soft tissues in the rear part of
the ammonite body looks very similar to that of modern
nautilids, with one exception: mantle fibers are not directed
forward as observed in Nautilus, but to the mid-ventral
line, probably to the ventral muscle.
Keywords Ammonoids Middle Jurassic Soft tissues 
Cadoceras Russia
Introduction
The study of fossilized soft tissue allows better under-
standing of the anatomy and biology of ancient animals.
Findings of soft tissues are particularly important for the
study of completely extinct groups, such as ammonoids,
which have left no descendants. Unfortunately, fossilized
ammonoid tissues are extremely scarce and many parts of
the ammonoid body (e.g., arms, hyponome) have never
been found. Nevertheless, several non-mineralized organs
of ammonoids such as gills, oesophagus, digestive tract,
cephalic cartilage with questionable eye capsules, mantle
tissues, and siphuncular blood vessels were found and
described (Lehmann 1967,1979,1985; Lehmann and
Weitschat 1973; Riegraf et al. 1984; Tanabe et al. 2000;
Doguzhaeva et al. 2004,2007; Wippich and Lehmann
2004; Klug and Jerjen 2012; Klug et al. 2012). Mantle
tissues can be considered as one of the rarest known types
of ammonite soft tissues: fragments of the mantle with
preserved muscle structure have been described only twice,
both times in the Late Triassic ammonoid Austrotrachyc-
eras (Doguzhaeva et al. 2004,2007). The structures which
are located at the rear part of the ammonoid body chamber,
such as ventral and dorsolateral muscle scars are well
studied (Doguzhaeva and Mutvei 1996; Klug et al. 2007),
but usually, only hard parts of the shell or occasionally
unstructured phosphatized remnants (Klug et al. 2007)
rather than the soft tissues itself are preserved.
This article describes the preserved fragment of mantle
tissue, imprints of the palliovisceral ligament and putative
remnants of gills, found in the rear part of the body
chamber of the Middle Jurassic (Lower Callovian)
ammonite Cadoceras stupachenkoi from Central Russia.
The findings of soft tissues and their imprints allow better
understanding of the structure of the rear part of the
ammonoid soft body.
Materials and methods
The specimen studied herein is a Middle Jurassic ammonite
Cadoceras stupachenkoi (Fig. 1). It is a cadiconic macro-
conch, which was found in Middle Jurassic deposits
&Aleksandr A. Mironenko
paleometro@gmail.com
1
Kirovogradskaya st. 28, 117519 Moscow, Russia
Swiss J Palaeontol (2015) 134:281–287
DOI 10.1007/s13358-015-0082-1
Author's personal copy

(Lower Callovian, Elatmae Zone, Stupachenkoi Subzone)
in the Unzha-river region, not far from the town of
Makaryev in Russia (Keupp and Mitta 2013: Fig. 2). The
specimen comes from a layer of calcareous sandstone
nodules, often phosphatized, with inclusions of pyrite (see
Keupp and Mitta 2013 for taphonomy and geological set-
ting). The diameter of the specimen is about 10 cm. Only
the posterior part of the body chamber with a small frag-
ment of the phragmocone is preserved. The aragonitic shell
layers were partially preserved, but removed for exami-
nation of the internal mould of the body chamber. The
specimen is housed at Moscow State University Museum,
Russia, with the collection number MSU 119.
The ammonite was studied using a binocular microscope
and a scanning electron microscope SEM TESCAN//
VEGA with a BSE detector at the Paleontological Institute
of the Russian Academy of Science in Moscow. It was
examined in an uncoated state in low vacuum conditions at
30 kV.
Results
A poorly preserved ventral attachment scar and the anterior
border of the annular elevation are located at a distance of
14 and 30 mm from the last siphuncle tube, respectively
(Fig. 1). Along the front edge of the annular elevation near
the ventral muscle attachment scar, a small, presumably
carbonized, piece of mantle tissue is located. It is 3–7 mm
wide, dark grey, and visible to the naked eye. The binocular
observation allows to recognize long branched longitudinal
fibers (Fig. 2). This mantle tissue is very thin with only one
layer of muscle fibers. All these fibers are directed to the
mid-ventral line of the body chamber. On the SEM images,
the remnants of dark carbonized tissues and small fibers
branching off from the large muscles are visible (Fig. 3).
Behind the anterior border of the annular elevation
(mantle myoadhesive band) and the mantle fragment,
imprints of the palliovisceral ligament are located (Fig. 4).
This area shows a double-layered structure: there are small
transverse stripes on the top layer and beneath them rough
and sharp transverse folds (Fig. 4b). There are no fossilized
soft tissues in these rough folds, but imprints of these tis-
sues composed of middle-grained sandstone, which fills the
body chamber.
The third type of preserved soft tissues is small branched
feather-like structures located in front of the annular ele-
vation. They are very tiny and mostly grouped in small
cluster, the size of the entire cluster is not more than 1 cm
2
(Fig. 5a, b). Nearby, separate branched structures are
located outside of this cluster at the rear part of the body
chamber. However, in the cluster, the structures are better
preserved and concentrated. The branched structures are
arranged in several layers. In SEM images, small trans-
verse ridges are visible in these objects (Fig. 5c, d). These
structures resemble small parts of gills.
Fig. 1 Cadoceras stupachenkoi with fossilized fragments of the soft tissues. aOverview over the studied specimen MSU 119/1. bScheme of the
specimen MSU 119/1
282 A. A. Mironenko
Author's personal copy

Discussion
Mantle tissue and imprints of the palliovisceral
ligament
Inside the ammonite body chambers, not only ammonite
body remnants can be found, but also the fragments or
intact shells of other animals, which lived inside empty
ammonite shell on the sea bottom, or were transported into
the empty shell by sea currents (Fraaye and Ja
¨ger 1995a,b;
Klompmaker and Fraaije 2012; Vullo et al. 2009). In
several cases, the clusters of small invertebrate fragments
were interpreted as ammonite crop content (Keupp 2000;
Ritterbush et al. 2014). In addition, different epicoles can
be attached to the inner walls of the empty ammonite body
chamber (Klug and Korn 2001). The scavengers which ate
ammonite bodies or animals which lived inside empty
shells left their traces, e.g., burrows, fecal pellets, etc.,
(Fraaye and Ja
¨ger 1995a). All of these findings can be
confused with the remains of the ammonite soft body.
However, there is no doubt that the fossilized fragment in
the Cadoceras body chamber is in fact part of the
Fig. 2 Fragment of the mantle tissue of the Cadoceras stupachenkoi. aOverview of the mantle fragment. Scale bar 2.5 mm. b,c, Longitudinal
muscle fibers in the mantle fragment. Scale bars 1 mm. dTwo muscle fibers. Scale bar 0.5 mm
Soft-tissue preservation in the Middle Jurassic ammonite Cadoceras from Central Russia 283
Author's personal copy

ammonite mantle (see Allison 1988 for possible mecha-
nisms of soft-tissue fossilization). Its structure (Figs. 2,3)
is very similar to the structure of the longitudinal muscles
of fossil coleoids (Allison 1988: Fig. 5B) and mantle
muscles of living nautilids (Mutvei et al. 1993: Fig. 9B),
but it does not resemble any traces of epicoles or scav-
engers. Its position on the anterior part of the annular
elevation fully corresponds to the attachment area of
mantle muscles (Mutvei 1957; Mutvei et al. 1993).
In the mantle tissue fragment of the Cadoceras, muscle
fibers are directed to the mid-ventral line of the body
chamber. In the rear part of the mantle of recent Nautilus
pompilius, mantle fibers are pointing towards the aperture
(Mutvei et al. 1993: Fig. 9B). It appears unlikely that the
direction of the fibers in the ammonite is a result of
postmortem shifting. Although the shell orientation of the
ammonite carcass on the sea floor during the decomposi-
tion of its soft tissues is unknown, all preserved soft tissue
remnants are located on the right side of the body chamber.
Therefore, this side was most likely lower during the burial
of the shell. In this case, if the muscles shifted downward
under the influence of gravity, they must have shifted to the
right side, not to the mid-ventral line of the body chamber
as it actually is preserved. Therefore, the preserved orien-
tation of the muscle fibers might represent the syn vivo
position. Nevertheless, more material with this kind of
preservation is needed to support this hypothesis. Possibly,
these muscles were connected to the ventral muscle which
was directed forward from the ventral attachment scar, as
was earlier suggested by other authors (Jordan 1968; Dagys
Fig. 3 SEM images of the mantle tissue fragment of the Cadoceras stupachenkoi.aLarge longitudinal fibers and carbonized tissue among them.
Scale bar 500 lm. bSets of small fibers. Scale bar 50 lm
Fig. 4 Area of the palliovisceral ligament. aOverview over the area of the palliovisceral ligament. bDetail of the sharp transverse folds and
small stripes in this area. Scale bar 2.5 mm
284 A. A. Mironenko
Author's personal copy

and Keupp 1998). If this orientation of the mantle muscles
represents the syn vivo-orientation, it would resemble the
connection of the inner mantle layers of coleoids with the
ventral mantle adductor muscle (Bizikov 2004). While the
coleoid mantle is thick, the ammonite mantle appears to be
very thin, containing possibly, only one layer of muscle
tissue similar to nautilids. Unfortunately, the direction of
the fibers, located far from the central part of the myoad-
hesive band, remains unknown.
The structure of the palliovisceral ligament of Cado-
ceras resembles that described from nautilids (Mutvei et al.
1993). In general, the entire rear part of the Cadoceras soft
body is very similar to the corresponding part of the living
and ancient Nautilida (see Mutvei 1957,1964; Mutvei et al.
1993; Klug and Lehmkuhl 2004) with the exception of the
ventral muscle attachment structure. As in Nautilus, the
ammonite mantle, which was attached to the myoadhesive
band, is thin, with clearly separate and distinguishable long
fibers. However, this similarity does not mean that the front
end of the ammonite mantle was identical to the mantle of
nautilids. Several ammonite shells have parabolic nodes
(Bucher et al. 1996; Doguzhaeva 2012) and adult apertural
modifications (e.g., lappets; see Makowski 1962), which
have never been observed in modern or ancient nautilids.
The presence of such structures in ammonite shells may
indicate that the ammonite mantle edge was different from
the mantle edge of nautilids and probably was more mus-
cular and complex. However, findings of preserved anterior
Fig. 5 Gill fragments of Cadoceras stupachenkoi. aOverview of the specimen with marked location of gill imprints. bDetail of the gill
imprints. Scale bar 2 mm. c, d SEM images of the gill imprints. Scale bars 250 and 500 lm, respectively
Soft-tissue preservation in the Middle Jurassic ammonite Cadoceras from Central Russia 285
Author's personal copy

parts of the mantle are needed to clarify these assumptions;
in the specimen studied herein, there are no traces of this
part of the mantle.
Presumable ammonite gills
The author considered several versions of the origin of
small branched structures located in front of the annular
elevation (Fig. 5): imprints of scavenger jaw apparatus,
which was used to eat the ammonite mantle; fragments of
the mantle tissue; fragments of the gills. The first version,
which interprets these structures as bite or radula marks of
scavengers, seems to be unlikely due to the shape and
layered structure of these objects. The idea that these
structures are remnants of decomposed mantle tissue can-
not be completely ruled out, but it seems unlikely, since all
these objects are about the same size and shape. It is more
likely that these fragments are remnants of the ammonite
gills. Probably, these gill fragments came to rest on the
shell wall after the local decomposition of the mantle.
Structures interpreted as ammonite gills were described
several times (Lehmann and Weitschat 1973; Lehmann
1979,1985). The findings of fossilized gills of coleoid
cephalopods are also known (e.g., Reitner 2009). However,
the microstructure of fossilized gills has never been
depicted and described. In the case of Cadoceras, if the
objects described herein are actually the remains of gills,
they are only small fragments, because the length of each
object is about 1–1.2 mm.
Recent Nautilida (Nautilus and Allonautilus) have two
pairs of gills, whereas all coleoids have only one pair. The
number of ammonoid gills is still unknown. Shigeno et al.
(2008) showed that the two pairs of Nautilus gills do not
form simultaneously, but successively. This adds additional
weight to the hypothesis that earliest cephalopods had one
pair of gills (Engeser 1996; Sasaki et al. 2010), whereas, a
second pair appeared later during evolution, likely as an
adaptation to low concentrations of oxygen in nautilid
habitats (Wells et al. 1992). Due to this assumption and
closer phylogenetic relationship of ammonoids and
coleoids (Engeser 1996; Jacobs and Landman 1993; Kro
¨ger
et al. 2011; Ritterbush et al. 2014), it is now widely
accepted that ammonoids likely have only one pair of gills.
Currently, it is impossible to clarify this question by the
examination of the herein described Cadoceras specimen,
as just a few parts of its gills are preserved. However, these
fragments can help to clarify the position of the ammonoid
gills: they were located (at least their posterior parts) very
deep inside the body chamber, not far from the last septum
(unless they were translocated post mortem). It may reflect
the great length of the mantle cavity of Cadoceras.This
fact should be taken into account for reconstructions of
ammonoid anatomy and calculations of the lifetime
orientation of ammonite shells (for recent calculations of
Cadoceras hatchling see Lemanis et al. 2015).
Conclusions
The fragments of the mantle, gills, and soft tissue imprints
preserved in the rear part of the Cadoceras stupachenkoi
body chamber indicate a similarity of the apical parts of the
soft body of ammonoids and nautilids. However, some
differences are observed: muscle fibers of the ammonite
mantle are not directed forward to the aperture as in
Nautilus, but to the center of the ventral side, likely to the
ventral muscle. The number of ammonite gills remains
unknown, but findings not far from the rear part of the
mantle of their fragments, indicate a large ammonoid
mantle cavity size.
Acknowledgments I am very grateful to Dmitry Buev (Moscow,
Russia) for donating the valuable Cadoceras specimen. SEM photos
were made with the generous help by Roman Rakitov (PIN RAS,
Moscow, Russia). I am also very grateful for the important and very
helpful comments from Christian Klug (Pala
¨ontologisches Institut
und Museum, Universita
¨tZu
¨rich, Switzerland), Rene
´Hoffmann
(Ruhr Universita
¨t Bochum, Germany), and an anonymous reviewer,
their comments helped to greatly improve the article.
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