Geology Reference
In-Depth Information
10.2.4.4 Cephalopods
still preserved after dissolution of the aragonite shell
crystals, compaction may result in the flattening of
cephalopod shells (Pl. 90/3) or replacement of the ara-
gonite biocrystals by phosphate, silica or pyrite. Finally,
the organic shell may disappear, leaving a mold that
can later be infilled by sediment of cements (Pl. 90/5).
In a meteoric environment, the aragonitic shells of ceph-
alopods are greatly affected by dissolution. Leached
shells with High-Mg calcite micrite envelopes create
molds; the molds are cemented by Low-Mg calcite crys-
tals coarsening towards the center of the cavity or by
blocky calcite.
Cephalopods are mobile marine mollusks that have
adapted to a swimming mode of life. Major groups are
distinguished by their soft parts and shell morphology:
Nautiloidea (Late Cambrian to Holocene) and some re-
lated Paleozoic groups, Ammonoidea (Early Devonian
to Cretaceous) and Coleoidea (Early Devonian to Ho-
locene) with the extinct belemnites (Carboniferous to
Eocene but most abundant in the Jurassic and Creta-
ceous where the group is used as index fossils).
Morphology:
The skeleton of nautiloids and am-
monoids consists basically of a straight, curved, spiral
or coiled calcareous cone. The cone is chambered by
curved, transverse partitions (septa) that unite the in-
ner shell walls. The lines of junction of the septa and
the shell walls (sutures) are simple, circular or undu-
lated in nautiloids and related groups, but fluted, crenu-
lated and complex in ammonoids. The first spherical
or ovoid chamber is called protoconch, the remaining
chambers of the shell form the phragmocone, and the
large unchambered space between the last septum and
the apertura is the body chamber. The chambers of the
phragmocone communicated via a tube (sipho) pass-
ing through a perforation in the septa and positioned in
the center or at the margin of the shell.
The belemnites are characterized by an internal skel-
eton consisting of the chambered phragmocone, which
was partly contained within a cavity at one end of a
solid calcitic guard (rostrum). The latter is commonly
the only preserved part.
Cephalopods in thin sections:
Fossil cephalopods
have coiled or straight chambered external shells (am-
monoids, nautiloids), or internal shells (belemnoids).
Coiled forms are usually planispiral forming thin-sec-
tion patterns comparable to those in planispirally coiled
gastropods (Pl. 89/4). However, cephalopod shells are
distinguished by having involute coiling (last spiral cov-
ers almost all previous ones).
In thin sections ammonites are represented by ran-
dom sections through fragments (Pl. 90) and sometimes
abundant sections of juvenile ammonites. Due to the
primary aragonitic mineralogy larger shells are usually
recrystallized or completely dissolved. Juvenile ammo-
nites i.e. embryonal or larval stages are more common,
particularly in Devonian and Mesozoic open-marine
wackestones. Sections of these very small fossils (dia-
meter of the shell about 0.5 mm) are characterized by a
large globular protoconch followed by one, rarely two
whorls subdivided by thin septa. Sections lacking septa,
can hardly be distinguished from those of trochospiral
dwarf gastropods. The occurrences of abundant juve-
nile ammonites can indicate the rapid death of plank-
ton.
Shell structure, diagenesis and preservation:
The
shells of nautiloids and ammonoids were aragonitic.
The wall is layered, exhibiting an outer prismatic layer
and inner nacreous and prismatic layers. The shells are
frequently recrystallized or leached and subsequently
infilled by calcite cement or sediment. Aptychi of am-
monoids and the rostra of belemnites consist of calcite
(Pl. 90/4, 6).
The diagenesis of cephalopod shells has been stud-
ied by Dullo and Bandel (1988): Preburial diagenesis
is characterized by the rapid decomposition of the or-
ganic tissue and the settlement of endolithic organisms,
which produces a highly porous structure or micritic
envelopes. The decay of organic shell material (e.g. of
the siphuncular tube) leads to accessibility for sediment
infilling. Aragonitic cements grow syntaxially upon ara-
gonitic microstructural parts of the shell. Burial diagen-
esis comprises the dissolution of aragonite and the loss
of the mineralized shell, often long before the sediment
is compacted. If the intercrystalline organic tissue is
Cephalopod limestones
Limestones characterized by abundant ammonoid
and nautiloid shells
are common in the Paleozoic and
Mesozoic (Pl. 90/1, 2). The limestones originated on
pelagic platforms, deep shelves or ridges of various ori-
gin (submarine volcanoes, drowned reefs, structural
highs), platform slopes and basins. Estimates of depo-
sitional depths of Paleozoic and Mesozoic open-ma-
rine ammonoid limestones are controversial and range
between some tens and several hundreds of meters to
several thousands of meters. There is good evidence,
that the depths of platforms on which Devonian cepha-
lopod limestones in Europe and North Africa accumu-
lated reached an order of several tens to about one hun-
dred meters (Wendt and Aigner 1985). Commonly these
carbonates were formed during times of reduced sedi-
