By Kevin Bylund
Note: Kevin welcomes discussion on this article in the Cephalopod Fossils forum.
The structure secreted by the mantle of cephalopods for protection or neutral buoyancy is called the Shell
. The complete shell is basically a hollow cone with two major parts, the Body Chamber
, or Living Chamber
, and the Phragmocone
. The opening on the large end is called the Aperture
, and the Apex
is at the tip of the small end. The shell or Test
that forms the cone is called the Shell Wall
Figure 1. Two lateral views of the shell of Nautilus, external on the left and internal on the right.
is the side the hyponome or siphon was
identified by the Hyponomic Sinus
an indentation in the shell to let the hyponome protrude.
is the opposite side. Adoral
, and Forward
are towards the aperture, Adapical
are towards the
adapertural and Posterior
is adapical. Lateral
between ventral and dorsal. Longitudinal
is in an anterior to posterior direction, and Transverse
in a dorsal to ventral direction.
Figure 2. Drawings of an imaginary coiled cephalopod shell.
lateral view on left (Part of the shell is broken away to reveal the suture lines on the internal mold) and apertural view on right.
Drawing of an orthoconic cephalopod shell and internal mold.
The body chamber was the part of the shell occupied by
the living animal. The edge of the aperture is the Peristome
chambers in coiled shells are termed Brevidomic
if they are less than ½
whorl in length, Mesodomic
if between ½ and ¾ whorls, and Longidomic
if more than ¾ whorls.
At maturity several kinds of modifications can occur on the aperture. Lateral Lappets
are projections from the lateral part of the peristome. A Ventral Lappet
projects from the venter. A Constriction
is a necking down; a Contraction
is a closing off. Sometimes the whole body chamber is Expanded
Drawings showing some of the modifications that can
occur on mature shells.
As the animal grew, it occasionally moved forward in the body chamber and secreted a Septum
at the back of the
mantle. This created a series of Chambers
, or Camerae
, called the Phragmocone. Some septa are deposited a short distance along the shell wall,
this part is called the Mural Part
. The Free Part
of the septum is between the mural part and the septal neck.
Figure 5 .
Parts of a Septal Suture line.
The septum is attached to the shell wall along a Suture
seen as a series of simple to complex lines on internal
molds. Parts of the suture line directed
adorally are termed Saddles
those directed adapically are termed Lobes
. Ortho ceratitic
shallow lobes and saddles. Agoniatitic
have broad lobes
and saddles with a narrow mid ventral lobe.
strong, mostly angular lobes and angular to rounded saddles. Ceratitic
have strong rounded saddles and serrated lobes. Ammonitic
have complex lobes and saddles.
Types of suture lines.
formulas are sometimes used to describe suture patterns (mostly
ammonoids, and only briefly described here).
The primary lobes are given a letter designation, E
for the External (ventral) Lobe, L
for the Lateral
Lobe and I
for the Internal (Dorsal) Lobe.
Adventitious Lobes, lobes forming later, between E
, are lettered A
accordance with their ontogenetic appearance.
New lobes appearing between L
are Umbilical Lobes, lettered U
numbered the same way.
In fossil cephalopod shells, anything that was in contact with the
siphuncular chord is considered part of the Siphuncle
. The non-living part of the siphuncle that
covers the living siphuncular chord is termed the Ectosiphuncle
it is composed of the septal neck and the connecting ring. The area, and any structures,
inside the ectosiphuncle are termed the Endosiphuncle
The Septal Neck
is where the siphuncle
passes through the septum. Septal necks
directed adorally are termed Prochoanitic
those directed adapically are termed Retrochoanitic
. Several other terms are used for
retrochoanitic necks. Acoanitic
necks are barely developed or
extremely short. Loxochoanitic
necks point inward at moderate lengths. Ortho choanitic
necks are directed adapically and are less
than ½ the
length of the chamber. Hemichoanitic
necks extend ½ to ¾ the
length of the chamber. Subholochoanitic
necks curve inward
just before reaching the next septum. Holochoanitic
necks reach to the next
septum or slightly beyond. Macrochoanitic
necks extend longer than
the distance to the next septum. Suborthochoanitic
necks are barely
are recurved, some almost touching the free
part of the septum.
Types of Septal Necks.
are tubular structures connecting the
septa or septal necks. Some rings are thin and
simple, others are thick and composed of two or three layers of
deposits. Connecting rings can be
convex or bulbous.
Several types of connecting rings (in red).
Some Cephalopods, mostly Nautiloids, deposited calcareous structures inside the shell, probably for buoyancy and attitude control. Cameral
were deposited inside the chambers, Mural
are on the shell wall, Episeptal Deposits
on the adapertural side of the septum and Hyposeptal Deposits
on the adapical side of the septum. Endosiphuncular
include cone shaped Endocones
, transverse partions called Diaphrams
are round structures
laying on the ventral wall of the siphuncle, Annulosiphonate
deposits are donut shaped deposits inside the siphuncle, and Parietal
looking like and attached to the inside of the septal neck.
Figure 9. Cameral deposits (in blue).
Endosiphuncular deposits (in blue).
Nautiloid shells can be Planispirally Coiled
(coiled in one plane) or straight, curved, open spiral etc., ammonoids
Planispirally coiled or have an open spiral are termed Heteromorphs
. Curved or coiled shells are Exogastric
if the ventral side, or Venter
, is convex and on the
side, and Endogastric
if the dorsal
side, or Dorsum
, is convex and on
the outer side.
Figure 11. Cartoon showing Exogastric and Endogastic coiling.
cross sectional shape, or Whorl Section
can be Round
like a lance point), Fastigate
towards the venter), Tabulate
a flattened venter) or some variation of each.
shells are shorter
laterally, and Depressed
Figure 12. Common whorl section shapes.
is one complete volution of a coiled shell. The
space enclosed on both sides by the last whorl is termed the Umbilicus
. Shells with a
wide umbilicus are termed Evolute
and shells with a
narrow umbilicus are termed Involute
. The umbilical
is where the shell wall attaches to the preceding whorl. The Umbilical
is between the umbilical shoulder and the umbilical seam. The Umbilical
is where the shell wall bends toward the preceding whorl. The Ventrolateral
is where the shell bends toward the venter, and the Side
, is between the
ventrolateral shoulder and the umbilical
Figure 13. Cross
section of a coiled shell showing parts and
Dimensions most commonly used for the description of coiled shells are the
, whorl width, W
umbilical diameter, U
, the umbilical
. Lately it has
become appropriate to have an
arrow pointing at the last septum, if visible.
Straight shells are Ortho cones
, curved shells are Cyrtocones
either of these could be long, Longiconic
, or short, Breviconic
shells that make at least one volution are termed Gyrocones
. Coiled shells
that touch or are just barely impressed by the preceding whorl are Tarphycones
are very evolute with many subcircular or depressed whorls. Involute to moderately involute shells with
subrectangular, compressed whorls are termed Platycones
. Shells that are
involute with subtriangular, compressed whorls are Oxycones
shells with an
oval whorl section. Spherocones
are subglobular with a small umbilicus and subcircular
subglobular with an open, angular, umbilicus. Planorbicones
are evolute with relatively few subcircular depressed whorls. Ancylocones
have open or closed, planispiral or helical early whorls followed
helical whorls. Shells that form two or
more straight shafts
are called Hamiticones
worm like shells are termed Vermicones
Figure 14. Irregular or Heteromorphic shell shapes.
Figure 15. Planispiral shell shapes.
All cephalopod shells are ornamented with at least Growth Lines
each one representing a former position of the peristome.
are usually radial folds of the shell so
they are equally apparent on internal molds, sometimes they are thickenings of the outer part of the
shell and don’t show on internal molds. Ribs directed radially are Rectiradiate
those inclined forward are Prorsiradiate
those inclined backward are Rursiradiate
Ribs can be Dense
, closely spaced, or Distant
widely spaced. Sinuous
snake across the flanks, Falcate
ribs are generally falcate, Projected
ribs are inclined forward on the outer portion. Branching
ribs have Secondary Ribs
from Primary Ribs
ribs have other ribs branching
from a single inclined rib. Intercalated
are ribs not
connected to other ribs. Bundled
ribs are connected at one dorsal
alternately connected at the dorsal and ventral
connected at both ends.
Figure 16. Type, direction and spacing of ribs.
are internal shell thickenings and usually only show on the internal mold as sinuous transverse grooves.
are small, usually longitudinal, raised portions of the shell separated by striae
, small grooves. If they are strong enough they will show on
and other Nodes
are present on some shells. Nodes on internal molds are commonly the bases of Spines
. Spines were usually formed hollow on the peristome and sealed later as the shell grew. Tubercles elongated radially are termed Bullae
and those elongated longitudinally are termed Clavi
A raised longitudinal ridge on the venter is called a Keel
. Keels can be Entire
. Sometimes a Furrow
or groove can be found on
each side of the keel.
A large, deep, longitudinal groove is called a Sulcus
, and can be found on the venter or in a lateral position.
Figure 17. Ornamentation other than ribs.
Arkell, W. J., 1957,
Introduction to Mesozoic Ammonoidea, in:
Invertebrate Paleontology, Part L, Mollusca 4, Cephalopoda-Ammonoidea,
by R. C. Moore, Geological Society of America and University of Kansas
Flower, R.H., 1964,
Shell Morphology, New Mexico Bureau of Mines & Mineral Resources
Memoir 13, p.
Miller, A. K., Furnish, W. M.
and Schindewolf, O. H.,
Treatise on Invertebrate Paleontology, Part L, Mollusca 4,
Cephalopoda-Ammonoidea, Edited by R. C. Moore, Geological Society of
and University of Kansas Press, p. L11-L20
Pojeta, J. Jr. and Gordon, M.
Cephalopoda, in: Fossil
by Boardman, R. S., Cheetham, A. H. and Rowell, A. J., Blackwell
Publications, p. 329-358
Teichert, C., 1964,
of Hard Parts, in: Treatise on
Invertebrate Paleontology, Part K, Mollusca 3, Cephalopoda-General
Edited by R.
C. Moore, Geological Society of America and University of Kansas Press,
Westermann, G. E. G., 1996,
Ammonoid Life and Habitat, in: Ammonoid
Volume 13 of Topics in Geobiology, Edited by Landman, N. H., Tanabe, K.
Davis, R. A., Plenum Press, New York, p. 607-707
Wiedmann, J. and Kullmann, J.,
Ammonoid Sutures in
Ontogeny and Phylogeny, in: The
Ammonoidea, Systematics Association Special Volume No. 18, Edited by
R. and Senior, J. R., Academic Press, London and New York, p. 215-255