Fossil Shell Morphology
The Terminology used in the Description of Externally Shelled Cephalopods (Nautiloids and Ammonoids)
By Kevin Bylund
Note: Kevin welcomes discussion on this article in the Fossils and History forum.
The structure secreted by the mantle of cephalopods for protection or neutral buoyancy is called the Shell or Conch. 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.
Figure 2. Drawings of an imaginary
Figure 3. Drawing of an orthoconic cephalopod shell and internal mold.
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 or Rostrum projects from the venter. A Constriction is a necking down; a Contraction is a closing off. Sometimes the whole body chamber is Expanded.
Figure 5. Parts of a Septal Suture line.
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, and
those directed adapically are termed Lobes.
Figure 6. 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 and
L, are lettered A and numbered
accordance with their ontogenetic appearance.
New lobes appearing between L and
I are Umbilical Lobes, lettered U, and
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.
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.
Figure 7. Types of Septal Necks.
Connecting Rings 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 straight, concave, convex or bulbous.
Figure 8. Several types of connecting rings (in red).
Cephalopods, mostly Nautiloids, deposited calcareous structures inside
shell, probably for buoyancy and attitude control. Cameral
Deposits were deposited inside the chambers, Mural
Deposits 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
cone shaped Endocones,
longitudinal Lamellae, transverse partions called Diaphrams, Rods are round structures
laying on the ventral wall of the siphuncle, Annulosiphonate
deposits are donut shaped deposits inside the siphuncle, and Parietal are deposits
looking like and attached to the inside of the septal neck.
Cameral deposits (in blue).
Figure 10. Endosiphuncular deposits (in blue).
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.
Cartoon showing Exogastric and Endogastic coiling.
cross sectional shape, or Whorl Section,
can be Round, Oval, Square,
like a lance point), Fastigate (tapering
towards the venter), Tabulate (with
a flattened venter) or some variation of each.
Compressed shells are shorter
laterally, and Depressed shells are
Common whorl section shapes.
A Whorl 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
umbilicus are termed Involute. The
Seam is where the shell wall attaches to the preceding whorl. The Umbilical
Wall is between the umbilical shoulder and the umbilical seam. The Umbilical
Shoulder is where the shell wall bends toward the preceding whorl. The Ventrolateral
Shoulder is where the shell bends toward the venter, and the Side or Flank, is between the
ventrolateral shoulder and the umbilical
Figure 13. Cross section of a coiled shell showing parts and common dimensions.
most commonly used for the description of coiled shells are the
diameter, D, whorl width, W, whorl
height, H, the
umbilical diameter, U, the umbilical
ratio, U/D. Lately it has
become appropriate to have an
arrow pointing at the last septum, if visible.
Planispiral shell shapes.
cephalopod shells are ornamented with at least Growth Lines,
each one representing a former position of the peristome.
Constrictions are internal shell thickenings and usually
on the internal mold as sinuous transverse grooves.
small, usually longitudinal, raised portions of the shell separated by striae, small grooves. If
they are strong enough they will show on
Ornamentation other than ribs.
Arkell, W. J., 1957, Introduction to Mesozoic Ammonoidea, in: Treatise on Invertebrate Paleontology, Part L, Mollusca 4, Cephalopoda-Ammonoidea, Edited by R. C. Moore, Geological Society of America and University of Kansas Press, p. L80-L100
Flower, R.H., 1964, Nautiloid
Miller, A. K., Furnish, W. M. and Schindewolf, O. H., 1957, Paleozoic Ammonoidea, in: Treatise on Invertebrate Paleontology, Part L, Mollusca 4, Cephalopoda-Ammonoidea, Edited by R. C. Moore, Geological Society of America and University of Kansas Press, p. L11-L20
Pojeta, J. Jr. and Gordon, M. Jr., 1987, Class Cephalopoda, in: Fossil Invertebrates, Edited by Boardman, R. S., Cheetham, A. H. and Rowell, A. J., Blackwell Scientific Publications, p. 329-358
Teichert, C., 1964, Morphology of Hard Parts, in: Treatise on Invertebrate Paleontology, Part K, Mollusca 3, Cephalopoda-General Features-Endoceratoidea-Actinoceratoidea-Nautiloidea-Bactritoidea, Edited by R. C. Moore, Geological Society of America and University of Kansas Press, p. K13-K59
Westermann, G. E. G., 1996, Ammonoid Life and Habitat, in: Ammonoid Paleobiology, Volume 13 of Topics in Geobiology, Edited by Landman, N. H., Tanabe, K. and Davis, R. A., Plenum Press, New York, p. 607-707
Wiedmann, J. and Kullmann, J.,
1980, 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
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