Fossil Shell Morphology

The Terminology used in the Description of Externally Shelled Cephalopods (Nautiloids and Ammonoids)

Figure 1. Two lateral views of the shell of Nautilus, external on the left and internal on the right.

Orientation

 Ventral is the side the hyponome or siphon was on, usually identified by the Hyponomic Sinus, an indentation in the shell to let the hyponome protrude.  Dorsal is the opposite side.  Adoral, Adapertural, and Forward, are towards the aperture, Adapical and Backward are towards the apex.  Anterior is adapertural and Posterior is adapical.  Lateral is between ventral and dorsal.  Longitudinal is in an anterior to posterior direction, and Transverse is 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. 

Figure 3. Drawing of an orthoconic cephalopod shell and internal mold.

Body Chamber

             The body chamber was the part of the shell occupied by the living animal.  The edge of the aperture is the Peristome.  Body 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 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 4. Drawings showing some of the modifications that can occur on mature shells.

 
Phragmocone

 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, and those directed adapically are termed Lobes.  Orthoceratitic Sutures are relatively simple having shallow lobes and saddles.  Agoniatitic Sutures have broad lobes and saddles with a narrow mid ventral lobe.  Goniatitic Sutures have strong, mostly angular lobes and angular to rounded saddles.  Ceratitic Sutures have strong rounded saddles and serrated lobes.  Ammonitic Sutures have complex lobes and saddles.

Figure 6. Types of suture lines.

Sutural formulas are sometimes used to describe suture patterns (mostly Paleozoic 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 consecutively in accordance with their ontogenetic appearance.  New lobes appearing between L and I are Umbilical Lobes, lettered U, and numbered the same way.

Siphuncle

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.  Orthochoanitic 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 recurved.  Cyrtochoanitic necks are recurved, some almost touching the free part of the septum.

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).

Deposits

Some Cephalopods, mostly Nautiloids, deposited calcareous structures inside the 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 deposits include 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.

Figure 9. Cameral deposits (in blue).

Figure 10. Endosiphuncular deposits (in blue).

Shell Shape

Nautiloid shells can be Planispirally Coiled (coiled in one plane) or straight, curved, open spiral etc., ammonoids not 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 outer side, and Endogastric if the dorsal side, or Dorsum, is convex and on the outer side.

Figure 11. Cartoon showing Exogastric and Endogastic coiling.

The cross sectional shape, or Whorl Section, can be Round, Oval, Square, Rectangular, Triangular, Lanceolate (shaped 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 shorter ventro-dorsally.

Figure 12. 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 narrow umbilicus are termed Involute.  The umbilical 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 shoulder.

Figure 13. Cross section of a coiled shell showing parts and common dimensions.

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.
            Straight shells are Orthocones, curved shells are Cyrtocones, either of these could be long, Longiconic, or short, Breviconic.  Curved 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.  Serpenticones 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.  Discocones have involute shells with an oval whorl section.  Spherocones are subglobular with a small umbilicus and subcircular whorls.  Cadicones are 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 by a hook.  Torticones have helical whorls.  Shells that form two or more straight shafts are called Hamiticones.  Irregular, worm like shells are termed Vermicones.

Figure 14. Irregular or Heteromorphic shell shapes.

Figure 15. Planispiral shell shapes.

 
Ornamentation

All cephalopod shells are ornamented with at least Growth Lines, each one representing a former position of the peristome.
            Ribs 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, and those inclined backward are Rursiradiate.  Ribs can be Dense, closely spaced, or Distant, widely spaced.  Sinuous ribs snake across the flanks, Falcate ribs are sickle-shaped, Falcoid ribs are generally falcate, Projected ribs are inclined forward on the outer portion.  Branching ribs have Secondary Ribs that branch from Primary Ribs.  Virgatotone ribs have other ribs branching from a single inclined rib.  Intercalated Ribs are ribs not connected to other ribs.  Bundled ribs are connected at one dorsal point.  Zigzag ribs are alternately connected at the dorsal and ventral ends.  Looped ribs are connected at both ends.

Figure 16. Type, direction and spacing of ribs.

Constrictions are internal shell thickenings and usually only show on the internal mold as sinuous transverse grooves.  Lirae are small, usually longitudinal, raised portions of the shell separated by striae, small grooves.  If they are strong enough they will show on internal molds.
            Tubercles 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 (smooth), Serrated or Clavate.  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.

Sources

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 Shell Morphology, New Mexico Bureau of Mines & Mineral Resources Memoir 13, p. 1-76

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 House, M. R. and Senior, J. R., Academic Press, London and New York, p. 215-255