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Characters and Anatomy

Molluscan Morphology

Molluscs are one of the most diverse groups of all the invertebrates and include numerous extinct and extant species they are predominantly a marine phylum although a few gastropods have evolved to inhabit freshwater and terrestrial environments.  The diversity of molluscs is extensive, but all species abide to a fundamental morphological plan.  To compare these similarities a hypothetical archimollusc has been created that displays all the basic features of molluscan anatomy (figure 1).

Hypothetical archimollusc

Figure 1 - Morphology of hypothetical archimollusc displaying all the basic features of molluscan anatomy. (Clarkson, 1998)

The Morphology of Ammonites

The shell or conch is the structure secreted by the mantle of ammonoids for protection and buoyancy.  The complete shell is a hollow spiral with two major parts, the body chamber and the phragmocone.  The shell or test that forms the cone is called the shell wall. The aperture is the area where the ammonoid would protrude from, the edge of this opening is the referred to as the peristome.  When maturity is reached modifications can occur to the aperture such as projections and nodules.  The opposite end of the spiral, the small tip and therefore oldest section, is called the apex.  Almost all ammonoid tests display planispirally coiling, those few that do not and have an open spiral are termed heteromorphs.

Morphology of Ammonoid

Figure 2 - A generalised ammonoid with the major anatomical and morphological characteristics (source: biodidac)

The body chamber is the part of the shell that would have been occupied by the living animal and is assigned a term depending on the morphology of the coiling.  The shells are termed brevidomic if they are less than 1⁄2 whorls, mesodomic if they are between 1⁄2 and 3⁄4 whorls, and longidomic if the shell displays more than 3⁄4 whorls. 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.

As the ammonoid 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.  The septum is attached to the shell wall along a suture, seen as a series of simple to complex lines on internal molds.  The parts of the suture line directed adorally (away from the mouth) are termed saddles, and those directed adapically (towards the mouth) are termed lobes.  The complexity of the sutures generally increased through time and can be used in taxonomy as a tool for the identification of species. 

                            Orthoceratitic Sutures – Relatively simple shallow lobes and saddles. 

                            Agoniatitic Sutures – broad lobes and saddles with a narrow mid ventral lobe. 

                            Goniatitic Sutures – strong, mostly angular lobes and angular to rounded saddles. 

                            Ceratitic Sutures – strong rounded saddles and serrated lobes. 

                            Ammonitic Sutures – complex lobes and saddles.

Types of ammonoid suture


           Internal mold showing suture                    

Figure 2 - The suture morphology in ammonoids generally became more complex overtime from goniatitic to ammonitic (diagram drawn using Microsoft paintbrush)

Figure 3 - An internal mold of an ammonitic suture in the species  Descophyllites ebneri from the Upper Triassic (photograph taken with permission of the University of Bristol)

In ammonoid shell all parts that are in contact with the siphuncular chord are considered part of the siphuncle.  The non-living part of the siphuncle that covers the living siphuncular chord is termed the ectosiphuncle and is composed of the septal neck and the connecting ring.  The area and any structures, inside the ectosiphuncle are termed the endosiphuncle.  While the septal necks are where the siphuncle passes through the septum, these are generally orientated convex towards the aperture. 

All ammonoid shells are ornamented with 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. Ribs pointing backwards are called rursiradiate while those inclined forward are prorsiradiate.  Ribs can be dense and closely spaced or distant and widely spaced depending on the morphological characteristics of the species.

Author: Nick Loughlin
Last updated: 18/11/06
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