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