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The Evolution of Cephalopod Molluscs


 

Paleontologically, it appears that the earliest cephalopods were shelled molluscs, which first appear in Cambrian rock layers, 500 million years ago (Monks, 2003). Beginning with a limpet, or similar limpet-like mollusc, evolution from primitive to advanced cephalopods probably occurred in stages. The limpet, at its back end, secretes a shell cap for protective purposes and partially hollows into rock surfaces. While capable of migration, it is fairly sedentary.

The first evolutionary step toward increased mobility of this limpet-like mollusc seems to have been an elongation of the shell by secretion along its peripheral edges to form a new partition. This partition formed as a new layer at the back end of the mollusc, and multiple chambers were secreted in each successive growth cycle, as with the nautilus shell. Each partition also had an opening for a siphuncle, like that of nautilus. These developments are easily seen in another fossil form, Orthoceras, which despite some significant differences otherwise resembles an uncoiled, external nautilus-like shell (Engeser, 1998).

Orthoceras fossils are most abundant in Devonian rock, about 400 Mya, and some achieved a remarkable length. Such shells would be unwieldy and easily subject to mechanical injury, so the coiled shells of the ammonites appear to be a subsequent evolutionary development permitting greater mobility. The ammonite, Goniatites, which first appears toward the end of the Devonian period, preserves buoyant chambers, partitions and siphuncle openings similar to Orthoceras. While not directly related, undoubtedly these two fossil groups descended from a common ancestral form.

Ammonite shells also have a strong resemblance to the nautilus shell, except for a marginally located siphon and an embryonic whorl at the apex of the shell. Ammonites continued, along with numerous developmental variations, and became the dominant cephalopod until the end of  the Cretacious Period (144 Mya).

Both the ammonites and Orthoceras are usually considered to be tetrabranchiate molluscs, but this is a questionable assumption (Monks & Palmer, 2002). In ammonites, the only tissue evidence that has been preserved is the radula, which is more like that of a living dibranchiate, not a tetrabranchiate cephalopod.

Ammonites became extinct, but their body plan and coiled shell constituted a very successful form, that persisted over several hundred million years.

Further "morphing" of the cephalopod body plan seems to have been in the direction of an actively swimming life style, unlike the grazing manner of limpets and ammonites. The enveloping of the shell by the mantle tissue, the eventual shrinking or disappearance of the shell, and the reduction of visceral organs from tetrabranchiate to dibranchiate form are all developments facilitating a more mobile, carnivorous life style that characterizes most living cephalopods. Reduction from an internal shell to only a remnant internal support structure is obvious among various fossil species of the Belemnoidea order. Similar evolution from shelled grazers to shell-less carnivores has also taken place in the Opisthobranchia subclass of living marine snails.

In considering the evolution of cephalopods, the particular examples shown here should not be looked upon as a direct lineage. The examples most likely represent polyphyletic taxa, arising from unknown ancestral forms. The drivers for evolutionary change, such as has occurred with the cephalopod body plan, appear related to major consequences of change in continental configurations between roughly 500 and 140 Mya (aggregation into Pangea and its subsequent breakup). Severe climate changes, reconfigurations of the continental shelves, reduction in estuaries and wetlands feeding the continental shelves, and alteration in oceanic current patterns all occurred during this interval. These major changes had profound impact on the niches and life styles of individual cephalopod species, as they did for most other species of marine life (Skelton, 1993).

Orthoceras_TH.jpg (113035 bytes)
Unidentified Orthoceras Species

 

AmmoniteB.jpg (121762 bytes)
Unidentified Ammonite Species
(probably late Devonian, of the Order: Goniatitida)

 

shell.jpg (62072 bytes)
Nautilus pompilius

 

Live_Ammonite_Composite.jpg (59198 bytes)
Hypothesized Live Ammonite
(after Neale Monks, 2003b)

 

LINKS -MORE INFORMATION:

Dr.  Neale Monks Evolutionary History

Neal Larson
Extinct Species

 


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