Ammonite Model

This model of the ammonite Douvilleiceras is a hybrid of several species found in the Queen Charlotte Islands, with Douvilleiceras spiniferum as a base. The shell model is almost 50 times life size, and accurate down to the protoconch's primary constriction. The model also displays some anomalies such as an extra node row that appears on the venter.

 Douvilleiceras is an ammonite from the Albian age of the Cretaceous period. This ammonite has been documented to reach no more than 46 cm, likely due to the amount of energy needed to create a complicated shell. The spines closest to the shoulder are proportionally longer in the umbilicus than at the aperture. This appears to be because each overlapping shell whorl creates a bigger shoulder, allowing the spines to grow protected to their full length. Environmental changes, such as shell breaks and layer delaminations, are exhibited on the surface of the shell.

The model's soft-body tissues are based on squid morphology. Suction cups seemed more likely than hooks, and one arm is hectocotylized, making it a male. The hectocotulus is a modified arm for sperm transfer to the female. In some cephalopods, such as the Argonaut, the arm is inserted into the female's mantle and detached. In the past, the detached arm led to an original misidentification as a parasitic worm, which was named Hectocotylus.

Evidence for whether ammonites had hoods is unclear. Retractor muscles are used by the modern Nautilus to withdraw into its shell. Retractor muscles leave a half moon scar on the inside of the shell. Some ammonite fossils do show retractor scars, but it is not evident in all ammonites. Regardless of retractor muscles, the ornamentation would be problematic for this particular ammonite. Looking again at the only model we have for confined cephalopod life, the contact points of the hood, mantle, and shell are smooth. The heavily ornamented shell of Douvilleiceras would be detrimental to a hood's movement.

In most cephalopods the siphon is a muscular tube, but the Nautilus has a flap-like siphon, with the ends overlapping much the same as a paper funnel. The overlap can be increased or reduced, varying the volume and force of thrust. This may be an adaptation to confinement. For this reason, a flap like funnel is used on the ammonite model. The funnel of my model may look small at first glance, but this organ is compressed as the tensed animal is starting to extend for attack .

Another possible adaptation could be a lack of grasping tentacles. Cuttlefish store their tentacles in pouches on either side of the buccal mass, and squid hold them within the arm mass. The grasping tentacles of modern coleoids are most commonly one and a half to three times the length of the arms. The extra mass of the grasping tentacles held within the shell would be obstructive, because of this my model only has eight arms.

A Douvilleiceras eye may not be as advanced as the customarily used cuttlefish eye, that changes focus by changing the shape of the eye. Instead I have given the ammonite a large simple eye like some deep sea squid. The Vampire squid and Spirula both have pupils that are very large in proportion to the eye. Some are so large that not much more than the pupils can be observed unless the eye is turned.

The Model can be seen at the Geological Survey of Canada book store, on the15 floor of 605 Robson Street in Vancouver. Five rolls of duct tape, 164 m of tin foil, 60 l of latex and 111 kg of modeling clay have gone into the sculpture. The project took approximately 650 hours.



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