This model emulates a subevolute ammonoid conch, showing soft ventral ribs and lappets. Lappets are projections of the aperture’s margins of the male ammonites, and they are hypothesized to function as reproductive structures, either for display or stabilization.
Ammonoids are a group of mollusks, more
specifically cephalopods (which include octopuses, squids, cuttlefish,
nautiloids, and others), that inhabited seas and oceans, first appearing in the
Devonian period, around 419 million years ago. They went extinct alongside the
dinosaurs during the mass extinction at the end of the Cretaceous period,
approximately 66 million years ago. Ammonoids were a highly diverse and
abundant group. At first glance, they resembled the modern Nautilus
species, as they possessed an external shell that housed most of the animal's
body. However, they are more closely related to coleoids, the group that
includes octopuses, squids, and cuttlefish. Like the modern Nautilus, ammonoids
had an external shell made of calcium carbonate (aragonite), subdivided into
two components: the phragmocone and the living chamber. The phragmocone was
divided internally by septa, which defined chambers connected by a ventral tube
called the siphuncle. The living chamber contained most of the animal's body
and was connected to the phragmocone via the siphuncle. The shell served not
only as protection but also as the animal's primary buoyancy mechanism. The
chambers contained air or another fluid of lower density than water, allowing
the animal to regulate its position in the water column by controlling the
amount of fluid in the chambers. This process is similar to how submarines fill
or empty their ballast tanks to dive or surface. As the animal grew, the number
of chambers increased, the size of the living chamber expanded, and
consequently, the size of the shell also grew. Generally, ammonoids had shells
that formed a flat spiral, but some species had uncoiled forms or deviated from
this typical morphology, showing a wide variety of shapes and ornamentations.
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