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Ammonites
The answer Hooke arrived at was simple: The ammonite would have a tiny
gas gland capable of filling the chambers with air at a pressure equal to that
of the surrounding water. With such a system, the ammonite would be unlike
a submarine: It could descend to any depth and never worry about implo-
sion, because its internal pressure would always match the external pressure,
which increases by 14-7 pounds with every 33 feet of depth descended.
Hooke's supposition, that ammonites (and the nautilus as well) were
capable of equalizing their internal gas pressure with the external pressure
through the use of a gas gland, turned out to be false. Instead, these two
groups of chambered cephalopods use (or used) a far less elegant solution:
Their internal parts are always at lower pressure than ambient (the water
pressure of the depth they inhabit at any given moment). In fact, studies
have shown that the chamber pressure is usually far lower. Nautiluses (and
probably ammonites) are thus susceptible to implosion when sufficient depth
is reached. They therefore must depend on the strength of their shells to
allow them to descend to great depths in the sea.
Comparison of shell interiors from nautilus (left) and ammonite
(right). Note the far greater complexity of ammonite septa.
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