Travel Reference
In-Depth Information
Ammonites have interested and mystified "natural philosophers"
since the Renaissance. In the 1600s, the British natutalist Robert Hooke il-
lustrated some of the copious ammonite fossils to be found at many locali-
ties around his native Britain and made the first informed speculations
about how they may have lived. He noted that these ancient shellfish had
shells partitioned by many calcareous walls (the septa), which divided the
shell into individual chambers. Hooke correctly deduced that the am-
monites from his collections were closely related to the living chambered
nautilus, known to him from shells brought from the far tropical Pacific
Ocean.
Ammonites and the nautilus both live within a chambered shell, and
the shell itself is cordoned off by the internal septa. It is thus reasonable to
assume that in many ways they shared a similar mode of life. But Hooke rec-
ognized that these two creatures differed in one great respect: The septa of
the nautilus, numbering about 30 in an adult shell, have a simple concave
shape. They are like small watch glasses fitted within the shell. Ammonites,
on the other hand, exhibited vastly more elaborate septa. Whereas their
middles might be only slightly curved, the margins of the ammonite septa are
crenulated into a complex series of curves and vaults that intersect the shell
as a very complex pattern. This intersection is called the suture. In nau-
tiluses, the suture describes a simple if slightly wavy line. In ammonites, the
suture is a very complex curve—a sine wave on LSD.
Hooke was most curious about how the nautilus and ammonites might
live. He thought that both animals used their shelled portions as a buoyancy
device and suggested that the internal, chambered portions in both were
filled with air. But air at what pressure? Hooke was a genius, a person eclipsed
in fame only by his immediate scientific antecedent in Britain, Sir Isaac
Newton. Hooke asked the following question and, in so doing, set the stage
for one of the most common approaches in functional morphology: If this
mechanism functioned as he thought, how could it best be engineered? Spe-
cifically, if some creature wished to have a gas-filled shell to use as a buoy-
ancy device, how could it most efficiently be built and most effectively work?
