iris.
AMMONITES floated through the world’s shallow oceans for more than
three hundred million years, and their fossilized shells turn up all around
the world. Pliny the Elder, who died in the eruption that buried Pompeii,
was already familiar with them, although he considered them to be
precious stones. (The stones, he related in his Natural History, were said to
bring prophetic dreams.) In medieval England, ammonites were known as
“serpent stones,” and in Germany they were used to treat sick cows. In
India, they were—and to a certain extent still are—revered as
manifestations of Vishnu.
Like nautiluses, to whom they were distantly related, ammonites
constructed spiral shells divided into multiple chambers. The animals
themselves occupied only the last and largest chamber; the rest were
filled with air, an arrangement that might be compared to an apartment
building in which just the penthouse is rented. The walls between the
chambers, known as septa, were fantastically elaborate, folded into
intricate ruffles, like the edges of a snowflake. (Individual species can be
identified by the distinctive patterns of their pleats.) This evolutionary
development allowed ammonites to build shells that were at once light
and robust—capable of withstanding many atmospheres’ worth of water
pressure. Most ammonites could fit in a human hand; some grew to be the
size of kiddie pools.
Based on the number of teeth ammonites had—nine—it’s believed that
their closest living kin are octopuses. But since ammonites’ soft body
parts are virtually never preserved, what exactly the animals looked like
and how they lived are largely matters of inference. It’s probable, though
not certain, that they propelled themselves by shooting out a jet of water,
which means that they could only travel backward.