I apologize for the delay. School
recently started, and I’ve been swamped with homework that, unfortunately, must
take top priority. In the meantime, I promised to elaborate on the finds my
family and I made during our trip to Montana. The pterosaur brain,
unfortunately, is still on hold—we’re waiting for input from our paleontology
professor friend—but I can still do a Things from the Shelf on the other main
item we found: ammonites.
These things are one of the more
common fossils found on the earth—so much so that they’re often used as ‘index’
fossils to date rarer ones—but I was still insanely excited every time I
cracked open a nodule of hardened mud and saw a spiral of mother-of-pearl
buried inside. The fossilization process involved in the making of what we were
finding is called the ‘mold and cast’ process, and it is exactly what the name
implies; the animal dies, is buried in mud at the bottom of the sea floor, and
over time the sediment surrounding it hardens into rock, forming the rounded
nodules that we were finding. The animal itself rots away within its casing of
stone, leaving its mother-of-pearl outer shell and a perfect mold of its body cavity.
In some cases, the cavity is filled completely with mud and becomes a complete cast
of the original animal. In others, only trace amounts of minerals seep into the
cracks, and the inside of the animal’s body cavity crystallizes like a geode. After
finding the nodules that encased these fossils, it was an easy matter to crack
the hardened mud balls open and reveal the ammonites, still preserved where
they’d fallen to the seabed millions of years ago.
The species that we were finding was
called Scaphites, and judging
by the leaf-patterned sutures that connect each chamber of its shell, it was
probably from the late Cretaceous. Ammonites first appeared 240-million years
ago, and later died out along with the dinosaur during the K-T extinction
65-million years ago. However, there still exists a species today—called the
nautilus—that is very closely related to the ammonite. There are only two
living species of nautilus, but they haven’t changed in 500-million years and
can give us a very close idea of how ammonites once looked and behaved.
Ammonites
were a rather bizarre species. In appearance, they most resembled modern land
snails, but unlike snails (who were from the class Gastropoda) their shell, or
mantle, was not completely hollow. To put it simply, snails live all the way up
their shells. Their body follows the contours of the spiral, occupying the
space with a network of nerves, organs and tissues. Cephalopods, by contrast,
only live in the ends of their shells. The entire shell is divided by walls of
shell called ‘septa’ into chambers the entire length of the spiral, and the
animal itself only lives in the outer chamber. The others are filled with gases
and connected with a strand of tissue called the siphuncle, and are used for
buoyancy and locomotion. If it chose, the animal could pump the chambers full
of water to sink or force air into them to rise, and if threatened, they could
jet gasses from a funnel-like opening near the mouth and propel themselves
backwards through the water. They also possessed ink glands, like modern squids
and octopi, and could release a cloud of blinding ink into the water as a decoy
when threatened.
Like
modern squid and octopi, ammonites were predatory. They ate small fish and
crustaceans, using their long, suckerless tentacles to grasp prey and force it
into a beak-like mouth in the center of a ring of around 90 tentacles. They
lived in schools, and were hunted by large, swimming reptiles like the
Mosasaur. They defended themselves with their shells, which were made of calcium
and were constantly growing; like the nautilus, they could draw themselves into
that shell in case of danger, and it is surmised that they may have possessed a
plated covering (called an ‘aptychus’) that fitted over the opening of the
shell. Nautilus are similar, having a leathery ‘hood’ rather than a plate.
Ammonites
are classified under the phylum Mollusca, the class cephalopoda, and the
subclass ammonoidea. They were named for the Egyptian god Amun—a local deity at
Thebes—because of the resemblance of their shells to the curling rams horns
depicted on the god, and the fossils were used at one time as discuses in
ancient Greek games. Today, they’re as common as dirt in certain sites
scattered across the world, but searching for them was still one of the
highlights of my trip to Montana. My thanks to Darell and Ward, for showing us
the sites, and to my dad, for dragging me out there in the first sites.
To
my readers: My thanks for your attention, and be sure to always stay curious.
You never know what you’ll find out.
Happy reading!
S.R. Koch.
Very well written and informative. Thanks!
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