Ammonites
I
remember the magic of finding a large Ammonite in my family's quarry – shell
light gray against the white limestone, and I was stunned by its coiled shape.
With
out a doubt, Ammonites are among the most abundant and beautiful of all
fossils.
From the Virtual Museum - http://www.fossilmuseum.net/EdResources/AmmoImages.htm
Although
not as glamorous as some of the dinosaurs, these extinct cephalopods are appealing and hold the
interest of all collectors and paleontologists.
Ammonites
(pronounced AM-uh-nites) were early mollusks, marine animals with no vertebrae but protected by a hard shell
(usually coiled) made from calcium.
As a
cephalopod, meaning “head foot,” they had tentacles and a large head, and while
ammonites are extinct, other cephalopods are still alive: squid, octopuses, and
cuttlefish.
While
living members of the cephalopods are fast-moving carnivores, catching prey
with their tentacles, we are not sure how fast ammonites moved. But as
cephalopods, they moved by squirting water through a siphon, a type of jet
propulsion.
Anatomy
Here's
what they looked like inside.
http://www/tonmo.com/science/public/ammonites.php
Protected by a hard shell (usually
spiral-coiled), ammonites regulated their floating through the use of many air
filled chambers, called phragmocones.
The walls of each chamber are called
septa; these walls were penetrated by a tube likee structure called a siphuncle
that probably regulated the air pressure, allowing the ammonite to rise and
fall in the water.
As you can see, the animal lived only
in the outer chamber.
A Brief History
Ammonites evolved from the early
nautiloids.
Milwaukee Public Museum Diorama
Nautiloids first appeared during the
subsequent Ordovician period (500-450 mya) and became a very widespread and
diverse group assuming the ecological niche of top predator following the
extinction of the Anomalocarids at
the end of the Cambrian.
Anomalocaris cnadensis by Yukio Sato. Cover of Simon Conway Morris' book
Nautiloids tended to have cone shaped
or long straight shells divided into chambers and evolved jet propulsion
through the use of its funnel ( hypernome ), a feature which all living
cephalopods have retained.
The ammonoids themselves evolved from
an offshoot of one of these extinct groups of straight shelled nautiloids - the bactritida - during the mid-Devonian
period and began to evolve the coiled shells we are so familiar with in fossil
collections today.
The bactritids are an obscure group and have been interpreted a transitional
form between nautiloids and ammonoids.
Three different basic forms of
ammonoids evolved
- Initially the goniatites, in the mid Devonian which lasted until the end of the
Permian (250 mya).
- Superseded by the ceriatites during the Triassic (250-200 mya).
- Then the ammonites themselves, coming to dominance in the Jurassic following
the extinction of the ceriatites (200-150mya).
From this point on the ammonites
dominated the world's oceans until their extinction at the end of the
Cretaceous period at the same time as the dinosaurs.
Along the way they become an extremely
diverse and complex group ranging from mere millimeters across, to truly huge
examples such as Pachydiscus
Seppenradensis which grew up to three meters in diameter.
http://www/feuer-stuerme-auf-kometen.de/sint/abb10.jpg
Ammonites evolved many different forms
and survived a number of major extinctions, and although they were in decline
before their final extinction, they were really one of life's success stories.
Extinction
So why did such a successful creature become extinct?
We're not really sure. The last
ammonite families disappeared at the end of the Cretaceous period along with
the dinosaurs 65 million years ago.
They were in decline at that point with
only a handful of surviving families, but the sudden disappearance of ammonites
along with the belemnites cannot really be attributed to background natural
extinction.
Various theories speculate that the
ammonite may have had planktonic larvae. Months or years of overcast skies
following a meteorite impact would have taken a devastating toll, crippling the
phytoplankton's ability to photosynthesis which would have had a knock on
effect along the food chain. On the other hand, many squids and octopi pass
through a planktonic stage and they managed to survive the meteor impact.
Another theory holds that the composition of the ammonite shell (aragonite or
mother of pearl) would have been extremely susceptible to the acid rain that
probably following the vaporisation of vast quantities of limestone into the
atmosphere followed the meteor impact.
The nautilus, being a comparatively
deep water animal, may have been afforded some degree of protection by the
meters of water between it and the surface.
On the other hand, perhaps the
ammonites were simply out-competed by the squids and octopi (coleoids) and, of course, the fish
which were dominant in the worlds oceans at that point. Perhaps all these
theories, and others, combined.
Whatever the reason, we are left with traces in the rocks and beautiful
fossils.
But there is one other thing.
Index
Fossils
Because ammonites are some of the most
widespread of all fossils and found throughout the world, they are extremely
useful to palaeontologists.
Since species of ammonite evolved and
became extinct so rapidly, researchers find them useful tools for dating rock
strata.
A paleontologist working in one part of
the world can determine the exact age of his sample by examining the precise
species of ammonite found in that layer and comparing it with other examples
elsewhere.
If an identical species is present in a
sample no matter how separated geographically, then the rocks must be of a
comparable date.
So, even though they are extinct, they
help us in dating other fossils.
And they do so with beauty and style.
Learn about Belemnites
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