A IS FOR AMMONITE
What's an ammonite?
Ammonites are extinct cephalopods, related to celebrated extant creatures such as octopus, squid, cuttlefish and the relative who they most superficially resemble, the nautilus. Sadly for us, the ammonite has been extinct for quite a while; they got iced along with the dinosaurs during the Cretaceous–Tertiary extinction event, which happens to be the most recent mass extinction event in the history of Earth. And when I say "most recent", I am implying that there have been more, because, well, there have been more.
Anyway, despite becoming extinct, ammonites had a pretty sweet run of it. Let's consult our handy-dandy Chart of Geologic Time:
I have colored the right vertical section of the chart in tasteful hot pink to show the 435 million year run of the ammonites. As you can see, dinosaurs of any real sort were only around for roughly half of that.
Also, if you've ever wondered what period of geologic time we're currently in, we're in the Cenozoic Era, Quaternary Period, Holocene Epoch; it's that very thin sliver of time topping the rest of the history. Cephalopods have owned the earth for far longer than any of us hairless primates have been trundling around, and it's likely that they will continue to do so after we're gone.
Cryptic quasi-apocalyptic suppositions aside, ammonites, like their descendants, were numerous. Currently, the worldwide biomass of squid exceeds the worldwide biomass of humans; that is, there is more living squid matter than living human matter. Bear in mind this is just for squid, this isn't even counting all the other cephalopods.
Ammonite fossils are distributed worldwide and make very useful index fossils, special fossils that can be used to date different strata of rock because they are specific to a particular time period. Given the long reign of the ammonite, in this case specific species of ammonite are used as the rock-clocks.
So what's great about them?
#1) They are named after the Egyptian god Amun/Ammon, who was often depicted with ram's horns. People thought that ammonite fossils resembled coiled ram's horns, thus they are called ammonites.
#2) They were ridiculously diverse in terms of shape and size. Along with the traditional nautilus-shell-looking ammonites, there were crazy variations. This gallery depicts a large variety, and this thread from good ol' TONMO (The Octopus News Magazine Online; if you have more than a passing interest in cephalopods, you need to join this site NOW) has a number of images of the nipponites, the squiggliest of ammonites.
Illustration based on nipponite fossils
#3) They are the source of ammolite, a rare and lovely gemstone.
Gem-quality ammolite is found in some ammonite fossils deposited in a region once covered in a shallow inland sea that stretched from "...Alberta to Saskatchewan in Canada and south to Montana in the USA.", according to Wikipedia other ammolite resources. It is iridescent like an opal and is the official gemstone of the Canadian province of Alberta.
Personally, ammolite (and opals, natch) is one of my favorite gemstones and there is no shortage of websites out there with examples of their ammolite jewelry. I think it looks better in fossil format, personally; patterns and colors can be viewed here at the Gem Society's page on ammolite, which provides much more comprehensive mineralogical information than I am providing here.
BONUS LINK: Here's a Russian site with all sorts of colorful ammonite fossils, including some with pyrite (fool's gold).
#4) They are a natural example of the spira mirabilis.
The spira mirabilis, the "miraculous spiral", is known by approximately 8 billion different names. These names include "logarithmic spiral", "equiangular spiral", "growth spiral" and others. For the specific mathematic properties of this curve, I suggest consulting Wolfram MathWorld's page on the topic. This site is messier, but has interesting examples of the curve in nature.
#5) They range widely in size!
Ammonites can be tiny...
....or very large.
I leave you with a few ammonite links:
-Site about large chalk ammonite fossils viewable at Peacehaven, in the UK. There are other interesting fossilized creatures there, too.
-Beautiful gallery that shows the diversity of ammonite shapes and shells from all over the world/time. At the bottom, there's a gorgeous iridescent blue shell that's not to be missed.
-This website can direct you if you're interested in purchasing ammonite fossils, or samples of ammolite. They are not cheap. More are available here.
-FAQ on Fossil Cephalopods from the venerable Cephalopod Page, presented by Dr. James B. Wood
-Article on nautiloids by Phil Eyden from TONMO.