Ammonites: A General OverviewBy Phil Eyden
Note: Phil welcomes discussion on this article in the Cephalopod Fossils forum.
Ammonites are extinct cephalopods and are among the most abundant and beautiful of all fossils. Although not as glamorous as Tyrannosaurus rex or as dramatic as Velociraptor, these extinct creatures are nonetheless quite interesting animals. Although most people have heard of them and they are probably the most widely recognised non-dinosaur fossils, it seems that ammonite information is not easy to come across, outside scientific literature. So here is a short overview of these ancient creatures - I hope you find it interesting!
First, a few facts about ammonites.
From where do we get the name 'ammonite'? It derives from ancient Egyptian mythology, from the oracle-god Amun who had the form of a ram. This deity was adopted into later Greek mythology as Ammon, the ammonite supposedly resembling the horns on either side of Ammon's head.
Ammonites are some of the most widespread of all fossils and are found throughout the world. They are extremely useful to palaeontologists as the thousands of individual species are easily identifiable to the trained eye. Since species of ammonite evolved and became extinct so rapidly, researchers find them extremely useful tools for dating rock strata. A palaeontologist 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 as most species only exist for a couple of million years or so. Due to the high turn over of ammonite species, sometimes zones in the strata have been identified as spanning less than a million years duration.
FIG. 1: Timeline: Major Cephalopod Events
The first molluscs appeared around 550 million years ago during the early Cambrian period, the period along with most of the basic forms of creature that exist today. Life was exclusively marine at this point and these early molluscs were extremely basic in their construction. The first cephalopods appeared in the late Cambrian and horn shaped shells of these small creatures, such as Plectronoceras, have been found containing basic chambers with a connecting tube called a siphuncle. Although we cannot be exactly sure what these creatures looked like, certain similarities in construction suggest they were the earliest ancestor of all later cephalopods including ammonites, belemnites, squid, octopi and cuttlefish. It seems likely that these early creatures crept along the sea floor possibly taking on a predatory role, though until we find a fossil of the soft parts nobody will really know for sure!
FIG. 2: Plectronoceras Basic Anatomy
FIG. 3: Plectronoceras Model
Ammonites evolved from the early nautiloids. 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. These animals tended to have cone shaped or long straight shells divided into chambers and evolved jet propulsion, suggested by a notch underneath the aperture of the shell that is assumed to have held the hypernome, (the siphon in which water is squirted out to move the animal through the water, a feature which all living cephalopods have retained). The nautiloids diversified into many different orders, some of them huge predators e.g. Cameroceras, which grew up to 10meters long, before most of them became extinct by the end of the Devonian period (350 mya). Only one order survived, the Nautilida from which our five species of Nautilus alive today are descended.
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). They were superceded by the ceriatites during the Triassic (250-200 mya). The ammonites themselves really came to dominance in the Jurassic following the extinction of the ceriatites (200-150mya).
FIG. 4: Devonian goniatite Tomoceras
FIG. 5: Structure Line Comparison
These three groups, the goniatites, the ceriatites and the ammonites all differed from each other in subtle ways though spiral coil shell shape appears superficially similar. They differed the shape of the body chambers, ammonites tended to have very complex divisions between their body chambers, while goniatites and ceriatites were much simpler. Goniatites had very simple shaped body chambers with zig-zagged or triangular suture lines, ceriatites were more complex with wavy sutures forming saddles and lobes, whilst ammonites had more complex sutures still, often forming very complicated and beautiful patterns indeed.
FIG. 6: Cleoniceras besairei (half polished)
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 millimetres across, to truly huge examples such as Pachydiscus seppenradensis which grew up to three meters in diameter. Such creatures would have rivalled Architeuthis in size! 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. We are aware of approximately 7,500 different species of fossil cephalopod, and doubtless many more thousands of species are yet to be discovered, or sadly are lost to us forever.
FIG. 7: Ammonite Basic Anatomy
FIG. 8: Oxynotoceras (Jurassic)
Unfortunately no ammonite has yet been discovered with the soft tissues preserved so we can only make educated guesses as to the physiology of the creature inhabiting the shell. However, the nautilus is a useful creature for comparison despite several important anatomical differences.
The shell of the ammonite tends to be thinner than the nautilus and would probably have been unable to withstand the pressures that the nautilus does. Some ammonites also bear structures known as lappets; these are flanges that protrude from the final chamber in front of the creature in adult male specimens [the microconch] and may have been used for sexual display. These features are not present on the larger female ammonites [the macroconch]. These structures suggest the creature to have been highly visual as such a feature would be invisible in the darker depths. From this it could be implied that the ammonite had good eyesight and was a fairly shallow water creature. Tiny teeth on the radula (tongue) of the creature have also been identified. These demonstrate a close affinity to most modern day octopi with 7 rows and two marginal blocks in contrast to the nautilus with 13. With this circumstantial evidence, it seems likely that the soft-bodied ammonite probably resembled a squid or octopus in form as opposed to the many-armed nautilus. Until a soft-bodied example is found, this can only be conjecture!
The ammonite creature probably occupied the last segment of the body chamber, as does the modern nautilus, and slowly secreted new chambers as the body grew. A connection was maintained with the previous body chambers with a structure called a siphuncle that stretched out like a thin tube from the rear of the animal passing through a tiny hole in each chamber. The siphuncle would draw water out of the connecting chambers and bleed in gasses thus keeping the creature buoyant in exactly the same manner as the modern Nautilus.
During its secretion each body chamber left a distinctive pattern at its edge known as a suture; this is distinctive in all species of ammonite and is a useful tool for identifying specimens. The aptychus is another interesting feature although its function is subject to much debate. It is a heart shaped structure, in form much like a bivalve, and is sometimes found in the final body chamber. It has been interpreted as either an enormous set of jaws or possibly a hood with which the creature could protect itself once withdrawn into its shell, assuming the ammonite could actually do this! It seems the jury is still out on this debate; the aptychus can be absolutely massive in some species, sometimes up to a fifth of the whole body length so it seems rather too large to be a set of jaws. On the other hand, it does not always fit the aperture of the final body chamber to be a lid!
Ammonites, of course, are generally spiral shaped but there are many variations. Jurassic species, as one may expect, tend to be simpler and less ornamented than Cretaceous species, many of the later types display ribs, ornamentation and clearly defined keels, features that are not common amongst their ancestors. By the end of the Cretaceous period, the ammonites had diversified into many different forms; some grew in crescent shapes, some had open whorls and others returned to a straight shell, reminiscent of the earlier nautiloids. These unusually shaped ammonites are known as heteromorphs. Still others developed spines possibly used for sexual display and/or to make it an uncomfortable meal for predators. The ammonite must have occupied many ecological different niches in the food chain, varying so widely in size and form.
FIG. 9: Various Illustrations
So why did such a successful creature become extinct? This is another question without an easy answer. The last ammonite families disappeared at the end of the Cretaceous period along with the dinosaurs 65 million years ago; it is true that they were an order 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 photosynthesise 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, all we are left with are traces in the rocks and beautiful fossils to tantalise us. Who knows? Perhaps one day a stunning surprise in the abyssal depths may be made by a marine biologist that will change that. Let's hope so!
Monks, N; Palmer P. 2002. Ammonites. The Natural History Museum.
Clarkson, ENK; 1998. Invertebrate Palaeontology and Evolution (4th ed.).Blackwell.
Moody, R; 1977. The Fossil World. Hamlyn
Walker,C; Ward D;1992 Fossils (Eyewitness Handbook) Dorling Kindersley
And a big thank-you to Tintenfisch!
Phil Eyden 4/29/03