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. However, the nautilus is a useful creature for comparison despite several important anatomical differences. 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.
The ammonites themselves really came to dominance in the Jurassic following the extinction of the ceriatites (200-150mya). They were superceded by the ceriatites during the Triassic (250-200 mya). Three different basic forms of ammonoids evolved, initially the goniatites, in the mid Devonian which lasted until the end of the Permian (250 mya). The bactritids are an obscure group and have been interpreted a transitional form between nautiloids and ammonoids. 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. Only one order survived, the Nautilida from which our five species of Nautilus alive today are descended. Cameroceras, which grew up to 10meters long, before most of them became extinct by the end of the Devonian period (350 mya). The nautiloids diversified into many different orders, some of them huge predators e.g. 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). 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. 1: Timeline: Major Cephalopod EventsĪmmonites evolved from the early nautiloids. 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.įIG. 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. 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.
Since species of ammonite evolved and became extinct so rapidly, researchers find them extremely useful tools for dating rock strata. They are extremely useful to paleontologists as the thousands of individual species are easily identifiable to the trained eye. This deity was adopted into later Greek mythology as Ammon, the ammonite supposedly resembling the horns on either side of Ammon's head.Īmmonites are some of the most widespread of all fossils and are found throughout the world. So here is a short overview of these ancient creatures - I hope you find it interesting!įrom 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.
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. Although not as glamorous as Tyrannosaurus rex or as dramatic as Velociraptor, these extinct creatures are nonetheless quite interesting animals. Note: Phil welcomes discussion on this article in the Cephalopod Fossils forum.Īmmonites are extinct cephalopods and are among the most abundant and beautiful of all fossils.