Alongside trilobites, ammonites are among the ‘must haves’ in any palaeontological collection. Professional geologists value them as among the best index fossils, many species having only existed for a relatively brief period of time (often a few tens of thousands of years) but in that time having become widely spread in terms of geographical distribution (often found in places as far apart as Britain and Australia). This is the basis of biostratigraphy: the way geologists can tell that sediments in two different places were laid down at the same time because they contain the same organisms.
Perisphinctes is a particularly widespread genus of Jurassic ammonite that may be found in places as far apart as the Europe, the Caribbean, India and Madagascar. They’re often very common fossils, and you’ll frequently see attractive Perisphinctes specimens being sold in fossil shops. But familiarity needn’t breed contempt, because while this ammonite might be a bit of a default specimen for nascent collectors, it’s a very typical ammonite that tells you a lot about this particularly interesting group of extinct organisms.