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.
To start with, Perisphinctes is a good example of a Tethyan ammonite. The Tethys Sea opened up during the Triassic as the two halves of the Pangaea supercontinent pulled apart. The northern half became Laurasia, and consisted of modern day North America and Eurasia, while the southern half, called Gondwana, eventually broke apart to form the African, American, Australian, Indian and South American continental plates. Around the margins of the Tethys were warm, shallow seas teeming with life, including many different types of ammonites, including the various different species of Perisphinctes.
Ammonites are molluscs, but unlike snails and clams, they were active animals capable of moving about quite quickly. Like all ammonites, Perisphinctes had an external shell that provided protection against predators, but inside the shell were a series of chambers that were partially filled with gas. As they grew ammonites were able to add new chambers and secrete more gas, ensuring that they always had approximately neutral buoyancy; in other words, if an ammonite stopped swimming it neither sank nor floated upwards, but instead hung there in midwater.
This much most geologists agree on; where things become more complicated is in interpreting ammonite lifestyle as it relates to the shape of their shells. Perisphinctes has what’s called an evolute shell, which means the whorls only slightly cover each other. So when you look at your Perisphinctes specimen sideways on, you’ll see a large, open spiral in the middle called the umbilicus. A widely held, but by no means ubiquitous belief is that the more open the umbilicus the less rapidly the ammonite was able to move. That’s because this part of the shell would have created a lot of drag as the ammonite moved through the water.
Another reason ammonites like Perisphinctes are assumed to have been slow swimmers is the presence of thick ribs on the flanks of the shell. There is an entire vocabulary invented by geologists for describing the types of ornamentation seen on ammonite shells because it’s so important for telling apart closely related species. But what you’ll see on a typical Perisphinctes is that the ribs fork part way down the flank, dividing into two or three finer ribs across the dorsal surface. One of the things you’ll see on some Perisphinctes specimens is that the ribbing becomes coarser on the later whorls compared to the earlier ones. This kind of change seems to have occurred once they became sexually mature, and is actually quite typical of ammonites generally. If you can’t see this on your specimen, it doesn’t necessarily mean yours was a youngster. It could well be that the adult living chamber was broken off, perhaps by a predator, so what became fossilised was the inner, gas-filled part of the shell that was a lot more durable thanks to its internal chambers.
Perisphinctes are also notable for exhibiting sexual dimorphism. Scientists studying assemblages of specimens all from a single horizon (or time interval) noticed that many ammonites, including Perisphinctes, seemed to occur in two basic sorts apparently living alongside each other. Often the earlier whorls were identical, but they grew to two different sizes and that the smaller ones often developed distinctive modifications to their apertures (called lappets) that the larger ones lacked. Lappets looked like flaps, extending outwards from either side of the aperture, but their function, if any, is uncertain. It’s often assumed nowadays that the smaller ones with the lappets were actually the males of the species, and that the lappets served no practical purpose beyond exhibiting the sexual maturity and vigour of the ammonite that carried them. But really, there’s no solid evidence to be sure of any of this, and it remains, at best, an hypothesis.
Can you tell if yours is a microconch or a macroconch? Well, if it’s a really big specimen, 8-10 cm across, it’s probably a macroconch. But small specimens could be juveniles of either sort, but if a small specimen exhibits the change in ribbing from fine to coarse on its out whorl it may well be a microconch. It’s pretty rare to see specimens with lappets intact because they were often broken off during the fossilisation process, but obviously if yours has them, it’s a microconch.
There are lots of other ammonites of course, including the bizarre heteromorphs that seem to defy logical explanation. But Perisphinctes is a good starting point, exhibiting all the key details that make the group so interesting, and definitely ones of those fossils that we like to call an essential collectible.