The tide went out, and then it went out some more and just to be sure it went out still further, till it appeared like the entire Bristol Channel had disappeared into the Western Approaches. It left behind ridges of rock with smooth flat surfaces gently dipping into muddy rock pools and a vast area of loose boulders. In amongst this grey expanse of unprepossessing foreshore a careful search for the right layers revealed a flattened fossil that looked like it had been ironed onto the rock. Preserved in muted but iridescent greens, blues and reds, the spiralled ammonite, Psiloceras planorbis, marks the lowest division of the Jurassic, the zone fossil for the base of the Lias.

The north Somerset coast was where I got hooked. My grandparents lived in Minehead and we visited frequently exploring my Dad’s childhood haunts. One of these was Doniford Beach, not far from the supremely muddy harbour at Watchet, and the home of Psiloceras planorbis. It was not long before I wanted to spend all of my time exploring this beach and the other fabulous sections of Lias at Kilve and the Triassic alabaster cliffs and bone-bed at Blue Anchor. Alabaster, crinoids, devil’s toenails (aka Grypaea arcuata), the very occasional coral and even a barely recognisable fish started to fill my bedroom. The best fossils though, were the ammonites.. There is something fascinating about a spiral, and the beautifully ribbed shells in their astonishing profusion of sizes and arrangements of ribs, keels and spines had, and still have a deep aesthetic appeal to me.

It seems I wasn’t the only person fascinated by ammonites. They have sparked curiosity down the ages and explanations of their form and formation have entered myth and religion as well as science.

When I was on my travels in Nepal I had some time to spare in Pokhara waiting for permits to allow me on a trek around Manaslu. Whilst exploring the wealth of shops for the many tourists there, I chanced upon some fine examples of Jurassic ammonites and belemnites for sale. These came from a location further up the Kali Gandaki River which runs around the western edge of the Annapurna Conservation Area through Pokhara and then away down into India where it joins the Ganges at Patna. These stones are known locally as shaligrama or saligrams and are said to be manifestations of the god Vishnu and to give potent spiritual blessings.

In contrast St Hilda took a more aggressive approach. Seeing that the proposed site for her abbey in Whitby was infested with snakes, she apparently sliced all their heads off with a whip and then cast them into the sea. The profusion of ammonites in the area is thus, easily explained. On a trip to Whitby just over a year ago, we went into a café cum museum for a cup of tea, tempted in not only by the tea but also by a display of the largest piece of jet ever discovered. Over tea a close inspection of said jet, showed that its surface was completely covered in the traces of hundreds of ammonites. A look at the handy explanation revealed that the ammonites were mostly of the species Hildoceras. Good to see this dominant saint’s name taxonomically affixed and so suitably ascribed.

Ammonites also came to the attention of the Roman’s nearest equivalent to a scientist, the observer and fanatical collector of information and stories to do with nature, Pliny the Elder. He noted the similarity of ammonites to the Egyptian God Amun’s ram’s horns calling them ammonis cornua (Ammon’s horns). The name stuck, and the word ceras, meaning horn now forms the suffix of all formal ammonite species names.

Ammonites have been around a while. They are part of the Cephalopod class of animals which trace their ancestry right back to the Cambrian period over 500 million years ago. The class of cephalopods includes the coleoids (octopus, squid and cuttlefish families), nautiloids (including the modern nautilus family) and the ammonoids. The ammonoids branch off from the ancestral coleoids at about the same time as the nautiloids, sometime during the Devonian period. In evolutionary terms the ammonites are, surprisingly, closer to the squid and octopus than they are to the modern Nautilus, despite the similarity of their spiral shells. This is a reminder that when understanding fossils as animals that the preserved hard skeleton or shell can mislead us from an understanding of the form of the actual animal.

Before the ammonoids evolved into the ammonites in their strictest sense (which made their appearance at the beginning of Jurassic) there were a number of earlier ammonoid models. The first were the goniatites, appearing in the Devonian and the second were the ceratites appearing in the Permian. Goniatites, which were most common during the Carboniferous period, would be the type of ammonoid that you might find in the succession of rocks underlying Hadrian’s Wall. This is particularly true of the western end of the Wall underpinned by rocks of the Pennine Coal Measures Group. Within this sequence the cyclic marine bands are occasionally the final resting place of these ammonoids. Here they provide a useful way of telling which marine band you are in depending on what type of goniatite is preserved. The other type of cephalopod to be found around the Wall, within the earlier Carboniferous sediments of the central section of the Wall, is the one shown in this month’s mystery rock. This is a type of othocerid found on the Northumberland coast just south of Berwick. These I describe when talking about them, as ammonites that have sneezed too hard and straightened themselves out. They are however an even more primitive branch of the cephalopod family which branched off late in the Cambrian period and survived until the mass dying at the end of the Permian period.

One of the fundamental features of all these animals is their chambered shells. These are used as buoyancy aids, with a tube called a siphuncle that runs all the way around each of the chambers to allow the animal to manage this. As the ammonoids evolved, the shape of these chambers became progressively more complex. This may have initially been as a way of making gas exchange and buoyancy management easier. It also may have had the knock-on effect of strengthening their shells, better protecting them from predators and allowing them to grow larger and swim deeper. Where the margin of these chambers meets the shell, marks out a line called the suture. These are complex and have a fractal beauty in the Mesozoic ammonites. Examining the sutures shows how distinctive they are between these different families. As an undergraduate I was shown the diagram of the different sutures of goniatites, ceratites and ammonites with the progressive and distinctive increase in complexity. One of the features of the orthocerid in last month mystery rock is that it has very simple, primitive chambers of straightforward curves and sutures lacking any sort of convolution. This marks it out as a cephalopod distinct from, but nonetheless related to the ammonoids.

The ammonoids were hugely successful, surviving multiple mass-extinction events including the great-dying at the end of the Permian period. Albeit, along with the dinosaurs (excluding birds!) they didn’t make it past the asteroid blast at the end of the Cretaceous Period, they lived through some 500 million years of pre-history. Part of their success was down to their rapid lifecycle in which they produced lots of eggs, grew fast and died young. This enabled them to evolve rapidly to exploit newly created (or emptied) ecological niches. This also meant that many ammonite species were only around for a relatively short geological time period. Tough on that ammonite species but handy for a geologist trying to figure out the age of the rocks they are looking at. The answer, particularly in the late Carboniferous period as well as the Jurassic and Cretaceous period was ammonite o’clock. In the case of my inspirational Psiloceras planorbis in its Liassic bed at the base of the Hettangian Stage of the Jurassic this equated to between 201.3 ± 0.2 Ma and 199.3 ± 0.3. Time to go home for a cup of tea and some of my Nan’s excellent seed cake.

@Northumbrianman

Attributions and Recommendations

For more on ammonites and their cephalopod family I can highly recommend “Squid Empire” by Danna Staaf.

The god Ammon depicted in a terracotta fragment is from the Museo Barracco, Rome. Picture of Jupiter Ammon, a combination of Amun and the Roman god Jupiter: By Lalupa – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1803828

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