Neale Monks (UK)
Alongside trilobites, ammonites are by far the most popular invertebrate fossils. Whether you’re an enthusiastic fieldworker or more of an armchair geologist, chances are that your collection includes a fair number of ammonites of one sort or another. These may well have names and localities, but details on the ecology of ammonites is often lacking, and relatively few popular books on geology say much about how ammonites lived beyond the fact they were marine invertebrates related to modern squids and octopuses.
As it happens, British geologists are well served when it comes to ammonites, with extensive exposures of Jurassic and Cretaceous strata accessible across the country, from the Kent coast to the Isle of Skye. Coastal exposures are the ideal, of course, but old quarries and other inland sites can be fruitful too, and the UKGE website is there to help anyone wanting to build up their own ammonite collection (see https://www.ukge.com/).
Belemnites are often found alongside ammonites, and being cephalopods as well, resembled ammonites in many ways. The biggest difference was that they were more squid-shaped animals, with the buoyant shell inside the animal instead of outside. They were probably quite good swimmers, but whether they foraged close to the seafloor like cuttlefish, or high up the water column like squid, remains unclear. It may well be that different belemnites did different things, and the variation we see in belemnite fossils reflects this.
But what, if anything, is the quintessential British ammonite? And if such a beast exists, would there be an equivalent belemnite to add to the core collection of British fossil cephalopods? What follows are some possibilities, either easy to collect in person or else inexpensively through the UKGE online fossil shop.
|Formation:||At Whitby, this species is very common in the Alum Shale Member.|
|Distinctive features:||This is the snake-stone ammonite par excellence, each whorl having an essentially circular profile, and the whole thing coiled tightly into a tight spiral. Ornamentation is very typical of Jurassic ammonites, with simple rib-like structures running neatly around the whorl without any sign of the keels or spines often seen on later ammonite varieties.|
|Ecology:||Ammonites with circular whorls and simple ribs are usually interpreted as having been slow swimmers. Producing too much drag to be fast and lacking the keel-like structures believed to provide stability, they probably bobbed about at a leisurely pace for the most part, hunting for whatever slow-moving prey (or even carrion) ammonites fed upon. Unspecialised as they may have been, these ‘basic’ ammonites were incredibly successful for tens of millions of years.|
|Notes:||Easy to collect, but also widely sold at fossil shows and online, this is definitely one ammonite likely to end up in pretty much everyone’s collection. In the past, specimens from Whitby might have heads carved onto them, supposedly to lend credence to the story that the Abbess Hilda had dealt with an infestation of snakes by turning them into stone.|
|Formation:||At Whitby, this species is most characteristic of the Whitby Mudstone and Alum Shale, but closely related species can be found in equivalent (Toarcian) formations, such as the Upper Lias of Somerset.|
|Distinctive features:||Compared with Dactylioceras, the whorl section is more flattened from side to side, and each whorl very slightly overlaps the preceding one. Unlike Dactylioceras, there is a definite keel along the ventral surface and the ribbing, while still quite simple, is much coarser. So, while Hildoceras may have been marginally more stable than Dactylioceras, its shell was still poorly adapted for swimming. In fact, these ammonites likely spent most of their time on or just above the seafloor, relying on their strong shells for protection, rather than trying to swim away.|
|Ecology:||Hildoceras were among the earliest ammonites to show sexual dimorphism, that is, morphological differences between males and females. When fully grown, the microconchs — the smaller of the two size classes — have lappets that extend forwards on either side of the aperture. These are assumed to be the males, although what the lappets are for remains a mystery. The simplest explanation is straightforward sexual display, the lappets having no other function beyond showing that the male is mature and healthy, rather like the tail of a peacock.|
|Notes:||It probably goes without saying that this genus of ammonites is named after St Hilda, the aforementioned abbess of Whitby. But associating ammonites with snakes is actually quite common. Similar traditions appeared in the Mediterranean, for example, albeit substituting St Hilda with St Paul, and the snakes being blinded before they were petrified, presumably explaining the lack of an obvious head on the ammonite fossil.|
|Formation:||This is one of the classic Lower Lias ammonites and can be found at the usual Lower Lias exposures in Dorset and Somerset. Marston Marble is a type of limestone famous for containing a high density of Promicroceras fossils.|
|Distinctive features:||A typical Jurassic ammonite in shape, with simple ribs, no keel and each whorl only slightly overlapping the previous one. At Lyme Regis, they are often preserved in the form of calcite crystals, making them very attractive fossils.|
|Ecology:||These small ammonites seem to have been short-lived but abundant, although, given their lack of streamlining, they were presumably poor swimmers. That being the case, they likely ambled about on or just above the seafloor, although our understanding of precisely what ammonites fed upon remains a bit vague.|
|Notes:||Scientists have found many specimens of this ammonite with serpulid worms on their shell, evidently in such a way that the worms lived on the ammonites when they were alive rather than after death. Such encrustations were useful for the worms, carrying them across the seafloor so that they could filter out particles of food from the water more effectively than if the worms were growing on something that didn’t move, like a rock. Unfortunately for the ammonite, these worms weighed them down and they seem to have lived shorter lives than their unencumbered brethren.|
|Formation:||Abundant in the Gault Clay at Folkestone, this is one of several Hamites species known from that locality.|
|Distinctive features:||A typical heteromorph ammonite, this species resembled a paperclip in shape, with three or four ‘shafts’ connected with tight, 1800 bends. Geologists rarely find complete specimens though, with fragments of varying length being much more common. Ornamentation is limited to sinuous ribs, although other heteromorphs, such as Turrilites and Scaphites, had spines.|
|Ecology:||The ecology of heteromorph ammonites remains contentious. Most scientists assume a planktonic lifestyle analogous to modern day cranchid squids, but there are so many different shell shapes to be seen among the heteromorphs that a variety of ecological niches is probable. Certainly, their open shells would seem to preclude active swimming, and some scientists have supposed they could float well enough to crawl across the seafloor using their arms and only drifting away when disturbed. So, instead of being planktonic, they may have been bottom dwelling hunters or foragers.|
|Notes:||The evolution of the genus Hamites is interesting in that different species appear to have been the precursors of later groups of heteromorphs. So, while there are Hamites species that are helical, ultimately giving rise to Proturrilitoides and the other Turrilitidae, there are also Hamites species that have hook-like body chambers that seem to be leading towards the Scaphitidae via Eoscaphites. Whatever else we can say about Hamites, they do seem to have been extremely widespread with a near-global distribution. Is this evidence of a planktonic lifestyle, whether as adults or juveniles? Nobody knows for sure.|
|Formation:||Very common in the Oxford Clay, but fragments are often reworked into the Pleistocene Boulder Clay.|
|Distinctive features:||Large, dense cylindrical or conical fragments; a ventral groove is often apparent on the tapering tail-end fragments, while the head-end fragments have a conical recess that enclosed part of the phragmocone.|
|Ecology:||Widely distributed in Boreal seas including quite cool regions, in life this belemnite was probably more like modern coastal rather than oceanic squid. The mantle cavity was not large, implying relatively little water could be ‘jetted out’ at any one time, so these streamlined animals may have been steady rather than rapid swimmers. The size and weight of the belemnite guard is difficult to explain. While the guard was evidently a counterweight to the buoyant phragmocone, modern squid manage without either, albeit with the need to swim constantly or otherwise find themselves sinking.|
|Notes:||As with all belemnites, by far the commonest remains are more or less complete pieces of the dense calcite rostrum (or guard). The whole rostrum was around 15cm to 20cm in length, implying this squid-like animal was rather large, maybe a metre or so in length. In life, the rostrum supported fins like those on squid, but the front few centimetres of the rostrum have a conical hollow that wrapped around the phragmocone, the gas-filled chambers that provided neutral buoyancy.|
|Did modern squid evolve from belemnites?|
|While belemnites do resemble squid or cuttlefish – the supposed ‘Neobelemnitidae’ of the Eocene – are a most perplexing group of fossils. It seems that belemnite-like guards evolved among certain cuttlefish- or Spirula-like cephalopods, and these account for some of the reports of Eocene belemnites. But other fossils supposed to be belemnites, such as Bayanoteuthis, are so ambiguous as to be difficult to assign to any particular group. In fact that fossil has even been identified as a coelenterate of some sort.|
|Formation:||Neohibolites of various species are found in a number of formations, perhaps most famously the Red Chalk at Hunstanton.|
|Distinctive features:||Where Cylindroteuthis has a cylindrical guard, Neohibolites has the more tear-drop shape typical of Cretaceous belemnites. There is also a more pronounced groove on the ventral surface. While not often preserved, Neohibolites developed an extended tip to the guard known as an epirostrum. The function of this needle-like structure is uncertain, though one suggestion is that it was an example of sexual dimorphism, since they only occur on fully grown specimens.|
|Ecology:||This genus of belemnite includes mostly small species, but they are very widespread, with species known from as far afield as Japan and Antarctica. However, they do seem to be associated with warm water environments and are not found in cold water deposits. This has led some scientists to hypothesise that belemnites as a group were actually better adapted to warm climates, and the gradual cooling that occurred across the Late Cretaceous caused the decline of the belemnites and their replacement by modern cephalopods such as squid.|
|Notes:||Belemnites have a rich body of folklore attached to them, including the idea that they were formed when lightning hit the ground. While often overlooked by collectors in favour of the flashier ammonites, belemnites are important fossils that have often been used for biostratigraphy, Neohibolites species included. They can be quite difficult to identify though, lacking the prolifically varied ornamentation seen on ammonites.|
The second part of this two part series can be found at: Further rummages through the cephalopods — ammonites (and others) from around the world (Part 2). It will by uploaded the the website on Friday, 10 July 2020.
Ammonites by Neale Monks and Philip Palmer, Smithsonian Books (1 May 2002), ISBN-10: 9781588340474.