Further rummages through the cephalopods — ammonites (and others) from around the world (Part 2)

Neale Monks (UK)

In first part of this two part series (Rummages through the core collection of British cephalopods (Part 1)), we looked at some of those British ammonites and belemnites that you’re likely to have in your collection. As we saw, even the most familiar species can have mysteries surrounding them, particularly with regard to things like diet and swimming abilities.

But, if you’re a fan of ammonites and their relatives, there’s a good chance you’ve picked up some specimens from other parts of the world as well. Indeed, one of the most remarkable things about ammonites in particular is how widespread some species are, which has made them among the most important groups for biostratigraphy. Put simply, biostratigraphy is the art of using fossils common to two or more geological formations to demonstrate the fact that they were deposited at the same time.

Ammonites

Being distinctive in shape and often very common, ammonites were among the first fossils to be used for biostratigraphy and identifying ammonite species from different deposits was pretty much all people did with them. Understanding them as living creatures was a minority pursuit until relatively recently and, to be fair, ammonites have proven to be quite difficult fossils to explain. Unlike, say, dinosaur or fish bones, ammonite shells tell us relatively little about what the muscles and others soft parts looked like.

In some ways, ammonite shells are like sharks’ teeth — abundant and very varied, but only very narrowly informative. It is generally accepted that ammonites were neutrally buoyant, much like modern nautiluses, and experiments with model ammonite shells has shown that differences in shape and ornamentation would have made some species better swimmers than others. But beyond that, many aspects of ammonite biology and evolution remain clouded in mystery.

Nautiluses and belemnites

Fossil nautiluses and belemnites are perhaps a bit easier to explain. For a start, nautiluses are still alive today, so one way to understand the fossil species is to compare them with the living ones. Similarly, while belemnites have been extinct since the end of the Cretaceous, the group to which they belong, the coleoids, are hugely abundant today in the form of cuttlefish, squids and octopuses. Belemnites share many features with squids and cuttlefish in particular, with which comparisons can be drawn.

Name:	‘Orthoceras’ sp
Age:	Devonian
Location:	Atlas Mountains of Morocco
Distinctive features:	Unlike the living species with their coiled, ammonite-like shells, orthocones have long, thin, tapering cone-shaped shells. Removed from the matrix, you may see a stack of discs that represent the now mineral-filled chambers of phragmocone that were originally filled with gas to help the nautiloid float. Commercially traded specimens are usually sold as polished sections with the chambers clearly visible.
Ecology:	Orthocone, or straight-shelled, nautiloids are assumed to have been nektonic animals, but beyond the fact they could swim, most aspects of their ecology remain obscure. Bryozoan fossils have been found that completely cover orthocone nautiloids shells, suggesting the nautiloids were actively swimming while alive. But the larger species would have been extremely ungainly, thanks to their size and the mass of cameral deposits needed to hold their shells horizontally. It is believed these larger species were more like flounders or rays, sitting on the seafloor unless disturbed.
Notes:	The popular Moroccan ‘Orthoceras’ are likely a variety of species, possibly Michelinoceras michelini and Orthocycloceras fluminense, the names tentatively given to the ones seen in fossil shops. Do note that partial forgeries are common, the most common trick being to carve the matrix in such a way that the fossil looks a lot bigger than it actually is. Sometimes slabs are made up of several smaller fragments cunningly glued together, while jumbo specimens may even be several smaller fossils glued end to end.

Name:	Goniatites sp
Age:	Devonian
Location:	Atlas Mountains of Morocco
Distinctive features:	Involute, moderately compressed ammonite, with gently curved ribs and weak tubercles close to the umbilicus.
Ecology:	Adult goniatites resemble ammonites, but recent studies have shown that the juvenile forms were often very different, with a much more globose shell shape than the adults. This has been interpreted as a way of ensuring adults and juveniles did not compete for the same resources, in much the same way as caterpillars feed on different foods to adult butterflies. In this case, hatchling goniatites drifted about in the plankton, whereas the adults actively swam about at the bottom.
Notes:	Goniatites are primitive ammonoids that evolved during the Devonian and lasted until the end of the Permian. Compared with nautiloids, they have considerably more complex suture lines, typically forming some sort of zigzag pattern. This is easily distinguished from the more curved suture lines usually seen on the ceratites that replaced them during the Triassic. Quite why we see this increasing suture line complexity from goniatites to ceratites and then finally ammonites remains uncertain. The two commonest explanations are that this was an adaptation to living in deeper water or else provided better defence against predators that were trying to crush their shells.

Name:	Arcestes sp
Age:	Triassic
Location:	Indonesia
Distinctive features:	Globose shell with a tight, involute coiling mode. Weak ornamentation, if any. The suture line is ceratitic, for example, consisting of back and forth arches, rather than the zigzag shape typically seen on the more primitive goniatites.
Ecology:	These early ammonites are difficult to explain. Their globose shells are strongly reminiscent of modern nautiluses, which might imply a similar ecological niche, for example, slow swimming close to the seafloor, while opportunistically feeding on carrion as well as sessile prey unable to swim away. On the other hand, this genus of ammonite is essentially worldwide in distribution, suggesting that the juveniles may have been planktonic, using oceanic currents to drift their way around the globe.
Notes:	Arcestes are members of an order of ammonoids known as the Ceratitida. These appeared during the Late Devonian and lasted until the end of the Triassic, when they were wiped out by what is known as the Triassic–Jurassic extinction event. Only a handful of ammonoids derived from the Ceratitida survived this event, members of a group called the Phylloceratina, and it is these that ultimately gave rise to the full flowering of ammonite diversity for which the Jurassic and Cretaceous are famous.

Name:	Garantiana garantiana
Age:	Middle Jurassic
Location:	Commercially collected in France, but widespread across western Europe as far north as the Hebrides.
Distinctive features:	Moderately involute and laterally compressed ammonite, with fairly straight, occasionally branched ribs. There is a clear groove running along the ventral part of the shell.
Ecology:	Garantiana is one of those ammonite genera for which sexual dimorphism has been suspected. Whereas Garantiana are supposed to have been the macroconchs, a different genus of ammonites, Pseudogarantiana, are thought to have been the microconchs. Certainly, the two genera are frequently found together, and while Garantiana are fairly conventional in appearance, Pseudogarantiana are smaller and sport lappets on either side of the aperture when fully grown. The usual interpretation is that the microconchs were the males, and that the lappets served no real practical purpose beyond displaying the maturity and fitness of the animal that bore them.
Notes:	Garantiana species are used for biostratigraphy across Europe and into the Caucasus region. This is reflected in the name of the Garantiana oil field some 100km off the west coast of Norway.

Name:	Cleoniceras sp
Age:	Cretaceous
Location:	Madagascar
Distinctive features:	Involute, moderately compressed ammonite, with gently curved ribs and weak tubercles close to the umbilicus.
Ecology:	Ammonite ecology is notoriously difficult to determine, but one line of evidence is the ratio of certain isotopes in shell material. The idea here is that temperature affects the ratio of, for example, the two isotopes of carbon or the two isotopes of strontium. By sampling ammonite shell material, where preserved, geologists can determine the ambient temperature at the time the shell was formed, which provides information on seasons and water depth. In the case of Cleoniceras, one study indicated ambient water temperatures around 18˚C, suggesting it didn’t live in the warm surface waters of its tropical habitat, but a bit deeper down the water column, around 100m or so.
Notes:	Cleoniceras belong to an important family of ammonites known as the Hoplitidae. These were common during the middle part of the Cretaceous (specifically, the Aptian, Albian and Cenomanian stages) and individual species were often geographically widespread but relatively short lived in geological terms, making them extremely useful for biostratigraphy.