Heteromorph ammonites were a group of externally shelled cephalopods that were particularly diverse during the Cretaceous period. Many species were abundant and geographically widespread and, for this reason, they have been used to date and correlate rocks.
Unlike regularly coiled ammonites, which underwent a steady decline in diversity through the Cretaceous, the heteromorphs continually produced new and often bizarre species indicating a certain level of success at occupying new ecological niches. Only at the final mass extinction, at the Cretaceous-Tertiary boundary, did the heteromorphs finally fail.
What makes a heteromorph?
Broadly speaking, heteromorphs are ammonites with shells coiled in something other than the normal way. Whereas most ammonites had shells that can be described as flat, closed spirals where each whorl at least partially enclosed the one before it, heteromorphs had shells that coiled in a variety of ways. Some were simply open spirals, while others were helical like snails, or consisted of approximately parallel shafts connected by tight bends, so that the resulting shell looked a bit like a paperclip.
At the most extreme, there was Nipponites. This is an ammonite with a shell formed from connected U-bends, each at an angle to the preceding one, resulting in something that looks more like a tangled ball of string than a proper ammonite!
But there is more to being a heteromorph than simply having a bizarre shell. Indeed, some heteromorph ammonites descended from forms with peculiar shells and re-evolved tightly coiled, spiral shells like those of regular ammonites. The Scaphitidae are perhaps the best-known examples of such heteromorphs. What helps to reveal even these non-heteromorph heteromorphs for what they are is the shape of their ‘suture lines’.
The suture line is visible where cephalopod fossils have had the outside of the shell worn away or removed, so that you can see the chambers and walls in between them. The line is the pattern observed when one of these walls is traced on the ammonite’s shell surface. Perhaps surprisingly, it varies quite a lot between species and can be used as a sort of fingerprint to identify fragmentary ammonites that otherwise don’t have enough detail to be identified by shape or ornamentation alone.
Ammonite suture lines generally curve forwards and backwards. The curves in the direction of the aperture are known as ‘saddles’, while the ones pointing backwards are known as ‘lobes’. Heteromorph ammonites have suture lines that are usually described as ‘quadrilobate’. This means that there is one lobe on the ventral surface, two on the flanks, and then one more on the dorsal surface. Compared with the suture lines of contemporaneous ammonites of the regular sort, the heteromorph ammonite suture line is distinctive and comparatively simple.
Quite what this means in biological terms is not clear, but a common assumption is that the more complex the suture line, the better able the ammonite was to swim into deep water. Unfortunately for this interpretation, the modern nautilus manages to live perfectly well at depths of up to 300m, yet its suture line is very simple, far simpler than even those of the heteromorph ammonites.
Theme and variation
Understanding precisely what heteromorph ammonites were doing in their native environments is difficult because nothing similar is alive today.
Among the modern cephalopods, the fact that their shells are completely different to those of modern nautiluses would seem to preclude any close ecological similarities and it is hard to see how anything with a heavy, external shell could have crawled about among rocky reefs in the same way as modern octopuses. Neither do heteromorphs seem likely to match the fast-moving squids of the shallow seas, since their shells would certainly produce a lot of drag.
About the only modern cephalopods that might hold clues to the lifestyles of the heteromorphs are to be found among the deep-sea squids. One in particular, Spirula spirula, is especially intriguing. This small (around 10cm) squid has a coiled shell inside its body and, in life, it hangs head-downwards. It is one of a ‘guild’ of animals known as ‘vertical migrators’. What this means is that while it moves very little in the horizontal plane, it moves up and down a great deal. During the day, it lurks at depths of over 1,000m but, by night, it rises into the top couple of hundred meters of the sea where it feeds on planktonic organisms of various types, such as small crustaceans.
Did heteromorphs do a similar thing? It is certainly tempting to explain the open shells of heteromorphs as devices that allowed them to drift about in midwater with their bodies held in an orientation favourable for catching food. By adjusting their buoyancy, perhaps they were able to rise and fall in the water column without expending much energy.
The problem is that heteromorph ammonites are not found in sediments indicative of very deep-water environments. Of course, their shells might have drifted from the open ocean inshore where they were fossilised, so by itself this doesn’t prove anything either way.
Probably the best evidence for the lifestyles of the heteromorphs comes from studies that connect the abundance of different types of ammonites to the ‘facies’, or geological environment, in which they are found. Recent studies of Albian heteromorphs found them to be most abundant and diverse in sediments indicative of fairly deep, turbulent, relatively phytoplankton-rich conditions. Regular ammonites, by contrast, were more commonly found in sediments typical of stable, relatively phytoplankton-poor environments. Does this imply heteromorph ammonites lived in the open sea feeding on plankton in much the same was as Spirula does today?
Some of the varieties that might be compared with Spirula are ptychoconic ammonites such as Ptychoceras. These ammonites had small shells comprised of three or four parallel shafts connected with tight 180o bends, with the resulting shell being somewhat similar to a paperclip in shape. Because of certain peculiarities in the construction of the shell, some authors have suggested that Ptychoceras had internal rather than external shells. If this was so, then these ammonites might have been even more like Spirula in habit than might otherwise be assumed. Regardless, they are ammonites that are typical of offshore rather than shallow-water environments.
Another interesting variety of heteromorph is the straight-shelled or orthoconic ammonite. Exemplified by types such as Baculites and Sciponoceras, these ammonites are often so common they have been used for biostratigraphy. Although, at first glance, they might be compared to the orthocone nautiloids prevalent in the Palaeozoic, they differ in a number of important ways. For a start, they lack the internal deposits inside the chambers at the apex of the shell that in orthocone nautiloids at least enabled them to swim parallel to the sea floor. Instead, orthocone ammonites appear to have floated with the apex pointing upwards. The assumption is that these heteromorphs moved up and down the water column like Spirula, rather than along the sea floor.
Most heteromorphs fall into another variety often referred to as the hamiticones, after one of the more important genera from this type, Hamites. While variable in detail, the basic plan here is that the shell starts off as an open spiral (sometimes flat, sometimes helical) and then opens up into a U-shaped living chamber. About the only thing certainly known about these ammonites is that they were very poor swimmers, but beyond that palaeontologists debate the functional significance of this particular shell design. Most likely, they were oceanic drifters that lived close to the surface, feeding on plankton, perhaps analogous to modern jellyfish.
Two other varieties of heteromorph are also worth mentioning. The helical heteromorphs such as Turrilites and Nostoceras are derived from hamiticone ammonites such as Hamitella through a process of paedomorphosis. Where Hamitella only had a helical shell when young, the adult living chamber being a large hook-shaped structure, an ammonite such as Turrilites retained helical coiling throughout life. The temptation here is to connect the retention of juvenile physical characteristics with a juvenile mode of life, but conclusive evidence is lacking. At best, all that can be said is that, while Albian hamiticone ammonites appear to have lived in offshore environments, the helical heteromorphs seem to have favoured shallower water inshore environments.
Finally, the scaphitids stand out as a group of heteromorphs seemingly reversing the trend of their relatives and favouring more tightly coiled shells similar to those of regular ammonites. The earlier members of this group, such as Scaphites and Eoscaphites, retain a hook-shaped living chamber broadly similar to those seem among the hamiticones, but the later scaphitids, such as Jeletzkytes, are much more compact and tightly coiled. The mode of life of these ammonites remains obscure – they seem to have favoured relatively deep water, but whether they lived close to the substrate feeding on benthic animals or moved about in midwater feeding on plankton, is not known for certain.
The last hurrah
The sheer diversity of heteromorph ammonites has intrigued palaeontologists for many years. Old textbooks would often cite them as an example of taxonomic senescence, the implication being that, by the Cretaceous, the once-vital ammonites had passed their prime and now exhibited only the outward signs of decadence and failure.
Quite the reverse would seem to be true – through the heteromorphs, the ammonites were occupying a wider variety of new niches and displaying a greater level of morphological diversity than they had ever shown before.
Where to find them
Unlike many of the other grand puzzles of the palaeontological record such as the Burgess Shale invertebrates and the Chinese dinobirds, the heteromorph ammonites have a rich fossil record and are easy to find. They have been found at most of the major exposures of Cretaceous rocks in England. Among the best places for amateur collectors are the Speeton Clay in Yorkshire, the Lower Chalk at Eastbourne, and the Scaphites and Crioceras Beds at Shepherds Chine on the Isle of Wight.
But one of the most interesting heteromorph ammonite faunas is to be found at the Gault Clay exposure at Folkestone. This formation spans the Middle and Upper Albian, a period of time during which the Turrilitaceae superfamily of heteromorph ammonites was undergoing a major phase of evolutionary expansion.
At the start of the Albian, there are only three families within the Turrilitaceae, the Anisoceratidae, Hamitidae and Ptychoceratidae. In fact, all three groups are known from the preceding Aptian stage as well, and the Ptychoceratidae appear to have their origins in the even earlier Barremian. For whatever reason, the Ptychoceratidae are much less significant in the Albian than they were in earlier times. Only two species are known from the Upper Gault, Psilohamites bouchardianus and Ptychoceras adpressum, and neither is likely to be found at Folkestone. Psilohamites is only known from tiny fragments and appears to have had a smooth, orthoconic form. Ptychoceras was also small and had a shell lacking ornamentation, but it had the rightly folded shape typical of the family.
Through the lower part of the Gault Clay, it is species from the Hamitidae and Anisoceratidae that are most frequently encountered. As a rule, all these ammonites are found as fragments a few centimetres long, but occasionally partial or even complete specimens can be found. The Hamitidae included species that grew to as much as 30 to 40cm in length, as in the case of Hamites maximus, one of the more common Gault Clay species. In common with many of the Hamitidae, this species started off as an open helical spiral and only subsequently did the shell unfold into its distinctive paperclip shape. Other species remained helical throughout their life, as in the case of Hamitella rotundus, another species that is reasonably common in the Gault Clay.
The Anisoceratidae were similar to Hamitidae in basic shape but unlike the Hamitidae that never had spines on their shells, the Anisoceratidae commonly had one or two pairs of spines along the lateral and ventral surfaces. Some of the Anisoceratidae were quite large ammonites by any standards, with some species of Anisoceras over 60cm in length. While only the bases of their spines are usually preserved, it isn’t difficult to imagine what these ammonites might have looked like in life – big, spiny animals surely ignored by most predators!
Like the Hamitidae, the Anisoceratidae had a tendency towards helical coiling. In most cases, the shells started off as open helices and then unfolded into flat open spirals or hooks. However, at least one genus, Pseudhelicoceras, contains species that were helical throughout life. Pseudhelicoceras robertianus is reasonably common in the Gault Clay, where its spiral shell and large spine bases make it instantly recognisable and one of the most distinctive ammonites in the entire formation.
One of the nice things about the fossils of the Gault Clay is their fine preservation – some specimens even preserve some of the original shell material. The suture lines are usually very clear and easy to trace, and this has made it easier to reconstruct the evolutionary history of the heteromorph ammonites. The Gault Clay has proved to be very useful for this, since it contains an abundance of specimens across many different families. While the Anisoceratidae were something of an evolutionary dead end, the Hamitidae gave rise to no fewer than four families that went on to be diverse and abundant throughout the later Cretaceous: the Baculitidae, Scaphitidae and Turrilitidae directly, and through the Turrilitidae, the Nostoceratidae.
Examples of the Baculitidae, Scaphitidae and Turrilitidae can be found at Folkestone. Species of Turrilitidae are probably the most commonly found though, at first glance, their tightly coiled, helical shells could easily be confused with snail shells. Proturrilitoides and Turrilitoides are both present in the Gault Clay, with Turrilites hugardianus being the species most frequently seen.
Eoscaphites represents the Scaphitidae and can be found towards the top of the Gault Clay. Although never common, it is easily overlooked because its tight coiling is much more like that of a regular ammonite than a heteromorph.
From the Baculitidae, only Lechites gaudini has been found in the Upper Gault, and only very rarely. Lechites are more common in the succeeding Upper Greensand formation. Indeed, it is only in the Lower Chalk that species from the families Baculitidae, Scaphitidae and Turrilitidae become truly common, while both the Anisoceratidae and Hamitidae enter a period of decline from which they do not recover, presumably being replaced ecologically by the newer groups.