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.