Ammonites have been studied intensively for the last 200 years but, between experts, there is very little agreement on what ammonites looked like or how they worked as living organisms. Could they float? Did they swim? How did they catch their food? How long did they live? Why did they disappear at the end of the Cretaceous? All these questions remain essentially unresolved.
In fact, ammonites are a quite poorly understood group of fossils in many ways. By far the majority of scientific papers written about ammonites concentrate purely and simply on what is known as primary taxonomy — is this ammonite species distinct from all the others so far discovered and, if it is, how can it be recognised reliably and where else can it be found? The reason most scientists concentrate on these questions above all others comes down to the usefulness of ammonites for biostratigraphy. Many ammonite species evolved and died out within fairly short periods of time, perhaps a few hundred thousand years, but their fossils are often abundant and, most crucially of all, often very widely distributed. So, if a particular ammonite species can be found in sediments at two different localities, it’s a good indication that those two sediments were laid down within the same, rather narrow period of time.
Just taking British palaeontologists as an example, virtually all the major scientists working on ammonites did so to further their studies of biostratigraphy: WJ Arkell, R Casey, MR House, MK Howarth, WJ Kennedy, HG Owen and LF Spath, to name just a few. While their publications have been hugely important in terms of ammonite systematics and evolutionary relationships, they wrote hardly anything at all about ammonite anatomy and ecology. You’d see much the same pattern looking at palaeontologists from other parts of the world and biostratigraphy, rather than biology, continues to be the focal point of most ammonite research, even among the younger generation of palaeontologists.
So what don’t we know about ammonites? What do palaeontologists argue about? That’s the focus of this article and you’d perhaps be surprised to know just how fierce some of these arguments can be.
Could they float?
It would seem self-evident that ammonites were neutrally buoyant. After all, modern nautiluses have very similar shells divided up into gas-filled chambers and they certainly do float in midwater. Their buoyant shells make it much easier for the nautiluses to swim about, so, unlike snails dragging heavy shells, nautiluses are comparatively speedy animals able to forage for food much more efficiently. In terms of design, ammonites have broadly similar shells with chambers and a siphuncle that connects them. Modern nautiluses use the siphuncle to replace the seawater in new chambers with gas and it isn’t unreasonable to assume that ammonites did the same thing (particularly since we know from fossils that they also possessed this tube). As they grew, they added new chambers to their shells, filled them with gas and, in that way, retained neutral buoyancy.
Sounds very neat and tidy, but not everyone agrees. German engineer, Klaus Ebel, has looked at ammonite shells and is convinced that they simply aren’t big enough to have provided neutral buoyancy. In a nutshell, to be neutrally buoyant, an ammonite must displace a greater weight of water than itself. Nautiluses clearly do this and, when examined closely, it can be seen that their hollow, gas-filled shells are rather large compared to their relatively small bodies. Ebel argues that ammonites didn’t have such a favourable ratio of body weight to shell size, so the weight of water they displaced wasn’t enough to counterbalance their body mass.
Ebel reconstructs ammonites as bottom-living animals, with shells that were buoyant enough to be lifted off the substrate, but not so buoyant that they pulled the animal up with them. Furthermore, he believes that the shape of the ammonite shell proves his point. As it grew, a typical ammonite shell continually ‘toppled over’ under its own weight, resulting in the classic spiral shape. Although Ebel’s interpretation of ammonite shell design is a minority viewpoint, it does at least fit in with what we can measure from ammonite shells, as opposed to vague assumptions that they worked in the same way as those of modern nautiluses. On the other hand, the distribution of ammonite fossils is a problem, because very many ammonite fossils are found in sediments laid down under poorly-oxygenated conditions and notably lacking in fossils of unambiguously benthic (bottom dwelling) organisms such as crinoids, clams or brachiopods. That’s not a problem if the ammonite shell sunk down from above, but it seems very unlikely that ammonites could have crawled about in such habitats.