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Ammonite maturity, pathology and old age<< Cephalopod Articles | By , Department of Paleontology, Natural History Museum, London
Dr. Neale Monks has long been a contributor to The Cephalopod Page. A version of this latest contribution will appear in a book he is working on and was inspired by a question from a keen and curious TCP reader. Many of the topics covered on TCP are directly inspired by questions from the public. A warm thank you goes to Neale and other TCP contributors as well as readers and fans that inspire us more than they know.
NOTE: Neals's book Ammonites is highly recommended. -Dr. James B. Wood
Mature ammonites are quite easy to identify. Sexually dimorphic ones develop the lappets or collars at maturity, as well as any distinctive ornamentation. Even species with little or nor dimorphism show signs of a slowdown in growth. The septae become bunched, as the new chambers become smaller. But being able to tell when an ammonite reached maturity isn't the same thing as being able to say how old it is. So how long did ammonites live for?
Obviously without any living ammonites to observe, so our estimates of the length of ammonite lifespans are somewhat vague. Living nautiluses may offer a clue. As they grow, nautiluses need to add new chambers to their shell to retain neutral buoyancy. In fact they are added at a rate of about one every month or so, and fully grown nautiluses can have over thirty chambers, indicating that they must take two or three years to reach maturity. If ammonites grew at a similar rate, then simply counting the number of chambers would give a reasonable estimate of their lifespans.
The problem is that we don't know just how typical the growth of the nautilus is compared with ammonites, or even extinct species of nautiluses. For a start, the living species of nautilus lives in fairly deep, cold water. High water pressure would reduce the rate at which chambers could be emptied, while low ambient temperatures would slow down its metabolic rate. The rate at which it could grow might therefore be rather low. Ammonites, in contrast, are commonest in warm, relatively shallow waters and so could have had the potential of much higher growth rates. Certainly living invertebrates, including cephalopods, grow fastest in warm, shallow water.
The great ammonite palaeotologist Otto Schindewolf published a report of a Jurassic ammonite encrusted with tubeworms. This paper was important because it tied together something we can observe today, the growth of tubeworms, with something unknown, the growth rate of ammonites. He noticed that on this specimen of the ammonite Schlotheimia an encrusting serpulid worm had grown its tube around the ammonite's shell in such a way that the two animals must have been alive at the same time. Schindewolf knew the growth rate of these worms today, and so could estimate the growth rate of the ammonite from this fact. He thought that each whorl of the ammonite shell took somewhere in the range of four months to three years to grow. A typical ammonite can have a dozen whorls, and so again we are looking at a lifespan of anything from 4 to 36 years. But again we must admit that while Jurassic species of tube worms might have grown at the same rate as their relatives alive today, we can't be certain.
Ammonite shells contain various minerals, some of which exist as isotopes. Isotopes are varieties of elements, such as oxygen, which differ in atomic weight but are otherwise chemically identical. An odd thing about the isotopes of oxygen in particular is that they occur in different proportions depending on the ambient temperature. By carefully analysing the composition of the shell it is possible to detect the relative abundance of each isotope of oxygen at the moment that piece of shell was being laid down. In a way, every bit of the shell is a thermometer set to the exact moment it was made! Palaeontologists have found that if pieces of shell are taken from points all along the shell, different oxygen isotope proportions are found. In fact, it seems that as the ammonite grew it was sometimes in warm water, sometimes in cooler water, and then back again to warmer water, with this cycle happening many times. The most obvious conclusion is that these are simply the seasons - the periods of growth recording cold phases having occurred during the winter, and the warmer phases in summer. By adding up these cycles, you can tell how many years the ammonite lived for. Borrisiakoceras, for example, a small Cretaceous ammonite, has been reckoned to have lived for about a dozen years.
All these age estimates greatly exceed those of the coleoids but are comparable to the living nautilus. It would seem, therefore, that ammonites lived relatively slow lives like the nautiluses, rather than the brief but frenetic ones of the coleoids.
A related and very interesting aspect of ammonite palaeontology is the study of old and sick ammonites. Specimens of particularly old ammonites, called gerontic specimens, look like mature specimens but often show extreme bunching of the septae, large size, and degraded or deviant ornamentation. Gerontic specimens are curious but not terribly informative; in contrast, pathological specimens are. These are ammonites which have sustained some degree of shell damage. In some instances this damage has not brought about the death of the ammonite, and the fossil shows signs of subsequent growth. Often these ammonites had been infected with some disease, or attacked by parasites, which they were either able to ignore or else rid themselves of. In other instances, they had been battered by some predator that was either too weak or inexperienced to kill the ammonite, and which had then given up its attack. Not all ammonites were so fortunate, and these left fossils which show lethal damage. Large marine reptiles and fish that preyed on ammonites have left their mark in this way. Their teeth leave bite-marks on the shells, and by matching the arrangement of these marks it is possible to deduce which exact species of reptile or fish fed on which ammonite, and even how they attacked the ammonite! By looking at a variety of fossils that show pathological damage it has been possible for scientists to establish where ammonites fitted into the marine food web.
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