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 there 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. 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.