Ammonite sutures in interior of shell much simpler than you would expect

      Late Mesozoic ammonite whose maximum diameter is 10.5 cm (4 in.);

       the specimen is from Madagascar

This blog is a follow-up on my previous blog, which also deals with ammonite sutures. What I want to show you now is something that I noticed years ago about suture lines. I have looked through many textbooks for a similar depiction, but I have never found any illustration of what I am showing here. The photograph above is of the exterior (partly worn) surface. Along the upper part of the photograph, you can see complex ammonitic sutures.

This second photograph shows one half of the interior of the same specimen that is illustrated above (i.e., the specimen was sliced in half). The bright yellow material on the right side is secondary foreign mineral material that replaced some of the chambers. Notice that in the upper part of the photograph, the partitions that form the chambers are very simple curves. They are not like the complex sutures lines show in the first photograph. The suture lines are complex ONLY at the junction between the chamber partitions and the outer shell wall.

The presence of complexity of the sutures at the junction with the wall, but not in the main parts of the chambers, begs the question "why." The answer is not clear because ammonites are extinct organisms. Some experts believe that the complex suture lines are related to the strengthening of the outer shell wall, so as to resist being crushed by hydrostatic pressure as the ammonite slowly swan (descended) into relatively deep depths in the ocean environment. The main parts of the chambers must have not needed the extra reinforcement.