That's rather a nice fossil! As far as I know, Titanites is the largest ammonite known from the UK and examples are also known from areas of Northern France. It is an upper Jurassic form and examples have been found up to 2 feet in diameter. Most of these larger ammonite forms seem to date from the Upper Jurassic or Lower Cretaceous.
Britain loses out to America in ammonite sizes (of course!); the North American ammonite Parapuzosia bradyi is known in sizes ranging up to 4.5 feet in diameter. Still larger was the Cretaceous period Pachydiscus seppenradensis from Germany which could attain a diameter of 6.5 feet, a truly monstrous beast.
Here's a link to an incredible example of Pachydiscus (scroll down a bit, you really can't miss it):
One really has to wonder about the lifestyle of these enormous ammonites. They would have been unable to swim very fast due to the sheer bulk of their shells and the unstreamlined shape they adopted. It also seems unlikely that they would have had ink sacs; there would be little advantage to be gained of producing a phantom image of itself in ink if the creature could only slowly move away.
I would imagine that the adult forms of these large ammonites would have been deeper water creatures as it seems that a stronger and larger shell may have been able to withstand stronger water pressures. Perhaps they lived during the day at a depth below the maximum diving depth of the marine reptiles and probably vertically migrating to shallower depths at night. Otherwise I can imagine they would be slow moving fodder at shallow depths where they could be easily spotted. Even a form such as Titanites would probably represent little trouble for a pliosaur.
I'm sure studies have been undertaken looking at the associated fauna found in the same deposits with these large species; it would be interesting to determine the depths they are thought to dwell. It would be also be interesting to know if there is any evidence of marine predation in these forms.
I suspect that Titanites and it's giant brethren would have made a difficult meal for a pliosaur, even a bruiser like Kronosaurus. A predator would have to be able to gauge the proper angle of attack, in order to get it's jaws around the thing. Pliosaurs didn't have much overlap between the eyes. Attacking Titanites from the side would likely spin the ammonite past the pliosaur's jaws; if that spinning were augmented by jets from the siphon, the result would be a target with an unpredictable trajectory.
If the pliosaur did everything right (attacked in the vertical plane, kept the ammonite in view and secured the prey on the first pass), the impact of its snapping jaws on the shell would break off quite a few teeth, and the pliosaur's reward would be a lump of meat much smaller than Titanites hulking profile would suggest. Why mess with a giant ammonite, when you could have a nice, fat fish (or squid, or cousin) instead?
I think you have made a very good point and I wish that I had thought of that. I had not really considered the dynamics of a pliosaur attack when I wrote the above. Would a marine predator really have much to gain through biting through a heavy shell to take a bite out of what amounts to empty chambers?
I would still consider the vast majority of ammonites and juvenile 'giant' ammonites as potential fodder; indeed a mass of belemnites were announced as ichthyosaur vomit recently. (details have been previously posted here). I still wonder if a rapid growth rate to attain a large size would be an evolutionary response to predation? I'd be curious to know if the number of body chambers in a fully mature example of some of the more massive species compares to some of the more typically sized contemporaries. In other words, did these large ammonites have a faster growth rate in order to achieve a size free from predation, or did they grow at a similar rate to their smaller cousins?
I still wonder if a rapid growth rate to attain a large size would be an evolutionary response to predation?...In other words, did these large ammonites have a faster growth rate in order to achieve a size free from predation, or did they grow at a similar rate to their smaller cousins?
The "arms race" model of development makes a lot of sense. The only gap I see in the sequence relates to the food-intake necessary to support the rapid growth of such a big organism. Wouldn't a Titanites need to spend most of it's pre-adult life foraging and eating constantly, thereby exposing itself to possible predation at the most vulnerable stage of its development? Seems to me, such a nascent beast would have to park itself in a steady stream of nutrients and oxygen and stay put, until attaining a size that could afford it a certain level of "operational immunity."
Phil, you may have covered this subject already, but does anyone have an idea of what likely constituted an ammonite's diet? Could a rapid growth rate be sustained by a lifestyle of passive predation, or would it have to work for it?
Back to Kronosaurus. Below is a link to an Australian paleontological expedition journal, with pictures of a 1m-long squid "pen." The authors speculate that the big squid would have been a likely prey item of Kronosaurus queenslandicus. Interesting stuff about the condtions that led to the pen's preservation, too.
That's an interesting question. Unfortunately with no images of soft bodied anatomy available to us mortals, we can only speculate as to the capacity of the creatures stomach. If the tough calcite aptychus was indeed a set of jaws then one can imagine the ammonite attacking small crustacea. If, however, the aptychus was some sort of clam shell door then presumably the ammonite would have had a beak composed of much softer material and would have feasted upon copepods and other marine planktonic sized fauna.
In most species of ammonite the oesophagus must have been of a tiny diameter, indeed, Architeuthis is only 1cm or so. It is difficult to imagine an ammonite tackling a hard-shelled prey animal if it had not the apparatus to break it up in order to digest it.
However, it is difficult to imagine these massive ammonite species existing on a planktonic diet; I would imagine them as bottom dwelling animals foraging for crustacea in the twilight zones, below the depth that the great reptilian marine hunters would hunt and with their proportionately large eyes to spot the shadows from above in the abyss...
Perhaps I am getting too romantic in my imagery here.........
The ratio of hard shell to soft body is so high in the ammonites, I wonder if they weren't habitually scavenging on the skeletonized corpses of "bony" animals. Not very romantic an image, but there would have been an ample food supply. Also, it would have been an ideal occupation for a slow-moving critter that needed the raw-materials to grow it's own armor.
Any living species, molluscs or otherwise, that consume bone?
Encumbered with a hard unstreamlined shell it is hard to imagine the ammonite as a voracious hunter. It would have had poor mobility, despite the hypernome, and may well have been a scavenger much like the modern nautilus. Perhaps the smaller species corkscrewed their way through the water seizing plankton and copepods with their arms (and tentacles?).
Until someone can examine the stomach contents of a fossil ammonite no-one will know for sure.
The belemnite was probably a different case entirely. I'm sure those occupied a similar niche to the squids, fast moving and deadly.
I'm sure you're right. If they've been found in Ichthyosaur guts (and vomit), they must have been living in the fast lane. They might even have been deadlier than squids; most belemnite reconstructions I've seen show an animal with an arm/mantle length ratio close to modern squids, but modern squids have a much lighter internal structure. The proportionally much heavier belemnite shell could have supported a heavier head and hook-bearing arm corona, and a more powerful siphon. If it weren't for all the pliosaurs, ichthyosaurs and honking-big sharks, I'd guess the belemnites could have developed into much larger forms.