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 said: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?
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.Clem said:... does anyone have an idea of what likely constituted an ammonite's diet?
Grrrrrr.Phil said:Until someone can examine the stomach contents of a fossil ammonite no-one will know for sure.
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.The belemnite was probably a different case entirely. I'm sure those occupied a similar niche to the squids, fast moving and deadly.