Toys!

Well, this ammonite certainly is 'for real', Tony, picture of an actual fossil attached. (Permission to reproduce from owner granted on his website). Ammonites in forms such as this are actually quite common in early Cretaceous deposits, though Polyptychoceras is particularly known from Japan and Canada. Most were quite small, i.e. 20cm long but a few grew larger. There really is nothing like them in the seas today, more's the pity.

Why exactly this happened, I'm really not sure, but bizarre shell forms suddenly appeared in many ammonite lineages throughout the Jurassic and Cretaceous. It is generally thought for these forms to have been prone to drifting about, rather than being powerful swimmers. As to why they toppled over at a certain growth stage, I'm going to have to some research, if anyone even has an answer to this. Perhaps this was something to do with sexual maturity? Alternatively, it may be something to do with the mass of the growing body and its effect on the balance of the shell in the water. One author, Ebel, believes ammonites such as this lived in a head-down position rooting and foraging through the seabed whilst drifting above it. He believes the change in angle of the shell is simply due to gravity pulling the shell down after reaching a certain length.

Interesting stuff.
 

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I read some chapter in a book in the Caltech Geology library about using the cameral fluid waterlines in the chambers to determine the swimming orientations at different growth stages. I have no idea about the author or title or anything though; I think it was rather old (50s, 60s, or 70s). I may be able to find it again, but I don't get on campus much lately, and very rarely am hanging out in the Geology library with free time...
 

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