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Discussion of the Reconstructions of Nautiloids and Ammonoids

ArchyNorth

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Great collection of illustrations Phil. Thanks for compiling them.

In your amazing joining text, you mention that some of them are believed to be bottom dwellers. My question is: How do we "know" where they lived within the water collum?

I know all paleontological theories are based off of scant evidence and some are pure conjecture, but I am wondering if there is "special" evidence for these fossils that can distinguish the swimmers from the bottom dwellers?

I know some fossils are found within the skeletons of free swimming animals (Icthyosaurs, Plesiosaurs, etc..) which suggests the Nautiloids and Ammonoids were taken mid-water; but this could also be explained by bottom hunting.

And maybe this is my own lack of experience with these fossils, but I am having a hard time seeing an Ammonoid with a shell over 2m across swimming freely.

Anyway, great post!! I'm always salivating over iformation like this. :smile:
 

Architeuthoceras

Architeuthis
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These are very nice reconstructions, most are just as I have had them pictured in my minds eye.

Only one, Pravitoceras, could be fixed, and I dont have a clue as how to fix it :hmm: The early whorls are coiled normally so the venter would be towards the outside (or the convex side) of the shell. At maturity it coils the opposite direction so the venter would now be on the inside (or the concave side). Most ammonites (according to a Nautilus model) are oriented with the funnel ventral, and the eyes dorsal (? is this the same with squid & octopuses), just the opposite of the reconstruction, unless the animal twisted 180 degrees in it's shell. Of course if you drew it that way it would look upside down. Anything that grew a weird shell like that was bound to be peculiar to start with.

Anyway, Thanks for posting these Phil, I did get a little work done today :wink:
 

Phil

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Hmm....I see what you mea Kevin. It would be interesting to see a Pravitoceras sectioned so that the path of the siphuncle could be traced. Unfortunately, the only source of information on Pravitoceras I had to go on was Google and almost all of the 25 or so sites that mentioned it were in Japanese, mostly offering specimens for sale. Hence my pitiful two sentences!
 

Phil

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My question is: How do we "know" where they lived within the water collum?

I know all paleontological theories are based off of scant evidence and some are pure conjecture, but I am wondering if there is "special" evidence for these fossils that can distinguish the swimmers from the bottom dwellers?

Hi Arthur, thanks for the kind words!

I took the environmental conditions for the ammonoids and nautiloids from a combination of Neale Monk’s book “Ammonites”(2002), Cyril Walker and David Ward’s “Eyewitness Handbook: Fossils” (1992) and Blackwell Publishing’s PaleoBase 2 (2003). From what I’ve read in these sources and others, I think fossil cephalopod depth estimates and environmental conditions are based largely on the following combination of factors:

1) Type of rock they are deposited in.
2) Shape of shell, thickness and complexity of sutures.
3) Angle of the living chamber aperture.
4) Associated animals
5) Flotation experiments using actual and computer models.

Type of rock


I have read that most ammonoids were not supposed to be able to withstand depths of deeper than 200m at maximum, any deeper then they would implode, though it is possible that a few particularly tough-shelled species such as Lytoceras may have been able to reach a depth of 800m (this depth seems to vary with which book one picks up, Lytoceras’ depth was apparently suggested by Westerman in 1996 but it does seem excessively deep). Most, of course, are thought to have lived in much shallower water than this. If, for example, deposits of ammonoid shells of a specific species are found in deep water anoxic black shales, as is the case with some of the Clymeniina and goniatites, then it follows that these animals probably lived higher in the water with the shells drifting down after death. Otherwise they would be living in conditions that were clearly unsuitable for.

On the other hand, other goniatites and some ceratites are common in limestone deposits which are thought to originate from coral reefs, this would indicate that they preferred shallow-water continental edge environments. Similarly, the Jurassic deepwater limestones of the Western Mediterranean are particularly rich in ammonites of Phylloceratina and Lytoceratina.



Shell form

Ammonites have a stronger shell form than nautiloids as in most species the septa are much more tightly packed together with complex suture patterns, but the shells tend to be much thinner. One would think that they could have colonized deep water habitats easier than nautiluses but this does not seem to the case and deep water fossils are rare. Two possible theories about this are that they were therefore able to produce shells much quicker than nautiluses using less shell-material giving them a faster growth rate, alternatively, the complex sutures could have been in response to predation as a complex ammonoid shell would be harder to break than a simple nautiloid shell.

Streamlining is obviously important in consideration of lifestyle, thin disc-like shells would clearly cut the water easier than a glob-shaped ammonite or one covered in spines due to increased drag. That not necessarily imply that all globular ammonoids were bottom dwellers though! Some may have buried into the sediment, possibly the bizarre triangular Solicylmenia may have done this.

The very shape of the odd heteromorphs does not seem to indicate a creature capable of swimming and it is thought these may have simply drifted on the tide. Even if they had a siphon (doubtful) the miniscule jet of water they could produce would have simply made them rock or spin in circles! Unable to control their direction, if at all, it is unlikely that they would have been active predators and probably just drifted about hunting plankton.



Angle of aperture/floatation studies

The angle of the aperture is very important in considering where in the water the animal would have lived. Using computer modelling, many ammonites have been reconstructed to determine exactly how they would remained buoyant in the water. Most heteromorphs have the aperture angled upwards as can be clearly seen in the nice drawing of Scaphites, this would indicate that these animals would not be capable of hunting in the sediment. Most more conventionally shaped ammonites were orientated in a similar manner to Nautilus.

There were exceptions, for example the Late Cretaceous ammonite Baculites is thought to have been orientated vertically with the head downwards and probably resting on the sediment; although it resembled an orthoconic nautiloid it lacked the counterbalancing mineralising deposits in the shell that orientated the nautiloids horizontally, thus the shell would have floated above the animal.

Many studies have been done on heteromorph ammonite orientation; here is a great link should you wish to read further:

http://palaeo-electronica.org/1998_1/monks/text.pdf


Associated animals.

Other fossils can help to indicate whether the animal lived in the water column or not. A mixture of corals, crabs or bivalves may indicate a bottom-dwelling lifestyle for example.

In short, researchers use a combination of the above factors in trying to determine habitats for these creatures. As you say, much is speculation but quite a lot can be inferred from rock-type alone!

(Hope that is of some interest. It took 2 hours to write as my computer crashed half-way through and I lost half of it!)

All the best,

Phil
 

ArchyNorth

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WOW!! :shock:

Ask and Ye shall receive!! Thanks for the detailed answer Phil.

For some reason, the rock type never came to mind when I was thinking of these fossils. Which is silly of course, as the matrix something is found in is as important as the fossil itself.

This fact should have kicked me in the teeth as I use it when I do any acheology work. It's all about context and Matrix = Environment.

Anyway, before I go hide my head from scholarly shame, Thanks so much for the detailed answer and your willingness to spend the time typing it out. Lots of info there and well written.

Cheers,
 

Architeuthoceras

Architeuthis
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ArchyNorth said:
WOW!! :shock:

Ask and Ye shall receive!! Thanks for the detailed answer Phil.

Ditto :notworth: :notworth:



(Westerman in 1996) also has a few reconstructions that are thought provoking. Some with tentacles and clubs reaching behind the shell to grab (or fetch) a fish, and a gyrocone akin to Crioceratites hanging on a sprig of seaweed like a seahorse with a large transparent umbrella like net between its arms. All copywrited (in that pricey book "Ammonoid Paleobiology") so I don't dare scan it.
 
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great info Phillipo ! & images as I'm sure we've all got millions of glossy pics of empty shells.
you've been bathing in Omega 3 again !

Archy, as you're obviousy an info Hoover I can strongly recommend "Ammonites" by Neal Monks as essential to your library, it's a great read.

Kevin, get back to work !

Keef
 

ArchyNorth

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spartacus said:
Archy, as you're obviousy an info Hoover ...

Hehe, you've noticed? :smile:

When I find a source of information on a topic that is immensly interesting to me, I tend to go digging for the answers to questions in my mind.
I can be a bit overzealous though. Just ask those I have PM'ed. :roll:
I just wish I could provide more to the topics than endless questions, and half-baked theories. :oops: Anyone have questions about sub-arctic archeology or cooking? :smile:

Anyway, Thanks for the book suggestion Spartacus and I will most assuredly be looking it up.

Cheers,
 

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