Heteromorphs

[Hajar]

Just a little group of heteromorphs...

 

[Architeuthoceras]

Very nice collection!

 

[Hajar]

Thanks Kevin.

What do you think mode of life was for this Polyptychoceras?

I quite like the repeated toppling model here as a way of making a shell with this shape: http://www.ebel-k.de/Ammoniten/Lifestyle1/Heteromorphs/heteromorphs.html,
but reconstructions elsewhere seem to always show a gently inclined to the horizontal position (e.g. http://www.tonmo.com/community/index.php?threads/3656/).

 

[nanoteuthis]

Hi Hajar and Kevin,

This is just the opinion of a non-scientist with no significant knowledge of evolutionary biology, but.... how do we know that these forms aren't just deleterious mutations of existing ammonite species, rather than normal representatives of distinct species?

We see so many deleterious mutations of modern species -- animals with two heads, or extra limbs, or one eye, etc. -- and if some future paleontologist happened to find their fossilized remains, perhaps s/he too would mistake them for normal representatives of distinct species, or perhaps late-stage forms of a species headed for extinction.

I'm just wondering if there's a possibility that the same thing is going on here.

Just curious,
Tani
(not-too-well-informed laysquid)

 

[Architeuthoceras]

how do we know that these forms aren't just deleterious mutations of existing ammonite species, rather than normal representatives of distinct species?

Hi Tani,
All representative examples of one heteromorph species look the same (there are no "existing" species that are not "mutated" the same way). For example, all Polyptychoceras specimens will bend their shell (the same angle and direction) at about the same ontogenetic time, so they all look the same.

 

[Architeuthoceras]

What do you think mode of life was for this Polyptychoceras?

I quite like the repeated toppling model here as a way of making a shell with this shape: http://www.ebel-k.de/Ammoniten/Lifestyle1/Heteromorphs/heteromorphs.html,
but reconstructions elsewhere seem to always show a gently inclined to the horizontal position (e.g. http://www.tonmo.com/community/index.php?threads/3656/).

Hajar,
I like the "gently inclined to the horizontal position" myself. Reconstructions showing the phragmocone lower than the head just seem to go against the intuitive idea of the phragmocone as a means of neutral bouyancy.

 

[Architeuthoceras]

Just for clarification, a few threads on heteromorphs and buoyancy

http://www.tonmo.com/community/index.php?threads/2853/

http://www.tonmo.com/community/index.php?threads/5645/

 

[monty]

I thought we knew the orientation of most ammonites from the lines showing the position of cameral fluid... is that the source of the orientation for these? Of course, that may only reflect the orientation shortly before death.

Are there good examples of the intermediate growth phases for these?

 

[Architeuthoceras]

I thought we knew the orientation of most ammonites from the lines showing the position of cameral fluid... is that the source of the orientation for these? Of course, that may only reflect the orientation shortly before death.

Are there good examples of the intermediate growth phases for these?

[FONT=&quot]I don’t know about these lines showing the position of cameral fluid, got any refs? I believe there are some pseudo-sutures or something that show there was liquid in the chamber, and the position of the siphuncle has been used to assume liquid contact at different orientations.[/FONT]

To get a good example of intermediate growth phases, just break the shell back from the aperture at any point along the growth lines and you can see what the shell looked like at that phase. It is hard to tell if a fossil shell is immature or just a broken or partial mature shell. Most shells will show approximated septa or other mature modifications at or near the aperture when it has reached maturity, if these modifications show on a small shell we can tell if it was a broken or partial mature shell, if not, it would just be a guess for the actual level of maturity.

Also, a paper commenting on the benthic life style of ammonoids and orthocones

 

[monty]

I believe the cameral fluid lines were referred to in an old-ish (60s?) book I found in the Caltech geology library. I'm not on campus too often when that library is open, but I'll try to find it again.