Discussion in 'Cephalopod Fossils' started by Hajar, Aug 5, 2009.
Just a little group of heteromorphs...
Very nice collection!
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/).
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.
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.
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.
Just for clarification, a few threads on heteromorphs and buoyancy
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="]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
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.
I just read MATSUMOTO and NIHONGI (Proc. Japan Acad., 55, Ser. B (1979), which is mostly about a Polyptychoceras specimen “set as an ornament at the entrance of a fish restaurant in Otaru (western Hokkaido), called Ginrin-so.”
They use another fossil to discuss mode of life and this is what they say:
“The specimen (about 200 mm in length in a restored condition) is probably an adult shell. Its body-chamber, comprising the last U-turned part, which is mostly dark coloured in the figure, is very long. The animal must have been very long like an eel. The volume of the body-chamber is somewhat larger than the total volume of the phragmocone. This must have been favourable for the benthic mode of life. Taking the septate part upside and the body-chamber below, Polyptychoceras could have kept a buoyancy, with some control through the siphon. Therefore, it may have taken an up-and-down locomotion to some extent. As the body-chamber is opened upward, the quick backward swimming by a jet-propulsion may be unlikely, if not impossible for the animal, but this situation must have been favourable for a slow and probably occasional up-and-down movements. To sum up, species of Polyptychoceras probably took a primarily benthic mode of life and were fond of sheltered places at the bottom of the sea-water.”
I'll read through the various buoyancy discussions (with their different assumptions about the size of the animal relative to body chamber) a bit later. Thanks for the links and I'm looking forward to understanding this better.
Here's a tiny heteromorph, about 3 mm long from a piece of matrix containing a couple of Polyptychoceras. The tightly coiled first-formed portion measures less than a mm across.
The photos are taken through the microscope and I stacked 14 images, each focussed at slightly different depths, using Combine ZP.
Looks like this is the one I should read: Okamoto, T. and M. Shibata. 1997. A cyclic mode of shell growth and its implications in a Late Cretaceous heteromorph ammonite Polyptychoceras pseudogaultinum (Yokoyama). Paleontological Research vol. 1/no. 1:p. 29–46.
Excellent. Compare Fig. 3 with the ammonitella I posted above!
On life orientation: "It is likely that every individual of this ammonite spent most of its lifetime with an upward-facing aperture." See the attached computer simulations. Good fun.
Looks like I have alot of reading to do
Your pics look just like one I did a few years ago. A Baculites ammonitella with the primary constriction and the beginning of the straight part of the shell (about 1mm lg.) probably post hatching as there are no chambers after the straight part.
That's a great image. I like it a lot.
Here are some pictures of the Muramotoceras, 6 cm long, with at least three other ammonite species accompanying. Okamoto (Palaeontology 31) has some fine reconstructions of how life orientation changed during growth.
Here's Okamoto's reconstruction of life orientation changes in Muramotoceras with the initial straight section pointing upwards, then a flip over with corresponding change in obliquity of ribs and finally the growth of the spiral enveloping the early straight section, now pointing downwards in a new stable orientation. He shows the same pattern for Eubostrychoceras.
Great specimen Hajar,
The paper shows one turnover event, but it looks like there was another from looking at your specimen. The straight part on yours looks like it turned over and assumed the same orientation as the first straight part. Is there any indication that the siphuncle moved inside the shell? Was ventral always ventral? Did this animal live upside down for awhile?
Just a little sketch to show the last turnover of the shell shown in the first fig. below (or above depending on your thread setup), fig B, this would be sometime after stage 70 on the stereograph.
Separate names with a comma.