Triradial constrictions, why?

Hajar

Haliphron Atlanticus
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#1
Here below is a funny looking globular goniatite, Parawocklumeria paradoxa, (14 mm) from the Devonian of Devon.

Is there a good explanation for this tri-radial pattern of constrictions?

Other goniatites, like the attached German Cheiloceras, don't show this tri-radial pattern, but have a regular spacing in the constrictions. Could it be a record of seasonal changes in growth rates or annual pauses in growth?
 

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Hajar

Haliphron Atlanticus
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#4
I found this in Korn 1995:

A remarkable diversification of several independent ammonoid lineages with high evolutionary rates occurred in the Late Devonian Wocklumeria Stufe. Many speciation events led to paedomorphic ammonoids that display a striking range of conch shapes, sculpture, and ornamentation. In the goniatite family Prionoceratidae, the transition from normal Mimimitoceras species to paedomorphic Balvia species provides an example of rapid size decrease combined with an early character developmental offset arising from progenesis. Adults of early Balvia species largely have the preadult ancestral morphology of Mimimitoceras, but later evolving species acquire distinct conch and ornamentation types. In the clymeniid family Parawocklumeriidae, evolution is characterized by the extension of tri-segmented and triangularly coiled whorls found only in juveniles of earlier species, to the adults of later species.
 

Hajar

Haliphron Atlanticus
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#5
Aha!

pg. 439 in Ammonoid Paleobiology (attached)

So, constant periodicities for lirae (daily), ribs, septa (monthly or fortnightly) and constrictions (annual) have often been assumed, but not confirmed. Page 440 has a table of assumed periodicites. On page 441: "The spacing of such features on the shell was probably controlled by the growth program, although it may have been subject to some environmental influence."

There is also a section on "detection of seasonal signals in morphology"
 

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Architeuthoceras

Architeuthis
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#7
Too bad they dont show a scale on those Foram pics (disregard the lack of scale on my E. quadratum photo :sly:). (Most) Forams are very small (.05mm-.5mm see this page) single celled organisms, hard to believe they can make those wonderful shells. The constrictions on ammonoids are seperate from the septa ( Parawocklumeria from GONIAT), where on those Forams, each chamber is expanded forming "pseudo-constrictions" between each chamber.

I just dont see it taking 3-4 years for an ammonoid to form 1 complete whorl. Assuming the constriction forming rate was constant and taking a guess of 10 complete whorls, an ammonoid wouldn't reach maturity until it was almost 30-40 years old.
 

cuttlegirl

Colossal Squid
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#8
Architeuthoceras;146272 said:
Too bad they dont show a scale on those Foram pics

It said that the cephalopod-looking one was 1000 microns. Wikipedia said that the largest foram recorded was 19 cm :shock:. I would like to see a photo of that one...
 

Hajar

Haliphron Atlanticus
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#9
Thanks Cuttlegirl. I also like these little forams that look like cephalopods. Here are some Cretaceous (Gault) examples from Kent, all about 1 mm or a bit less across. There are larger ones in the local Eocene, the attached yellow-orange one being 11 mm across.

Kevin, does your ammonoid have 4 constrictions for each of the 10 assumed whorls? I wouldn't be surprised if it didn't. Do you have an explanation for what the constrictions might record?

Hirano (1981) estimated an age at maturity for a Gaudryceras at 20 years using the annual constriction assumption (7 whorls).

Inferred ages at maturity for other ammonoids are shown ranging from 2 to 7 years in Table 1 of Ammonoid Paleobiology. All the estimates contain assumptions which might not be true and there's a statement that "there is no evidence to suggest that the time interval between the formation of successive lirae, ribs or constrictions was consistent during ontogeny.."

That said, the trilobed goniatites are very distinctive and the forms with four or five regularly spaced constrictions per whorl seem very suggestive of periodic pauses in growth.
 

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Architeuthoceras

Architeuthis
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#10
All ammonoids (?and nautiloids) have a Primary Constriction, supposedly formed while in the egg, waiting to hatch, so later constrictions could be some kind of pause in growth (?time/seasonal/ecological) or just some kind of holdover genetic thing. Constrictions could also be just another form of shell ornamentation with no correlation with time or environment. The case of Parawocklumeria looks to me like some kind of progenetic deal. IMHO :smile:
 

Architeuthoceras

Architeuthis
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#12

Inner whorls (11mm dia.) of Sulcogirtyoceras ornatissimum showing the start of the constrictions. The first complete constriction lower left, and smaller partial constrictions on lower half of last whorl. Notice that the small nodes on the inner whorls start to become transverse clavi in between the constrictions, then start to form the ventro-lateral sulcus just anterior of the last constriction (on top).


Middle whorls (25mm dia) of another specimen (latex cast) showing the constrictions getting closer together (relatively), and finally disappearing. Adult shells are smooth, oxyconic, with no ornamentation.


A forerunner of S. ornatissimum, Girtyoceras meslerianum (~10mm dia), with equally spaced constrictions.

Could this show that constrictions are just part of the ornamentation and not related to time or ecology?
 

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Hajar

Haliphron Atlanticus
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#13
You could well be right Kevin (I have no biological training), but it still doesn’t feel like a satisfying answer to the “why?” question.

This Korn 1992 paper (http://app.pan.pl/archive/published/app37/app37-021.pdf) illustrates Devonian prionoceratids with a variety of constriction patterns (most with three, but some with four per whorl) and notes that the shell constrictions are accompanied by an apertural shell thickening.

There’s Mimimitoceras alternatum in which “radial shell constrictions only in early stages, in adults as internal shell wall thickenings.” Then there is Mimimitoceras liratum in which “a terminal stage with a much more distinct ventrolateral salient of the growth-lines and shell constrictions is added.”

He says, “Progenetic Balvia displays conch morphology of ancestral Mimimitoceras juveniles. with distinct ornamentation types that were added terminally.”

In “Ammonoid Paleobiology”, pg. 424-425 several lines of evidence supporting the interpretation of constrictions as representing discontinuities in growth are presented. What governs the pattern of the constrictions is still a puzzle to me.
 

Hajar

Haliphron Atlanticus
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#15
These two arrived yesterday, sent by a Polish friend. There's a little 12mm Parawocklumeria and a larger 19mm Wocklumeria, both from the Famennian of the Kowala quarry. The Wocklumeria shows the triangular inner whorls and the globular outer whorls.
 

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DWhatley

Cthulhu
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#17
You get the most interesting stuff. How's the live version of your collection thought coming?
 

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