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Dorsal Siphuncles

Nauti-guy

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#1
In most cephalopods the siphuncle is central or ventral, the primtive condition as found in plectronocerids, ellemerocerids, and endocerids, e.g. being ventral. In some such as the Trocholitidae (Tarphycerida) and in a few pseudorthocerids it is instead dorsal of the center, in some along the dorsum.

Any ideas as to the significance of this difference. Does it indicate anything about the orientation of the animal with respect to the shell and what evolutionary pressures might have influenced it.
 

Architeuthoceras

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#2
A specimen of Plectolites, Trocholitidae (Tarphycerida), with a dorsal siphuncle. From the Ordovician Juab Limestone in Utah.


I have always thought of the siphuncle location as just random in different families. It seems far fetched to view Plectolites or other coiled forms with dorsal siphuncles as endogastric (so the siphuncle would actually be ventral). Some ammonoids (the Clymeniids) had dorsal siphuncles, and Rayonnoceras, the giant Carboniferous actinocerid? orthocone, had a dorsal (above center) siphuncle.

These workers think that all ammonoids would have had a dorsal siphuncle if not for the end devonian extinction event.
 

monty

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#3
I've read some analysis of reconstructions of the animals' orientations from cameral fluid lines, siphuncle position, and such. I am not near my books and notes, but I seem to remember that the placement of the siphuncle and necks had to be arranged such that part of the fleshy bits of the animal are in contact with the fluid (or, at least, with something that can "wick" the fluid to it) in order for the fluid regulating mechanism to work, at least assuming it was similar in ammonites to modern nautilus (I think it's some osmotic/ionic process, but I don't remember the details. I think Ward's Natural History of the Nautilus has a detailed description.)

It might be interesting to compare to Spirula since that's the only living animal with a marginal siphuncle. There seems to be some evidence that the siphuncle anatomy (and hence physiology, but who knows how) was rather different in some extinct lineages, though.
 

Nauti-guy

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#4
'Archituthoceras' Great picture of a Plectolites section, better than what we have down here from Flower's collection. But I don't think siphuncle position is random in different families, but has some morphologic/funtional reason.

Monty, I tend to agree, at least we approach the problem from the same point, that of siphuncle function in Nautilus. Royal Mapes (Ohio State) brought the wicking aspect to my attention in personal conversation on a visit to the NMMNH&S. Since then I have speculated that the siphuncle postition (ventral, central, dorsal) at least in coiled cephalopods indicates where the last septum was relative to the coiling. As to why it would be high in some and low in others, or in between is another matter.
 

monty

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#5
Nauti-guy;125656 said:
Since then I have speculated that the siphuncle postition (ventral, central, dorsal) at least in coiled cephalopods indicates where the last septum was relative to the coiling.
I'll see if I can find the reference when I get back home in a few days. I remember someone had worked this out in some detail... My recollection was that it wasn't just the last chamber, although obviously that's the one with the highest rate need for evacuating fluid.
 

Architeuthoceras

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Random was probably a poor choice of words, variable may have been better. For example, Aturia, if i'm not mistaken (and I may well be) is the only relatively recent nautiloid with a dorsal siphuncle.

Shell shape also has a lot to do with body chamber length, thus location of the last septum. And fluted septa as in ammonoids and Aturia may have trapped fluid away from the siphuncle depending on orientation. Which brings up the question, does the shape of the septum drain fluid towards the location of the siphuncle at it's normal (living) orientation? At all orientations until the fluid is gone? And does this have anything to do with the location of the siphuncle?
:hmm:
 

monty

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#7
Architeuthoceras;125690 said:
Random was probably a poor choice of words, variable may have been better. For example, Aturia, if i'm not mistaken (and I may well be) is the only relatively recent nautiloid with a dorsal siphuncle.

Shell shape also has a lot to do with body chamber length, thus location of the last septum. And fluted septa as in ammonoids and Aturia may have trapped fluid away from the siphuncle depending on orientation. Which brings up the question, does the shape of the septum drain fluid towards the location of the siphuncle at it's normal (living) orientation? At all orientations until the fluid is gone? And does this have anything to do with the location of the siphuncle?
:hmm:
When I was visiting esquid yesterday, I hung out at the library while she was in class, and found they had Lehmann's The Ammonites: Their Live and Their World that has an overview of some of this.

In particular, it has a chart after Trueman 1941 that shows the orientation of ammonites of various shapes with the assumption that the body cavity is heavy (it doesn't say how they decided on the density and shape of the animal) and then comparing the center of buoyancy with the center of mass. comparing that to siphuncle position might be enlightening. Also, where the funnel us relative to, say, the midpoint between centers of buoyancy and mass might be interesting, since that's tuned to linear horizontal movement without rocking or twisting.

It also is interesting that the system depends on osmotic difference between chamber fluid and seawater, yet the salt levels in ancient oceans was a lot lower than here. Perhaps this should be brought up in the discussions of "why are there no freshwater cephs," in that perhaps the ability to concentrate seawater was vital to survival in the chambered cephs, and no-phragmacone coleoids are new enough that they haven't adapted to fresh water, or the chambered ones' dependence on easy access to salt may have "locked in" other evolutionary constraints.

Anyway, there is also, in Nautilus (Lehmann p. 163):

The whole [siphuncle] structure is covered by a thin organic layer consisting of conchiolin, which also covers the whole interior of the chamber[... and] is absorbent like blotting paper so that it can take up water over its entire surface
So the fluid level doesn't have to be exactly at the septal neck line... which makes some sense, in that the orientation of the neck changes as the whorl goes around...

In Spirula, it is similar, despite different septal neck geometry (Ibid. p. 165).

The question arises as to how the process of emptying can continue when the level of fluid is so low that the siphuncle is no longer in contact with it, since the newly formed chamber originates in the upper part of the shell. This is where the permeability of the conchiolin membrane plays a role, because it is capable of absorbing fluid like a wick.
In spirula, the septal neck is very short, and near the top of the most recent chamber, so this wicking is extremely significant, and (unfortunately) therefore the position of the neck in the chamber is probably not a good indicator of the natural orientation or cameral fluid level.

Lehmann also says (Ibid. 169)

In the ammonites, the position of the siphuncle within the chambers, disregarding the first 1-2 whorls, is always marginal. For short or very long body chambers this means that the removal of the fluid from newly formed chambers is rendered more difficult because after only partial emptying of the chamber the siphuncle would no longer be in contact with the fluid. Presumably the interior organic membrane lining the chambers was of a similar composition to that of the nautilus and was thus capable of absorbing fluid like blotting paper. On the other hand, the ventral position of the siphuncle had the advantage that the largest possible effective surface area was exposed to the greatest effect.
No suggestion of how this may or may not relate to the original question is intended to be implied or inferred. :read:
 

Architeuthoceras

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#8
monty;125726 said:
No suggestion of how this may or may not relate to the original question is intended to be implied or inferred. :read:
But basically it suggests that the position of the siphuncle in the chamber has no significant advantage for fluid removal.

Interestingly the paper previously linked noted that the dorsal siphuncle of the Clymeniids was larger than those of coeval ammonoids with ventral siphuncles, girth vs length so to say, equalizing surface area. Will have to look at nautiloids and see if the same holds true.
 

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