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Cast And Mold Confusion Conceptually Cured

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tracer

casts and molds had been bugging me. might seem elementary to some, but it was messing me up. i was thinking that a mold was a negative impression of the outside of a fossil, and that a cast was therefore a filled impression of a fossil that used to be there. which was kind of right, but one thing was throwing me. internal molds. i felt like they should be called "casts". but what i wasn't thinking was that they are internal molds if they only represent the internal surface of a fossil. they're casts if they fill the actual void where the fossil itself used to be.

i guess.

don't be casting aspersions over this. somebody else somewhere might have wondered too. prolly shoulda thrown in a picture or two but i'm suffering from lethargy.

p.s. - even though i'm the forum fool, i usually don't like admitting this kind of stuff because i'm not sure how goofy it makes me sound, but i thought i could chance it on a day when two other guys had the poor judgement to admit that they "get" what i'm trying to say around here. sheesh.

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Auspex

Thank you for casting light on a moldy subject!

I guess that means an in-matrix steinkern is a mold-in-a-mold?

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erose

steinkern [′shtīn‚kərn]

(geology)

Rock material formed from consolidated mud or sediment that filled a hollow organic structure, such as a fossil shell.

The fossil formed after dissolution of the mold. Also known as endocast; internal cast.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

I've heard steinkerns described as being molds but that is not correct. They are casts for which the mold was the inside of the clam, snail, brachiopod, etc.

What I find interesting is that they sometimes are formed before the sediment they are deposited in is formed. I have a bunch of those big Cretaceous gastropods (Lunatia pedrnalis) with worm tubes and oyster spat attached to the steinkern. How much time do you think went by from the death of the original snail to the point when it was finally inundated by mud and buried?

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tracer

steinkern [′shtīn‚kərn]

(geology)

Rock material formed from consolidated mud or sediment that filled a hollow organic structure, such as a fossil shell.

The fossil formed after dissolution of the mold. Also known as endocast; internal cast.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

I've heard steinkerns described as being molds but that is not correct. They are casts for which the mold was the inside of the clam, snail, brachiopod, etc.

What I find interesting is that they sometimes are formed before the sediment they are deposited in is formed. I have a bunch of those big Cretaceous gastropods (Lunatia pedrnalis) with worm tubes and oyster spat attached to the steinkern. How much time do you think went by from the death of the original snail to the point when it was finally inundated by mud and buried?

ok, so then i was right, and now i'm wrong. rats. ok, i'll just call it a "steinkern" if it's a moldicast of the inside of a shell, and i'll call it a cast if it's a moldicast of the mold of the outside of the shell. it's just difficult calling things "steinkerne" because i always have to do it with that accent that "frau bleucher" used in "young frankenstein".

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Guest solius symbiosus

Generally, a mold is negative and a cast is positive. Though, there are negative surfaces of both. I have an image of a bivalve, somewhere, that I started illustrating. I'll finish the illustration, and post the pic.

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tracer

Generally, a mold is negative and a cast is positive. Though, there are negative surfaces of both. I have an image of a bivalve, somewhere, that I started illustrating. I'll finish the illustration, and post the pic.

(watch this, ya'll...)

sooo....solius....if i take some plaster, and form it around a moebius strip, will i have a cast or a mold of the moebius strip?

(ta da!)( :P )

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Auspex

My favorite steinkerns (and anecdotally the mystery that spawned the name) form when an articulated bivalve is buried in, and fills with, sediment; after the sediment becomes stone, the bivalve's shell dissolves, leaving the "stone seed" (internal mold) rattling around loose in the rock (external mold).

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Guest solius symbiosus

(watch this, ya'll...)

sooo....solius....if i take some plaster, and form it around a moebius strip, will i have a cast or a mold of the moebius strip?

(ta da!)( :P )

Hmmm... taking the equation for the surface

x = [R+cos(1/2T)]cosT

y = [R+cosS(1/2T)]sinT

z = sinS(1/2T)

where S is an element of [-w,w] and T is an element of [0,2π)

Gives an equation:

-R²y+x²y+y³-2Rxz-2x²z-2y²z+yz²=0.

For the surface.

Now, if we take the anti-derivative of the equation for the perimeter

∬=(x′²+y′²)¯½

and integrate, we get:

=[1/(16)w⁴cos⁴(1/2t)+{[R+wcos(1/2t)]cost-1/2wsin(1/2t)sint}⁴+{Rsint+1/4w[sin(1/2t)+3sin(3/2t)]}⁴]½

Hey! Hold on a dog gone minute... you don't use calc to define a fossil

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Terry Dactyll

Tracer...... I see your point lol.... the ammonites I find consist of both a sediment infil of the body chamber then sometimes a calcite replacement of the floatation chambers....so theres two things going on....I think the original shell degrades after a while and calcite fills all the void of say the space where the 2-3 mm shell would of been... (ok im lost) have I got casts of the ammonite in calcite and the actual shell was the mould....

erose....How much time do you think went by from the death of the original snail to the point when it was finally inundated by mud and buried?

Im finding ammonites that have only preserved as half's with the keel acting as the edge... weve discussed maybe the top half of the shell was exposed and rotted, making it to weak to preserve compaction, while the underside was pretected from degredation by the sediments.... timescale... your guess will be as good as mine....but shell is quite tough material, I would imagine quite a while to atually rot down.... although this process could continue once initiated after burial perhaps....

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tracer

Tracer...... I see your point lol.... the ammonites I find consist of both a sediment infil of the body chamber then sometimes a calcite replacement of the floatation chambers....so theres two things going on....I think the original shell degrades after a while and calcite fills all the void of say the space where the 2-3 mm shell would of been... (ok im lost) have I got casts of the ammonite in calcite and the actual shell was the mould....

erose....How much time do you think went by from the death of the original snail to the point when it was finally inundated by mud and buried?

Im finding ammonites that have only preserved as half's with the keel acting as the edge... weve discussed maybe the top half of the shell was exposed and rotted, making it to weak to preserve compaction, while the underside was pretected from degredation by the sediments.... timescale... your guess will be as good as mine....but shell is quite tough material, I would imagine quite a while to atually rot down.... although this process could continue once initiated after burial perhaps....

i have not read it anywhere, but i believe that being rapidly infilled, as well as covered over, in a hypoxic or anoxic environment, is only part of the equation for complete preservation. i think the presence of the decaying animal itself, and bacteria, and minerals in the environment which can be converted/precipitated out by the bacteria, are big factors. i continue to be very interested in the way concretions form sometimes very dense rock around organic nuclei.

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erose

There is still a very distinct difference between a steinkern and a "cast" fossil. The steinkern is a cast of the original "interior" of the animal. But a cast will be of the exterior.

I have collected many a fossil from the Devonian in NY & PA where the fossil is a perfect cast of the exterior of the animal with very fine detail. But there is no "shell" material. The cast is made from exactly the same mud as the surrounding rock. What is amazing to me is that the original animal had to have dissolved away completely, left behind a "mold" sturdy/crisp enough to hold the detail and then still allowed some sort of infiltration of fine sediment to fill the void. Then of course there are all kinds of fossils preserved somewhere in between. I have attempted to read books on taphonomy and it just makes my brain hurt.

So going back to steinkerns covered in other fossils: Were they buried, fossilized, shell(mineralized or not) then dissolved away, then re-exposed on the bottom of the sea before being colonized, and then later reburied awaiting one of us folks to drag them home? How much time could that represent? 100s, 1000s, Millions?

I think contemplating the Mobius strip is easier...

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Nicholas

Well I was curious, and now I'm even more confused than I was before...

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Harry Pristis
On ‎12‎/‎1‎/‎2009 at 7:24 PM, erose said:

 

steinkern [′shtīn‚kərn]

(geology)

Rock material formed from consolidated mud or sediment that filled a hollow organic structure, such as a fossil shell.

The fossil formed after dissolution of the mold. Also known as endocast; internal cast.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

I've heard steinkerns described as being molds but that is not correct. They are casts for which the mold was the inside of the clam, snail, brachiopod, etc.

What I find interesting is that they sometimes are formed before the sediment they are deposited in is formed. I have a bunch of those big Cretaceous gastropods (Lunatia pedrnalis) with worm tubes and oyster spat attached to the steinkern. How much time do you think went by from the death of the original snail to the point when it was finally inundated by mud and buried?

 

 

Succinct and accurate.

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DPS Ammonite
"On 12/1/2009 at 4:24 PM, erose said:

 

steinkern [′shtīn‚kərn]

(geology)

Rock material formed from consolidated mud or sediment that filled a hollow organic structure, such as a fossil shell.

The fossil formed after dissolution of the mold. Also known as endocast; internal cast.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

I've heard steinkerns described as being molds but that is not correct. They are casts for which the mold was the inside of the clam, snail, brachiopod, etc.

What I find interesting is that they sometimes are formed before the sediment they are deposited in is formed. I have a bunch of those big Cretaceous gastropods (Lunatia pedrnalis) with worm tubes and oyster spat attached to the steinkern. How much time do you think went by from the death of the original snail to the point when it was finally inundated by mud and buried?"

 

 

"Succinct and accurate." per Harry.

 

 

Edit: I know that internal molds are steinkerns. Are internal molds with external casts on top (M and N) and external casts (R) also considered steinkerns. Even though I have heard of people describing as fossils that had an external cast on the surface, I don't know if the definition of steinkern should be that broad. What do others think, can a fossil described as a steinkern include a cast on top?

 

 

 

Harry and Eric, here is a better set of definitions:

 

Here's a diagram from that shows what a steinkern, "stone kernal" is.  (Twenhofel, William & Shrock, Robert, Invertebrate Paleontology. New York: McGraw-Hil Book Company Inc. , 2 Ed., 1953). A steinkern can be a mold, a cast or a combination of the two. The definition of a mold is the material formed against all surfaces of the original fossil/ organism (or cast) regardless of whether the mold was formed against the interior or exterior of an organism or if the mold was formed against a concave or convex surface. A cast can only be formed against a mold.

 

Looking at the diagram, J and E are interior molds and steinkerns. M, N, and R  are exterior casts and steinkerns. M and N also contain interior molds within. G is an exterior and interior cast of the shell. Without the center filling I don't think that most would call G a steinkern. At best you might be able to call G a hollow steinkern since it contains a component of a steinkern, a cast.

 

 

 

 

Mold def 1.docx

Mold def 1.pdf

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