What is surrounding this Crinoid stem?

[solidus]

3 hours ago, Mahnmut said:

As Ludwigia said, not all seafloor is full of fossils. Think of a modern beach, there are spots with lots of shells, but also wide stretches with barely any, and only very occasionally a bone or other vertebrate remains. Depending on depth, oxygen- and nutrient content the seafloor is even emptier than the average beach. And depending on acidity and other factors, not all calcitic remains get preserved. In low pH sand, there are hardly any bones left when graves are dug up after 30 years.

There are places where shells and other fossils make up the better part of tall rocks, but it all depends on many factors.

Best Regards,

J

Got it. So there is a lot more to the creation of a fossil than animals dying on the sea floor and getting buried. Learning more every day!

[BobWill]

8 minutes ago, solidus said:

Got it. So there is a lot more to the creation of a fossil than animals dying on the sea floor and getting buried. Learning more every day!

You should also keep on mind that much of the sea floor from hundreds of millions of years ago has been uplifted with tectonic forces. It can then be exposed by erosion and sometimes transported to new places. Think of the marine fossils on top of Mt. Everest. 

[BobWill]

On 8/29/2023 at 3:42 PM, solidus said:

So let me try to visualize whats going on here. Is the actual screw-like part just limestone that replaced a cavity that was INSIDE of the Crinoids stem itself? And the white part on the outside replaced the actual organic material of the Crinoid? So is the screw part essentially like what you would get if you filled a cast mould with plaster, and the white part is the mould? Is the stem actually like a hollow screw-like tube when it was alive? In that case the white portion is the actual animal and the dark screw portion is just its hollow inside?

 

Sorry for so many questions... just trying to wrap my head around it.

The white part on your sample is what we usually find as crinoid stems but yours is not as well preserved. It is a replacement fossil and the dark part is a duplication fossil (an internal mold) formed by sediment that filled the inside after the soft tissue was gone. The stem is actually a support structure like the shell of a snail but it had a "skin" on the outside and the soft tissue where the dark part was now on the inside. Since soft tissue only fossilizes very rarely the "shell" or white part is the more common fossil but the internal mold is a fossil too.

 

We think of the stem as the original material and the hollow, now darker, part as being "cast" by it using the word cast as a verb. However in paleontology, if you use the adjective "internal" before the noun "cast" or "mold" that duality is reversed because it is an internal surface being reproduced. If you had the dark part separated you could pour plaster over it and reproduce a copy (cast) of the inside of the original stem. That makes it a mold and this reversed duality is why people disagree about the term. You will not find a textbook focused on paleontology that calls a steinkern (another term but one we all agree on) an internal cast but oddly enough some geology textbooks do. That might add to the confusion. 

 

We should avoid using the term "screw" to describe the shape because that suggests the ridges spiral around the object. On a crinoid stem they would be concentric circles that separate segments instead. Stem segments are sometimes found as dis-articulated disks. If you could remove enough of the matrix to follow a ridge all of the way around you could confirm this.

Edited September 1, 2023 by BobWill

[solidus]

1 hour ago, BobWill said:

The white part on your sample is what we usually find as crinoid stems but yours is not as well preserved. It is a replacement fossil and the dark part is a duplication fossil (an internal mold) formed by sediment that filled the inside after the soft tissue was gone. The stem is actually a support structure like the shell of a snail but it had a "skin" on the outside and the soft tissue where the dark part was now on the inside. Since soft tissue only fossilizes very rarely the "shell" or white part is the more common fossil but the internal mold is a fossil too.

 

We think of the stem as the original material and the hollow, now darker, part as being "cast" by it using the word cast as a verb. However in paleontology, if you use the adjective "internal" before the noun "cast" or "mold" that duality is reversed because it is an internal surface being reproduced. If you had the dark part separated you could pour plaster over it and reproduce a copy (cast) of the inside of the original stem. That makes it a mold and this reversed duality is why people disagree about the term. You will not find a textbook focused on paleontology that calls a steinkern (another term but one we all agree on) an internal cast but oddly enough some geology textbooks do. That might add to the confusion. 

 

We should avoid using the term "screw" to describe the shape because that suggests the ridges spiral around the object. On a crinoid stem they would be concentric circles that separate segments instead. Stem segments are sometimes found as dis-articulated disks. If you could remove enough of the matrix to follow a ridge all of the way around you could confirm this.

Thanks so much for that, very informative. I got the feeling that the usage of the terms mold/cast/etc were a bit contested based on some of the replies.