So why are crinoid stem fossils so...full of stems?

[FossilizedJello]

Hey everyone, just wanted to start a mini discussion about this. I just find it weird that there are tons of fossil clusters consisting of just stems. Then occasionally you see a full crinoid fossil with the stem and crinoid itself.
So my point is that usually you see a cluster of stems or just one crinoid with a single stem. How come these clusters with tons and tons of stems dont contain the organisms and just the stems? The obvious answer would be difficulty in fossilizing. But it just doesnt make sense as you would think you would see more in these stem clusters.

[Tidgy's Dad]

Many crinoids had very long stems containing hundreds or thousands of columnals. A lot of the smaller circles you lay find are sometimes the remains of 'rootlets' or cirri (arms). 

The calyces (heads) of some crinoids are very small and difficult to differentiate from the stems, 

Calyces are made up of plates which often separate after death and may be difficult to spot. 

Yes, preservation bias may also be a factor, sometimes the calyx plates are thinner and more fragile than chunky stem supports. 

[FossilizedJello]

26 minutes ago, Tidgy's Dad said:

Many crinoids had very long stems containing hundreds or thousands of columnals. A lot of the smaller circles you lay find are sometimes the remains of 'rootlets' or cirri (arms). 

The calyces (heads) of some crinoids are very small and difficult to differentiate from the stems, 

Calyces are made up of plates which often separate after death and may be difficult to spot. 

Yes, preservation bias may also be a factor, sometimes the calyx plates are thinner and more fragile than chunky stem supports. 

Ah this makes a lot of sense. So basically besides the fact that there are thousands of columnal stem segments, when there head does get fossilized. its not even that much apparent from the segments. Very cool. Thanks for the lesson ;).

[Tidgy's Dad]

Some crinoids are free swimming and don't have stems at all, others have short stems, but where you find these crinoidal limestones, yes, they can be composed largely of crinoid stem ossicles. 

[jpc]

I believe crinoids are quick to disintegrate when they die, so the real question might be, why do even find complete ones occasionally?  There is something special happening in the deposits where complete crinoids are commonly found.  I can think of four off hand... Waldron Shale of Indiana, Holzmaden in Deutschland, the Chinese ones you see for sale, and the floating crinoids from the Solnhofen deposits.  

[RuMert]

BTW with Cyclocrinus, whose stems can easily be found in Callovian/Oxfordian deposits in our region, no calyx has ever been discovered and nobody knows what it looked like (if it did exist). This mystery drove some people insane

[jdp]

something to keep in mind is that wave action will mix up fossils and then cause them to accumulate with other fossils of the same size. This is called "sorting" and is an important pre-depositional process that shapes a lot of these fossil-dense accumulations. This is why you'll find so many crinoid columnals in some rocks.

[Mediospirifer]

Think of a stemmed crinoid as being like an intricately beaded necklace, with a pendant composed of bits of glass glued together. When it dies, all of the connective tissues (thread and glue for the necklace analogy) decay first, and any wave action will scatter them. There are hundreds or thousands of beads (stem columnals), and a few dozen (or less) bits of glass (calyx plates) that are difficult to recognize. Only in circumstances where the animal was buried before disarticulation are there complete calyxes still present.

 

I've found a lot of tiny crinoid columnals when searching for microfossils, and a few similarly tiny holdfasts. I'm not sure I'd recognize an isolated calyx plate if I found one.

 

They're fascinating creatures! 

 

[LabRatKing]

6 hours ago, FossilizedJello said:

Hey everyone, just wanted to start a mini discussion about this. I just find it weird that there are tons of fossil clusters consisting of just stems. Then occasionally you see a full crinoid fossil with the stem and crinoid itself.
So my point is that usually you see a cluster of stems or just one crinoid with a single stem. How come these clusters with tons and tons of stems dont contain the organisms and just the stems? The obvious answer would be difficulty in fossilizing. But it just doesnt make sense as you would think you would see more in these stem clusters.

Have a look at an extant species of crinoid...then picture thousands of these animals in a vast carpet along the primordial seafloor.