Why are crinoids usually the only type of fossil in an area?

[Digi]

As the title states, why do crinoids often show up alone as the sole type of fossil in an area? There's a quarry near my house I've been to a few times that has crinoid bearing marble. I've found a few crinoid stem pieces in rocks, but I don't really understand why they're the only type of fossil (unless you count chert.) The area is Permian-Devonian, so if crinoids fossilized shouldn't plenty of other common marine animals have fossilized too? Or am I thinking too narrowly and there's a good chance they DID fossilize and I simply have to look a little deeper? I'm not too knowledgable on the paleozoic so I appreciate any answers.

[Kane]

Just like modern marine areas, organisms favour certain depths and conditions. For example, if you swim in the ocean, you will encounter different fauna in deeper areas than you would in shallower, near-shore areas.

[Tidgy's Dad]

Also some crinoids formed vast 'forests' and had stems composed of hundreds and hundreds of columnals, plus all the arm ossicles, so crinoidal limestone is sometimes made up of mainly these, though other fossils may be present in some if one keeps looking hard enough.  

[Digi]

That makes sense, thanks to the both of you. I take it these areas end up comprised of mostly crinoids since they're more prone to fossilization (all the columnals and ossicles) along with how prevalent they were. 

[digit]

Great question!

 

As the two replies above already point out, some of the reasons may be that crinoids often occur in habitats where they are very dense (nearly carpeting the bottom) and the well mineralized columnals likely have a reasonably high preservation rate. A similar argument occurs for fossil shark teeth. Sharks have lots of teeth (throughout their lives) and crinoids are composed of lots and lots of columnals. Shark teeth are the most mineralized part of the animal and so 99.9% of shark fossils are teeth. While crinoid calicies and pinnules from the crown are often found, the many segments in the elongated stalks (the columnals) may outnumber other parts in the fossil record.

 

There are many crinoid species still extant and divers often spot them if they know enough to identify them. Most of the shallow-water species have no stalks to speak of but are mostly a crown of tiny segmented pinnules (feather-like arms) around a central body with small "feet" that grip onto the substrate and allow mobility (yes these ones can crawl around and often walk up to the tops of reef promontories to take advantage of a good current bringing food bits their way).

 

There are also deep-water crinoids that much more resemble the stalked crinoids of the fossil record. Divers (who don't have access to a deep-diving submarine) rarely see these type of crinoids--the ones often referred to as "living fossils". I know a scientist, Chuck Messing, who studies echinoderms and specializes in these deep water "sea lilly" types. He's taken small subs down to a few thousand feet to visit colonies of these at various locations of Honduras and the Bay Islands. I don't remember the details but he's discovered from repeated visits over time that these deepwater forms grow exceedingly slowly (a common characteristic for organisms growing at the edge of a habitable zone) and may live for many decades or even centuries.

 

It's possible that these stalked forms may have grown at depth in the past--though they may have grown in a wider diversity of habitats back hundreds of millions of years ago and been out-competed in the shallow habitats and only dominate the depths these days. They were once a very common life form and may, as Kane mentioned, simply have dominated certain habitats to the exclusion of many other organisms. Most commonly, I associate crinoids with brachiopods and the odd trilobite. There are many examples of hash plates with this holy trinity of fossil types (much less tasty than the Cajun/Louisiana Creole mix of onion/celery/bell pepper).

 

Simple questions like this often spur a good conversation and exchange of ideas. While we may never settle upon "the correct" answer, it does provide a good topic to muse upon for a while.

 

 

Cheers.

 

-Ken

[Digi]

Fantastic addition! Thanks, Ken! This reminded me of a section of hash plate I receive from a local professor where that holy trinity was 100% apparent. I have hopeful plans to head back up to the quarry on the 23rd, I'll keep an eye out for some not so crinoid shaped "crinoids" 

[FossilDAWG]

You should also consider that water currents can wash away finer sediment and concentrate crinoid ossicles, forming deposits that almost entirely are composed of crinoid bits.  In Ontario there are some Silurian formations that consist almost entirely of hundreds of feet of crinoidal limestone.

 

Don

[digit]

12 hours ago, FossilDAWG said:

In Ontario there are some Silurian formations that consist almost entirely of hundreds of feet of crinoidal limestone.

Wow! You'd need a really big display case for that.

 

Don brings up the added great point of the accumulation and concentration of crinoid columnals due to currents, storms and/or gravity causing masses of these to settle into crinoid rich deposits. That would help to explain the incredible density seen in some hash plates or formations. There would have to be something else going on here as well if these deposits are virtually entirely crinoidal to the exclusion of brachiopods, trilobites or other items like rugose corals and bryozoans which are also represented in formations of this age. Physical processes would surely be involved in forming these dense aggregations but might not explain their monotony.

 

 

Cheers.

 

-Ken

[FossilNerd]

22 hours ago, FossilDAWG said:

You should also consider that water currents can wash away finer sediment and concentrate crinoid ossicles, forming deposits that almost entirely are composed of crinoid bits.  In Ontario there are some Silurian formations that consist almost entirely of hundreds of feet of crinoidal limestone.

 

Don

Wow! That is crazy! I have seen a crinoidal layer over a meter thick in the local Borden Formation, but in this case, that is like comparing apples to oranges.

[Mark Kmiecik]

On 7/19/2020 at 10:23 AM, digit said:

Wow! You'd need a really big display case for that.

Yes, I believe they call that one "earth". 

[Tomasz]

On 7/18/2020 at 5:07 PM, Tidgy's Dad said:

Also some crinoids formed vast 'forests' and had stems composed of hundreds and hundreds of columnals, plus all the arm ossicles, so crinoidal limestone is sometimes made up of mainly these, though other fossils may be present in some if one keeps looking hard enough.  

Paleozoic crinoids were far more diversified and abundant comparing to later geological periods. They have never achieved such richness after the end of Permian.

[Tidgy's Dad]

2 hours ago, Tomasz said:

Paleozoic crinoids were far more diversified and abundant comparing to later geological periods. They have never achieved such richness after the end of Permian.

Correct. 

Same as brachiopods.