Armored fish scales?? Weird Armor or Crab - has internal structure

[kate_rose]

So this material is from the Clayton formation in Arkansas.  It is Paleocene in age.  We are finding a lot of these flat pieces which I at first thought might be crab shells but now I think they must be scales from armored fish.  Sometimes they are very thin and small - these examples are larger ones.  If I am correct does anyone know if we can tell which fish they are from? (tape measure in view is in mm)

 

The lone piece is from the same matrix as the others.  After cleaning it up I would have guessed it was just a larger piece of armor but while I was removing it both ends broke off and it has some internal structures too.  Also it seems to have tunnels that run through it that make me think of passages for nerves or blood vessels (bone? - it sure doesn't seem like it because it seems to have a shell).  Here are pictures of its external and internal structures.  The tube-like piece connects with one of the holes in the outside shell.  I have taken pieces of all 4 exposed internal surfaces.  It is not quite symmetrical which makes me think it isn't a crab . . .  The fourth pic where a lot of the white matrix is visible is the back.  I have been reluctant to clean it up since it is a bit fragile.  

 

Note:  pics 5 & 6 (with the tunnel-like structure) have a bit of rubber cement on the surfaces - I had to retake the pics and had already begun to glue them back together

 

[kate_rose]

OK after a bunch of internet reading and carefully re-reading the paper which describes this site I think the smaller plates are crab shells (based on a post by dirtdauber here - also Clayton formation.  At least one is even Costacopluma texana but I think the middle one is not.  Do we have any decapod experts around?

On 7/22/2012 at 1:00 PM, dirtdauber said:

I appreciate the help. Seems to most closely resemble the Caloxanthus sp. in Armstrong's paper. Is that your opinion? I don't find a lot of decapod material at this site. Thus far, only a few small crabs, Costacopluma texana and the one hexapodid crab, along with a few claw fragments of crab or shrimp. I find about one crab per visit at this site. I'd be glad to send you one or two examples of the C. texana crabs if you desire. Pictured are all of the C. texana and claw fragments I've collected from that location thus far.

George

Now I am not at all sure about the other thicker pieces and especially the thing with internal structure.  Could that be a crab too (it certainly looks like one at first glance to me)?  Maybe I need to pick away at the bottom to see what it is hiding under the matrix still attached there. . .  I left my dental pics when we moved and now I am really regretting it.  Kate

[dirtdauber]

As stated in my message, I think the crab carapaces in the first photo are all Costacopluma grayi. Don't know about the other pics. -- George

[Plantguy]

Interesting finds....I've only rarely run into crabby material so I'm of know help but they seem to have crabby character....wondering if @MB might have some thoughts or expertise in this area. 

Regards, Chris