Sideritetized micro Brachiopods From Cretaceous Anthills in Az.

[Arizona Chris]

HI all,

 

One of the most unusual preservations of fossils in the Dakota / Mancos shale here in Arizona is by infilling of the molds in the shales by an iron mineral called siderite.  It is black, shiny and sub metallic and has a heavy feel.  Finding a big razor clam preserved this way for example has always been exciting - the heft and appearance are breathtaking.  However, we have never found brachs in either shales or sandstones.  Mostly huge molluscs such as oysters, ammonites, and gastropods.  So when we started pulling out super tiny complete brachs out of the sides of the anthills recently, it was a super find!  They are so small, the size of a pinhead to thumbtack, and are smooth and very generic looking.  Its going to be very hard to nail down the identification on these because of this, but I thought Id post what we have so far.  

 

Out of a gallon of anthill gravels, we found roughly a teaspoon of these brachs.  (Tablespoons of shark teeth!)  So here is what we have so far, at 10x through the microscope:

[Fossildude19]

These are neat, ... and tiny! 

I'm sure people would love to see the teeth as well. 
Any other fossils found in the matrix collected? 

Thanks for posting these. 

Regards,

[Al Dente]

Interesting. I think these are clam molds and not brachs. The symmetry doesn't look right for brachiopods.

[FossilDAWG]

Very cool little fossils.  I agree that they are clams, not brachiopods.  Check out Corbula and related genera (such as Crassicorbula).  Corbulids are unusual for bivalves in that one valve is larger and rounder than the other, which is flattened and fits into the larger valve.  Corbulids are also small as a rule; 1 cm wide would be a large specimen.  They are extremely abundant at some of the Cretaceous sites I collect.

 

Don

[PFOOLEY]

36 minutes ago, FossilDAWG said:

...Corbulids are unusual for bivalves in that one valve is larger and rounder than the other, which is flattened and fits into the larger valve...

 

evolutionary origins of novel conchologic growth patterns in tropical american corbulid bivalves.pdf

[Arizona Chris]

I suspected that they were brachs because there is not only the right/left symmetry and not top/bottom, and the hole on many of them for the pedicle to emerge.  I have never seen a clam with such an arrangement.  Have you ever seen a clam with a pedicle opening?

[Al Dente]

1 hour ago, Arizona Chris said:

I suspected that they were brachs because there is not only the right/left symmetry and not top/bottom, and the hole on many of them for the pedicle to emerge.  I have never seen a clam with such an arrangement.  Have you ever seen a clam with a pedicle opening?

 

What you are looking at is actually a steinkern of the clam, the shell is dissolved away. The right/left symmetry isn't really present on your specimens. If you look closely, one side sticks out slightly more than the other and is not a mirror image of the other side like you would get with a brachiopod.

[Arizona Chris]

Would that then imply that these clams were buried alive, since both valves are still intact and not opened?  I can go with that because we have found quite a few molluscs in the Pennsylvanian Naco formation that have been preserved that way.  

[Al Dente]

3 hours ago, Arizona Chris said:

Would that then imply that these clams were buried alive, since both valves are still intact and not opened?  I can go with that because we have found quite a few molluscs in the Pennsylvanian Naco formation that have been preserved that way.  

 

Buried alive is a possibility or they could have died and not been disturbed by currents or burrowing organisms.