Eagle Ford Bouldin Flags Calcite Mold - Unknown Fossil

[LSCHNELLE]

I found this crystalline calcite replacement fossil mold after breaking open the width of a 2" to 3" thick Eagle Ford limestone layer loose fragment.  The rock broke along the circular arc of the fossil mold.  The mold is somewhat mushroom shaped with a small inoceramus clam attached to the side. It is about 4.25" wide.  A full circular arc might be more than 6 inches in diameter.  The mold appears to be fragmented and hollow on the top of "mushroom shaped" side.  The narrower bottom of the mold also flared out a little, but not as much as the top.

 

I think that the thin base layer cutting at 30 degrees to the mold is an oyster shell hash layer that it was deposited with - although at the mold top the hash layer to be unusually smooth faced.  Some of the calcite mushroom lip broke out on the other fragment (the 4th photo shows it upside down on the bottom). The lip was not likely to be easily recovered.  So, I cut that face of the limestone fragment back from the lip so that the two can fit together where you can still see the concave fossil surface inside.  I worked off the convex outer matrix of the mold and the micrite limestone matrix until there was little else left but the mold. The oyster hash (or other) layer and the small inoceramus are also attached.

 

The limestone layers in the outcrop area has some 15mm or less tooth width sized Ptychodus and up to 12 to 20 mm long cutter shark's teeth.  There were also two ammonite molds (10" to 12") preserved in similar crystalline manner.  There are a few shark verts and fish/ray teeth also.

 

The few Paleontological Society of Austin folks I showed it to at the recent meeting could only see the concave face before I had carved it out more.  They would not try to guess its origin.

 

My first inclination was that it is a large vertebrate bone fragment.  That might have been just well wishing.  A giant inoceramus hinge plate or other large invertebrate (like ammonite or nautiloid) interior mold seems more likely.  Any educated guesses?  Thanks in advance for your time.

 

 

[erose]

The Balcones fault zone includes many calcitic veins. I have found stuff like that in both the Eagle Ford and the Austin Chalk groups around Austin. Just for yucks turn the (white) lights off and hit it with some UV. 

[LSCHNELLE]

Thanks for checking it out. I need to get a UV light to make that happen.  I know the pictures are difficult to make out. But, it essentially like a 6" to 7" Portobello mushroom top with a 4" to 5" stem diameter.  The mushroom top is tilted and attached to an cemented rock layer just 3/4" thick. That could be a hardened fossil beach shell layer that the fossil mold fell into with a limey mud layer that I chipped away from the mold.  The mushroom top is chewed away on part of the top side.