A Cross-section of Something, Perhaps?

[K-boy Cachers]

This peculiar thing caught my eye while I was out looking for urchins, clams, gastropods and all of the other bountiful fossil blessings of Central Texas. This was in an intermittent creek cut in the Comanche Peak formation, Lower Cretaceous period, western Bell County, Texas.

 

The scale in the background is inches (sorry, no metric device readily available). The oval shape of the fossil is 1.5 cm by 1 cm.  All of the lines you see making up the fossil are crystalized sediment within the limestone matrix. My 8 year old was excited about how "sparkly" it looked under the flashlight.

 

In a couple of the pictures you can see what appears to be a very small section of the side of the fossil.  I am stumped on this one. My hunch is that i'm seeing the inside of an organism that we  typically get to see the outside of. But i don't know what the insides of the urchins look like.  Seems too oval to be a cross section of phymosoma texanum. Maybe it was a plant or coral? Could it be a flattened-out, crystalized Parasmilia?

 

 

[hemipristis]

It appears to be a cross-section of a solitary rugose coral

[Rockwood]

Sure does.

Transported by a river perhaps ? It wouldn't be Cretaceous.

There are solitary scleractinians though.

[TqB]

I'm sure it's a scleractinian, so presumably Cretaceous (the matrix looks like chalk) and Parasmilia is certainly a contender.

 

The order of insertion of the  septa is the principal difference from rugosa although often difficult to make out in practice. Here, however, you can clearly count 12 long septa which will be the first two sets of 6. Then these are added to in all the spaces with another 12 then 24.

(Rugosa are more complex, with some spaces always unoccupied so the symmetry is actually bilateral.)

 

I think it's a textbook example.  

[Rockwood]

Son of a gun ! The septa do seem to bifurcate. Don't they ?

 

[TqB]

39 minutes ago, Rockwood said:

Son of a gun ! The septa do seem to bifurcate. Don't they ?

 

No bifurcation, just insertion - they're simple ridges on the base and sides of the epithecal cup, formed between the soft mesenteries.

[Rockwood]

34 minutes ago, TqB said:

No bifurcation, just insertion - they're simple ridges on the base and sides of the epithecal cup, formed between the soft mesenteries.

Is this a bit contrary to appearance ? Determined by serial sectioning. Or am I just slow to absorb the concept ?

[TqB]

1 hour ago, Rockwood said:

Is this a bit contrary to appearance ? Determined by serial sectioning. Or am I just slow to absorb the concept ?

Yikes, I'm not well up on scleractinians! Looking it up in a couple of books (Treatise and a textbook - online is a bit confusing), some septa can temporarily bifurcate at the outer edges depending on what sort of mesentery they're deposited in (there are exocoels and endocoels, giving rise to exosepta and endosepta) and they separate later, so sorry about that!

 

Not in all genera though and you always end up with a sequence that goes 6 + 6 + 12 + 24 + 48 etc. septal ridges, adding as you move up the cup. (Usually to not more than 8 cycles, 768 septa.)

 

Here's what can happen with the bifurcating ones, also resulting in pillars called pali:

 

[K-boy Cachers]

Thank you all for your input.  

 

Dear Central Texas fossil hunters,  what are your thoughts and have you seen many coral specimens like this in the Comanche Peak layer?

[erose]

Definitely a coral. There are a number of species that it could be. Parismilia (spelling?) is one. But what is nice is that if you can get your hands on the right reference paper (see below) having that section would make an Id a real possibility.

 

Corals of the Cretaceous of the Atlantic and Gulf Coastal Plains and Western Interior of theUnited States, by John West Wells, Bulletins of American Paleontology Volume 18 1931-1933

[RyanNREMTP]

Looks like an accurate identification has been made. I like the fossil. I would have been excited to pick that one up as well.