Exogyra ponderosa. Large find!

[John525]

These are some amazing specimens I found in Texas last year in one area of a river that runs through some privately owned land of some friends of Mine. All of them were half buried with the top of the shell downwords in  hard River sediment/rock. Looked like a bunch of circles when I discovered them as the river was super shallow that week. A chisel and hammer would pop them right out. I work on cleaning them up from time to time. There's Alot left to do! Hope you all enjoy. 

[Tidgy's Dad]

Very impressive beasties! 

Nice.

[Wrangellian]

Wow! They look like big, thick abalones.

It sounds like that was their natural orientation, if the other valve is dissimilar and these were all found in the same orientation. It's a good idea as a collector to make notes of things like that.

[DPS Ammonite]

All the shells that I see are the much larger bottom shell. They Exogyras literally grew their way off the muddy bottom. The top shell was much thinner and flatter allowing the bivalve to lift the top shell to feed. Do you see any of the top shells. @John525 ?

[Ludwigia]

Remarkable finds. I think I'm seeing the top shell in the second photo.

[ricardo]

Great Exogyra! Thanks for show it!

 

This one is a smaller “cousin” from our Cenomanian, but not so big as yours

Articulated specimen. Tentúgal Fm, C level (collected 2019/05/25) .

 

Coimbra, Portugal.

 

 

 

 

 

[John525]

@ricardo I like your speciman! Everyone else: maybe I used the terminology wrong in regards to top shell. The way I found them they were all laying half buried in the sediment like it is in my hand in the 2nd picture. (kind of interesting they were all buried like that). If the top shell is the thin part that would lay inside the cavity as @DPS Ammonite mentioned, then I only found one out of the whole lot with that piece still attached. I can see if I can find that one in the bin they are stored in later on. 

[Mark Kmiecik]

Great find. Congratulations.

[Wrangellian]

15 hours ago, John525 said:

@ricardo I like your speciman! Everyone else: maybe I used the terminology wrong in regards to top shell. The way I found them they were all laying half buried in the sediment like it is in my hand in the 2nd picture. (kind of interesting they were all buried like that). If the top shell is the thin part that would lay inside the cavity as @DPS Ammonite mentioned, then I only found one out of the whole lot with that piece still attached. I can see if I can find that one in the bin they are stored in later on. 

That's how I understood you originally. That would tend to indicate that these are the bottom valve. Please do post a pic of the one with the top valve intact when you find it!

[BobWill]

The fossil of these oysters can end up pointing either way when the animal dies.  Unlike Brachiopods, they actually have a right and left valve, not a top and bottom. The thicker valve is the left one. Since the right valve is thinner it doesn't preserve as well so we see it less often. Which way they orient in life can also depend on what is growing along with them on the sea floor and what they push up against as they grow since one valve is permanently attached to the substrate.

[Wrangellian]

I guess the heavier valve would tend to be the bottom one, all else being equal. Of course turbulence etc could disrupt that before or after death but that would be the exception, no?

[John525]

@BobWill Thank you for that awesome information. @Wrangellian I just searched my bin and I actually have 2 of them with the 2nd valve. Here are some pictures of the 2. I will have to post them separately though. 

[John525]

[John525]

[John525]

[John525]

[Wrangellian]

Those are keepers.. Thanks for showing.

[BobWill]

9 hours ago, Wrangellian said:

I guess the heavier valve would tend to be the bottom one, all else being equal. Of course turbulence etc could disrupt that before or after death but that would be the exception, no?

Since they are attached to something solid when alive the plan is to only ever move the right valve. If there is enough turbulence to disrupt that they are living in the wrong neighborhood

[Wrangellian]

OK now I'm confused.. Which valve is the one attached to the substrate, the thick one or the thin one?

[DPS Ammonite]

The larger thicker valve rests on the floor unattached. When younger, the thicker valve attaches usually to a shell such as an Inoceramus. The creature hopes that it can grow larger and upwards faster than the mud can accumulate.

[JohnJ]

1 hour ago, DPS Ammonite said:

When younger, the thicker valve attaches usually to a shell such as an Inoceramus.

 

...Or ammonite. 

[BobWill]

On 6/1/2019 at 8:21 PM, DPS Ammonite said:

The larger thicker valve rests on the floor unattached. When younger, the thicker valve attaches usually to a shell such as an Inoceramus. The creature hopes that it can grow larger and upwards faster than the mud can accumulate.

I am confused by this comment John. First you say the thicker valve is unattached then you say the thicker valve attaches. Do they attach and then become unattached later in life? It seems like I always see the attachment point on the left valve. I know that to determine which is which you hold any bivalve with the hinge line above and the beak pointing away and for most oysters this makes the larger valve the left one.

[DPS Ammonite]

I think that some exogyras are attached early in life and later detach or their host disintegrates. Most of my exogyras show small or no attachment scars, suggesting that they live in the mud unattached when older.

[KJAG]

Just now, KJAG said:

 

Found a bunch about 20 miles east of San Antonio, but the only one I found with both valves.  Sticking out the side of a newly-formed erosion ditch.

 

 

[KJAG]

I should mention this specimen (and many others found east of San Antonio) were extracted relatively close to the surface in heavy clay, not within or near any limestone/rock formations (although these formations exist miles to the northwest along the Balcones Fault Zone).