Exogyra ponderosa. Large find!

[erose]

7 hours ago, KJAG said:

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).  

 

Yes, Exogyra ponderosa is an index fossil for the younger Upper Cretaceous and in Texas it is most often found in less consolidated strata of the Upper Austin and Taylor Groups.

 

For what it matters...I have some large Exogyra that do show a small attachment scar on the larger left valves. But most do not. On one of my first visits to Texas I encountered a similar bed of E. ponderosa all sitting in life position (right valves up) in a rather sandy layer above the Dessau Formation.  It was a shallow erosion gully and they were maybe a foot give or take between but you could easily imagine them all alive under that warm clear Western Seaway. 

[Fossildude19]

That's a nice find!

Thanks for sharing it. 

[Jared C]

never knew how interesting these familiar fossils could be. What a great thread

[Plax]

On 6/10/2019 at 11:16 PM, BobWill said:

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.

The spat fall on something solid and as the Exogyra grows that attachment becomes less important. Am pretty sure that most oysters start out like this. Oystermen salt suitable areas with old shells for spat to settle on and grow. The initial impression of the settlement object is called bioimmuration. Almost all my Exogyra have at least a hint of it and never thought of it until it was pointed out to me quite recently. Left and right valve is applicable of course but upper and lower are also, in a descriptive sense. 

  We must keep in mind that in many Exogyra abundances the aragonitic shells are not preserved. The habitat would have been shelly mud if we could see it all.

[BobWill]

9 hours ago, Plax said:

The spat fall on something solid and as the Exogyra grows that attachment becomes less important. Am pretty sure that most oysters start out like this. Oystermen salt suitable areas with old shells for spat to settle on and grow. The initial impression of the settlement object is called bioimmuration. Almost all my Exogyra have at least a hint of it and never thought of it until it was pointed out to me quite recently. Left and right valve is applicable of course but upper and lower are also, in a descriptive sense. 

  We must keep in mind that in many Exogyra abundances the aragonitic shells are not preserved. The habitat would have been shelly mud if we could see it all.

Yes, I may have read a different meaning into John's post than what he intended. I had answered the original question about which valve attaches and I believe another question arose so I made sure there was no confusion about left/right vs. upper/lower since readers would encounter taxa descriptions using the former. Certainly in the case of seeded substrate of old shells you would call the attachment point an example of bioimmuration but not one occurring on anything geologic. An example of bioimmuration is also considered a duplication fossil so it needs to copy something biological.

Edited October 13, 2021 by BobWill

[Plax]

On 10/13/2021 at 6:29 PM, BobWill said:

Yes, I may have read a different meaning into John's post than what he intended. I had answered the original question about which valve attaches and I believe another question arose so I made sure there was no confusion about left/right vs. upper/lower since readers would encounter taxa descriptions using the former. Certainly in the case of seeded substrate of old shells you would call the attachment point an example of bioimmuration but not one occurring on anything geologic. An example of bioimmuration is also considered a duplication fossil so it needs to copy something biological.

Quite right. I haven't noticed exogyra attached to anything but marine life but then I'm looking for bioimmuration.  Have of course seen modern oysters attached to concrete and all other manner of things. Bioimmuration is a great way of seeing what aragoniitc fauna was present in the beds where they are gone physically.