Help ID Texas Cretaceous Bivalve. - strange pointy clam

[JamieLynn]

Hi Eveyrone! I know that ID'ing bivalve steinkerns is difficult if not impossible. But these are such an unusual shape. I have not found anything in my hunting - and I've found a LOT o bivalves. These are from Blanco TX - Glen Rose formation. They look like a bit like Trigonia (or pterotrigonia) in shape but not quite so "fortune cookie" curved , but have the really dramatic "underside" of something like an Artica.  I do not have the Houston Gem and Mineral Society book on Bivalves, so if anyone does...would you mind seeing if you can find this one?  It is really intriguing me! Thanks!! 

 

[DPS Ammonite]

Check out Trigonia and related clams that are common in the north Texas Cretaceous rocks. 

 

Get a copy of “A Field Guide to Fossils of Texas” by Finsley. It has picture two species that are similar.

 

https://www.google.com/search?q=trigonia+texas&tbm=isch&ved=2ahUKEwjX1aL954XkAhWYnZ4KHX8uA7QQ2-cCegQIABAC&oq=trigonia+texas&gs_l=mobile-gws-wiz-img.3...6057.6833..7275...0.0..0.238.557.0j2j1......0....1.........0i24.r2kYQtp332w&ei=jM1VXdffDJi7-gT_3IygCw&bih=454&biw=320&client=safari&prmd=inmv&hl=en-us

 

 

 

 

[JamieLynn]

Yes, I recognized that it is similar to Trigonia (in my original post I mention that) but they always have very strong ribbed designs. As you can see, these are quite well preserved but do not have the Trigonia ornamentation.  I have the Field Guide to Fossils of Texas. That is my first go-to. I am not seeing any in there that look similar. Can you please tell me which genus species you believe these might be? 

[DPS Ammonite]

15 minutes ago, JamieLynn said:

Yes, I recognized that it is similar to Trigonia (in my original post I mention that) but they always have very strong ribbed designs. As you can see, these are quite well preserved but do not have the Trigonia ornamentation.  I have the Field Guide to Fossils of Texas. That is my first go-to. I am not seeing any in there that look similar. Can you please tell me which genus species you believe these might be? 

You have an internal mold that preserves little of the outside ornamentation of a Trigoniid. Without the surface ornamentation it will be hard to nail down a species. It is best to find literature for species that occur in the Glen Rose and then make your best guess.

 

Some locals may be able to help you: @erose @JohnJ @BobWill @Uncle Siphuncle

[El_Hueso]

Wow! That's a great find. It's always cool to find unusual or uncommon forms of fossils. Was this a recent find? Ive been thinking of going back to Glen Rose soon and wondering how the rivers look right now.

[grandpa]

OK, so the one on the bottom in picture #2 and on the left in picture # 4 is, I do believe, an Arca simondsi (Whitney, 1937), which is pictured in the HGMS Texas Cretaceous Bivalves 2 book on page 67.  I'm still looking for the other one - I know that little fella, I just have to find him to refresh my memory.

[grandpa]

2 hours ago, DPS Ammonite said:

It is best to find literature for species that occur in the Glen Rose and then make your best guess.

I keep trying to fit a Trigonia ID to the 2nd specimen as DPS Am suggests above and just can't make a fit.  The Trigonia sp. from the Glen Rose include only 5 sp. (according to HGMS Bivalves 2).  They are:  T. gordoni, T. stolleyi, T. taffi, T. wendleri, and T. whitneyi.  None of those species matches the physiology of the specimen in question (even ignoring the ribbing).  The physiology that separates this specimen from a Trigonia sp. (to my eye, at least) is the wide space between the beak peaks on this specimen (vs. a tendency for T. species to have beaks that curve in toward each other) and the large muscle scar in the open gap of the shell.  I know these features together with the overall shape are diagnostic of something I've seen/collected before; I just can't place what it is.  I sure hope someone comes up with an answer because my head is starting to hurt.  When they do, I bet it will be obvious.

[JamieLynn]

2 hours ago, grandpa said:

OK, so the one on the bottom in picture #2 and on the left in picture # 4 is, I do believe, an Arca simondsi (Whitney, 1937), which is pictured in the HGMS Texas Cretaceous Bivalves 2 book on page 67.  I'm still looking for the other one - I know that little fella, I just have to find him to refresh my memory.

 

I assumed they were the same thing....You think they might be two different species?  And thank you for your help looking them up the bivalves book. I just haven't had the funds to buy that one. I have Gastropods and Ammonites, but not the Bivalves! 

[JamieLynn]

3 hours ago, El_Hueso said:

Wow! That's a great find. It's always cool to find unusual or uncommon forms of fossils. Was this a recent find? Ive been thinking of going back to Glen Rose soon and wondering how the rivers look right now.

 

Yes, found it this week. Yeah, the Glen Rose is my backyard, and I"ve been collecting for at least three years and I've never seen one of these! Even if it is something as "lowly" as a bivalve, finding something new to me is so great!  I don't do much hunting in the rivers, but it's been pretty dry, so might be good! If you ever want to meet up and hunt, let me know! 

[grandpa]

11 minutes ago, JamieLynn said:

You think they might be two different species?

I do.  The shapes are different enough, and one seems to have a muscle scar behind beaks while the other does not show that scar.  Although an end-on photo of that end of each specimen would be helpful.

 

14 minutes ago, JamieLynn said:

And thank you for your help looking them up the bivalves book.

You're quite welcome.  It was fun, and I relearned something.

 

11 minutes ago, JamieLynn said:

Even if it is something as "lowly" as a bivalve,

Oh no, don't cast dispersion on the bivalve.  Texas has such a wealth of Cretaceous bivalves that it is a boon for any serious collector who wishes to explore its diversity. 

[JamieLynn]

i'll get you a pic in the morning! Thanks again! 

[JamieLynn]

I hope these end on photos will help- thanks so much @grandpa

 

[grandpa]

That does help.  Your last photo showing almost identical morphology of the muscle contact scars (along with further research on my part which turned up no other option) now has me convinced that your initial assumption was correct and that we are dealing with two specimens (albeit somewhat deferentially eroded) of the same species, which I still believe to be Arca simondsi.  I hope you find this helpful, and again - appreciate the bounty of diversity in Texas' Cretaceous Bivalves!!

[JamieLynn]

Very helpful! And yes I will never disparage a clam again.... hahhahah!!!