Unknown Devonian mollusk

[TNDevonian]

I have 11 specimens of this, and all of them are incomplete. I am posting an example that shows the salient features that apply to all. This is from the lower Devonian Birdsong Shale member of the Ross formation in Parsons, Tennessee. All specimens are 30~50 mm ovals consisting of growth rings only, and any hinge area is missing from all. The rings have no radial features or ornamentation. They are very shallowly concave and seemingly have no prominent apex The growth rings center from what appears to be a muscle scar, and the outer ring is always slightly thicker and wider. The shell is quite thin at about 1 mm. I can post other photos, but this one has all the key features that are perplexing me.

 

 

[fossilnut]

My first look and thought was that this was an operculum from a gastropod. Are gastropods listed from that formation? Wait to hear from others.

[Fossildude19]

Any completely exposed specimens handy?

 

[TNDevonian]

When I first found these, that was my thought too, but the more I looked at them, I managed to talk myself out of it. The largest gastropods associated with this formation are generally too small to have this as an operculum, and I have never seen one of these in association with gastropods, especially large ones which are rare to find.

[TNDevonian]

Fossildude19,

 If you are referring to a complete shell free of matrix, no these are very fragile and would never erode out intact. They barely survive being held by the matrix which is usually quite fragile itself. 

[FossilDAWG]

My first impression is of a sort of pectin-like bivalve, but I do not have a more specific ID to suggest.  My experience with the Birdsong is limited, although to diversity makes it a fun formation to collect.

 

Don

[TNDevonian]

Don,

 

I have been unable to contact a person who I know must have intimate knowledge of the Birdsong, so I floated this balloon here. There are only a couple of Devonian bivalves that I can find from anywhere that share the simple growth lines without any radials, and they all have distinctly pointed apexes that aren't central, deeper concavity, and are not oval.  None are listed in the Birdsong. And yes, collecting in the Birdsong is like shopping in a massive seafood market!

[Fossildude19]

20 hours ago, TNDevonian said:

 There are only a couple of Devonian bivalves that I can find from anywhere that share the simple growth lines without any radials, and they all have distinctly pointed apexes that aren't central, deeper concavity, and are not oval.

 

 

Have you considered that perhaps it is not pelecypod, but possibly a diplostracan? (Formerly conchostracans)

 

[TNDevonian]

 

3 hours ago, Fossildude19 said:

 

 

Have you considered that perhaps it is not pelecypod, but possibly a diplostracan? (Formerly conchostracans)

 

Tim,

Yes, but the size would be remarkable, and usually their shells are not marked with distinct growth lines as these are. The same for clam shrimp, and for both, the concavity would probably be more distinct. Having a specimen that would explain the sweeping curve outward as it approaches the supposed hinge area would be a help.  It's a quandary, and it has had my full attention for several months. Thanks for your  attention and observations

[TNDevonian]

On 10/24/2022 at 12:32 PM, Fossildude19 said:

Any completely exposed specimens handy?

 

Here are some other examples that show the fragility and features common to all:

[GordonC]

Consider Orbiculoidia Sp.    In our area (Port Jervis NY)  two  species were identified in the Glenerie Formation, lower Devonian ( lies between the Helderberg Group and the Tristates Group.)  Here a fine specimen of Orbiculoidea Ampla (Hall) collected by @Jeffrey P (gallery_13044_1993_510109.jpg)

Edited October 28, 2022 by GordonC

[TNDevonian]

10 hours ago, GordonC said:

Consider Orbiculoidia Sp.    In our area (Port Jervis NY)  two  species were identified in the Glenerie Formation, lower Devonian ( lies between the Helderberg Group and the Tristates Group.)  Here a fine specimen of Orbiculoidea Ampla (Hall) collected by @Jeffrey P (gallery_13044_1993_510109.jpg)

 

I have considered it, but in general, the Orbiculoidea are all based on concentric, round growth rings around a central apex (hence the name), and also do not have that eccentric curve away as these do as they approach the area I assume is a hinge. The distinct tapering outer margin, and the offset origin point for the growth lines point away from it being a known Orbiculoidea. There are also no ridges between growth lines as in the example offered. Thanks for looking.

[Tidgy's Dad]

They do look very much like lingulate brachiopods to me.

Maybe not Discinoidea itself, but another Discinoid or even a member of the Obolidae. 

 

[TNDevonian]

On 10/27/2022 at 9:17 PM, Tidgy's Dad said:

They do look very much like lingulate brachiopods to me.

Maybe not Discinoidea itself, but another Discinoid or even a member of the Obolidae. 

 

I can't find any examples of either that have details that match closely enough to assign it to them . The un-ridged growth lines, general shape, lack of radials, tapering outer rim that then turns away from the concentric origin are all points that have negated most comparisons that I can find. It is an extremely thin shell that has made finding a complete specimen seemingly impossible, and all are missing key elements. The initial photo in this post shows just how odd the configuration is. I thank you for you suggestions, and will continue to try to find matching images or descriptions.

[Tidgy's Dad]

Yes, I don't know of a brachiopod from this formation that matches. 

But nothing else I know from the formation matches either. 

 

The thinness, preservation, colour and patterning match closely to several brachiopods that I know of. 

Although, as I have said previously, I do suffer from brachiopareidolia.

 

[TNDevonian]

1 hour ago, Tidgy's Dad said:

Yes, I don't know of a brachiopod from this formation that matches. 

But nothing else I know from the formation matches either. 

 

The thinness, preservation, colour and patterning match closely to several brachiopods that I know of. 

Although, as I have said previously, I do suffer from brachiopareidolia.

 

And there's the rub. It could easily be overlooked or dismissed as part of a known brachiopod unless you accidentally realize that it has details that don't fit with any of the usual pieces parts in this formation. It took me a while to realize its oddness, and once seen, more are looked for and found. If you examine the photos, you will see what appears to be a right handed and a left handed valve with the tapering part of the outer rim mirror imaged. The concavity of either is very meager, 

[Tidgy's Dad]

On 10/29/2022 at 6:59 PM, TNDevonian said:

And there's the rub. It could easily be overlooked or dismissed as part of a known brachiopod unless you accidentally realize that it has details that don't fit with any of the usual pieces parts in this formation. It took me a while to realize its oddness, and once seen, more are looked for and found. If you examine the photos, you will see what appears to be a right handed and a left handed valve with the tapering part of the outer rim mirror imaged. The concavity of either is very meager, 

Hmmm.

Or ventral and dorsal valve if it's a brachiopod, not left and right.