Bivalves or Brachiopods?

[Pippa]

I'm in need of your help. 

I'm trying to learn how to recognize and maybe differentiate brachiopods from bivalves in the worn fossil rocks I find on Lake Michigan's beaches. 

If indeed it is one or the other, how do you tell which? 

In the photo below, what looks like the shaft of an arrow, is it the foot or stalk? 

OTOH, if it's neither b-pod nor bivalve, what is it?

 

 

Often I find just bits and pieces in these tumbled rocks.  I suspect some of them to be parts of shells, but I'm not quite sure. 

The shape within my blue circle below is visible over and over on many of my finds. Is thisa part of a shell? Or not?

 

 

This fossil turns the corner on the same rock as above. It's visible on both sides of the fossilized rock, which is about 5mm wide.

I've combined photos of both sides of the rock onto one pic. The total length of this fossil is just about 2.5 cm.

I assume that's a cross section of a shell showing the insides of the two valves?

 

 

 

[Ludwigia]

Just google "brachiopod anatomy" and "bivalve anatomy" and you'll find lots of information to your question. It is only in extremely rare cases that soft parts like foot and stalk are preserved. Your impressions are all of parts of the shell. Most shown are brachiopods, with exception of the fossil in the 3rd & 5th photos of which I'm not sure.

[Pippa]

6 hours ago, Ludwigia said:

Just google "brachiopod anatomy" and "bivalve anatomy" and you'll find lots of information to your question. It is only in extremely rare cases that soft parts like foot and stalk are preserved. Your impressions are all of parts of the shell. Most shown are brachiopods, with exception of the fossil in the 3rd & 5th photos of which I'm not sure.

Oh, I did. Thing is, it doesn't seem to help me much. Other than basic outer shape that looks like it might be a shell. How do you tell from the above that it's a b-pod vs a bivalve?  Bivalves tend to be symmetrical vs b-pods that mostly have different sized valves. 

 

You made me think though. If the stalk and the insides don't get fossilized (duh, I should have realized that )  maybe what looks like the "stalk" in the first photo is just a random bit of another fossil? 

[grandpa]

The rule of thumb for distinction between Brachs and Bivalves is:   brachs have left/right symmetry of the front valve and similarly of the back valve, which might be of a much different shape (usually is) compared to the front valve.  So, draw an imaginary line down the center of the valve, top to bottom, and the left side should be a mirror image of the right side.

 

Bivalves have front valve/back valve symmetry.  Both valves look and are shaped alike.