Strophomena brachiopod from Ordovician of Indiana

[connorp]

I collected this Strophomena planumbona brachiopod recently in the Liberty Formation (Late Ordovician) near St. Leon, Indiana. What caught my eye are the "bumps" near the muscle scar. I've collected and seen a lot of Strophomena brachiopods, but can't recall having ever seen these before. What exactly are they? @Tidgy's Dad

 

 

[Mediospirifer]

Interesting!

 

While I have several Strophomena specimens, they don't usually have that part of the inner shell exposed. I can't say I've seen that before... and a quick check of my "Fossils of Ohio" guide shows a smooth interior. 

 

I have seen brachios with isolated bumps where something intruded and became encased in the shell material. Check out this thread for a wild variety of what can go wrong for a brachiopod: LINK.  One interesting cause of interior lumps is parasitic intrusion by sponges! (Okay, I expect the brachiopod wasn't as happy to host them as I am to find them. They make interesting fossils!) Your specimen's bumps look smaller than those, and form a suspiciously regular pattern. I wonder if yours had a disease or inflammation that caused it to secrete excess shell? Brachiopod with a rash?

 

@Brach3would be interested in this, and may have some ideas, if he's available. 

 

[Crusty_Crab]

I agree, great question! When answering questions like these, I like to start with a brainstorm of what I can observe. One thing that I see is that the "bumps" seem to coincide along the axes of the costae. Perhaps it might be related to formation of the costae (which are generally a feature of the exterior morphology)?

 

 

I consulted my copy of Treatise on Invertebrate Paleontology H Brachiopoda Revised, Volume 1, which appears to show those "bumps."  

 

 

Perhaps the origin of the bumps is the intersection between the costae and the growth lines?

 

 

 

 

[Tidgy's Dad]

Strophomenates have pseudopunctate shells, which means they have calcite rods forming in the ' secondary' inner shell layers but not extending to the outer 'primary' layer. Often,  these rods produce bumps, papillae or plugs on the inside of the valves. In strophomenids, these are rounded lumps, in chonetids, sort of cones, and in non-chonetid productids are sometimes internal spines. They show up much more clearly in some specimens than in others. 

Here are some showing on a Strophomena filitexta from my collection.

 

And inside a Sowerbyella  valve:

[Tidgy's Dad]

Oh, @connorp, I forgot to mention. 

It has been proposed that these little bumps also performed a function for the organism. It has been suggested that they trap food particles.