Fossil Seafloor?

[FedEx]

I found these on a beach in Mississippi where I normally find late Miocene fossils. The first piece has little bumps all over it which I think are bivalve steinnkerns, and it has that prominent shell fragment embedded in it. The other pieces don’t have much going on except for burrows(?), and some of the burrows appear to have little bivalves in them. I’m calling these holes burrows because that’s what they look like, but it’s weird that they’re not filled in. The burrows and the (now dead) bivalves inside might be modern. Piece 5 also has a couple weird circular patterns - maybe traces? All of the pieces were found right next to each other, and they’re all made of limestone. I’d just like to confirm this is a piece of seafloor and not modern somehow because I’m used to finding shark teeth. I have no clue how these ended up on a sand beach. 
 

There are a lot of photos, so I labeled them all. I might have went a little picture happy, but I’m nothing if not thorough.

 

Piece 1 obverse

 

Piece 1 reverse

 

Bivalve and little gastropod steinkerns

 

More bivalve steinkerns

 

Close up of embedded shells

 

Bivalve shell in burrow

 

Close up of shell frag

 

Piece 2 obverse

 

Piece 2 reverse

 

Piece 3 obverse

 

Piece 3 reverse

 

Piece 4 obverse

 

Piece 4 reverse

 

Burrows

 

Piece 5 obverse

 

Piece 5 reverse

 

Circular pattern

[FedEx]

After a little more research, these are definitely piddock clams. They’re common in the region, and I’ve actually seen their handiwork before on beach rocks. Ignore the bivalves in the burrows. They’re cool little clams but very modern.