|Ecphora fossil on left, Chesapecten on right, collected along the Potomac River
The first fossil ever described from North America also happens to be the state fossil of Virginia: Chesapecten jeffersonius. It was first described from the shores of Virginia in 1673 and is a Pliocene Scallop. While I have found many pieces of this fossil, I have not found a complete one. I must admit that the one pictured here is a fossil Chesapecten, but not the species that is our Virginia state fossil. The real deal has 9-12 ribs and the whole one I have pictured has more than that, but otherwise looks the same. It is another fossil scallop in the same genus and very similar, but not the exact one.
The Maryland state fossil is a predatory snail, the Miocene Ecphora gardnerae. It has distinctive color and ribs, so I think I'm safe in saying I have the right ones pictured. It was recently reclassified, with the old name being Ecphora quadricostata, again referring to the 4 most distinct ribs, or "costae."
Both of these can be found along the shores of the Potomac and some other large waterways by beach combing. The hard part is finding a good location and then finding a whole piece. I have not been lucky enough to find either states' official dinosaur - yet. I also have not found Washington DC's official "state" dinosaur, the "Capitalsaurus." If I ever do, that would most certainly be a great and appropriate post for Capital Naturalist.