I did some googling and found this site that dates them to the middle Eocene, which is 40-47 Million Years Ago (MYA). I currently have a very nice Venericardia (Venericor) densata sitting on my desk. It’s pictured in the bottom right of that chart. It looks pretty much like a clamshell, but made of rock.
These little bits of rock excited my little sister, and I’m absolutely sure that they’d make a fantastic science lesson. Kids really do love fossils, so what better way to education them about biology than a field trip to a local deposit (no matter where you live, there’s something within a reasonable drive, even if it’s just a museum). If kids can actually find fossils themselves, date them using the same techniques as paleontologists, and look for related organisms that are still alive, then they’ve just learned a wonderful lesson in paleontology, evolution, biology, and science in general. Plus they’d almost certainly love it.
A simple field trip could spark a lifetime love of science. People often ask what we need to do to improve science education, and from now on I’m going to answer “fossils”.