Bivalves vs. brachiopods
Bivalves and brachiopods are both types of “sea shells.” both have shells composed of two valves, but the organisms inside the shells are quite different. Typically, the two valves of a bivalve are mirror images of each other (termed equivalved). Their valves are symmetrical along a plane through the hinge. Some bivalves are unequivalved (for example, modern oysters). In contrast, brachiopods are symmetrical across valves, but not along the hinge. Brachiopods have dissimilar valves, but each valve is symmetrical along a line midway across each valve, perpendicular to the hinge. Although bivalves are much more abundant than brachiopods today, in the Paleozoic Era, when most of Kentucky’s bedrock formed, brachiopods were much more abundant than bivalves.
Another difference between bivalves and brachiopods is the manner in which their valves are connected. Bivalves have two valves connected by a soft-tissue ligament and in many cases, a variety of protrusions called “teeth” (cardinal teeth, lateral teeth), or dentition, along the hingeline. Teeth on one valve match small grooves on the other valve. In contrast, articulated brachiopods have valves connected by a ball-and-socket mechanism along the hingeline.