The Rhynchonellida

Rhynchonellids look a bit like little nuts. Their hinges come to a point, a condition paleontologists call non-strophic. They are often ridged as shown here. The commisure, the line between the two valves or shells, is zig-zagged, as can be seen in the somewhat unusual asymmetric rhynchonellid Rhactorhynchia. The earliest fossil rhynchonellids are from the Ordovician period. During the Mesozoic Era, rhynchonellids were the most abundant brachiopods. A few species still exist today.