The Ordovician (about 444 to 488 million years ago) was a time of abundant trilobites and diverse echinoderms. Some of the echinoderm classes present then are still to be found in modern seas, but most are extinct, as are the trilobites.

The shallow marine fauna is very well represented in the Upper Ordovician (Early Caradoc, roughly 450 to 460 million years ago) Bromide Formation of Oklahoma, USA. This is thought to be the most diverse echinoderm fauna in Earth history. This small collection brings together a representative selection. It mostly comes from the Poolville Member of the Bromide Formation exposed in the Dunn Quarry, Criner Hills, Carter County, Oklahoma. Descriptions of this exposure can be found in Sprinkle (1982, ed). The matrix in the exposed succession here consists of alternating light grey to tan biomicrite with shaly micrite in thin beds. Echinoderms occur in thin intervals or biostromes throughout the succession.

The Bromide fauna is highly illustrative of the first major faunal diversity peak of the Palaeozoic (Raup & Sepkowski 1986, Sepkowski 1990) peaking between an earlier Ordovician radiation and the end Ordovician extinction. It is also an excellent example of exceptional preservation of articulated echinoderms and arthropods beneath storm-deposited layers of muddy sediment.