THE BIOGEOGRAPHY OF ORDOVICIAN EDRIOASTEROIDS (ECHINODERMATA)

Edrioasteroids have a fossil history spanning over 300 million years, from the Early Cambrian through the Late Permian, reaching their diversity peak of 20 genera during the early Late Ordovician. Ordovician edrioasteroid faunas have been reported from every continent except Antarctica, Australia, South America, India, and sub-Saharan Africa. We suggest this may be a collection bias as new genera recently described from Morocco indicate that additional fieldwork will likely enhance the faunal record from these seemingly deficient areas. Here we collect geographic distribution data for Ordovician edrioasteroids from published faunal reports and plot the occurrences on paleogeographic maps.

Floian Stage edrioasteroids include the edrioblastoid Lampteroblastus from Utah, the pyrgocystid Argodiscus and the isorophid Anedriophus, both found in Morocco. Isorophids Fanulodiscus, Archaepyrgus, Deltadiscus, and edrioasterid Paredriophus have been found in Dapingian age rocks from Utah and Nevada. Two new subclades, rhenopyrgids and cyathocystids, are first recorded in Darriwilian age rocks from Russia while Argodiscus rarus makes an appearance in the Prague basin. North American faunas are dominated by edrioasterids, pyrgocystids and lebetodiscine isorophids during the Sandbian while isorophine isorophids and pyrgocystids make up the Peri-Gondwanan faunas. Katian Stage edrioasteroids are well known from a variety of localities with the most diverse faunas found on the North American continent where isorophids, especially Carneyella, Isorophus, Isorophusella, Streptaster, Cystaster, Lebetodiscus, and Cryptogoleus, dominate. Isorophusella and Streptaster are known from North America and Peri-Gondwana. Hirnantian records are sparse with only the cyathocystid Cyathotheca known from Sweden.

Our findings show that of 38 Ordovician genera, 23 are known only from North America. Shallow equatorial seas surrounding the Laurentian continent in the Ordovician northern hemisphere provided conditions necessary for the formation of hardgrounds and shell pavements, both appropriate substrates for the obligate encrusting edrioasteroids. Cooler waters surrounding the southern Peri-Gondwanan terranes offered only siliciclastic environments with shells as the only hard substrate for attachment.