THE FISH COMMUNITIES OF THE HARDING SANDSTONE (ORDOVICIAN, COLORADO): IMPLICATIONS FOR THE FOSSIL RECORD OF FISHES THROUGH THE ORDOVICIAN

The Great Ordovician Biodiversification Event has become a signature feature in the Paleozoic diversification of many groups of organisms. Although the fossil record of fish is sparse, sources of data are now available from Laurentia (notably North America) and circum-Gondwana (Australia, Bolivia, Argentina and latterly Oman) which provide insight into the environmental preferences adopted by fishes during this time interval. Bone beds from these localities are notably dominated by heavily armored ostracoderms (principally the astraspids and arandaspids in Laurentia and Gondwana respectively), but micro-fossil sampling from a variety of sedimentary facies has revealed a significant diversity of scale morphologies, some of which appear to attributable to comparatively derived groups including thelodonts and chondrichthyans. The Harding Sandstone (Sandbian, Colorado) is used as an exemplar of diversity within a shallow marine Ordovician embayment, where a rich abundance of sedimentological, ichnological and invertebrate paleontological data can be drawn on to place fish distributions into discrete environmental settings within a sequence representing a suite of shallow water depositional settings from shoreface through to quieter lagoonal conditions. Fish from the Harding embayment can divided into generalists, shoreface, and lagoonal specialists and thus document one of the stratigraphical earliest environmental segregation within fishes in the same palaeogeographic basin. This distribution pattern has implications for sampling of other Ordovician localities, and, to a certain extent, restricts the recoverable diversity due to preservation biases of particular facies.