PRIMITIVE FISH FROM THE MIDDLE ORDOVICIAN WINNESHIEK LAGERSTäTTE OF NORTHEAST IOWA, USA

Among the variety of previously unknown fossils from the Middle Ordovician (Whiterockian, Darriwilian) Winneshiek Lagerstätte, primitive fish materials are of particular note. Because of the limited number of Ordovician fish fossils reported worldwide, the new Iowa material will expand our understanding of jawless fish at a poorly-known but critical time in early vertebrate evolution. The Winneshiek fish fossils are preserved as thin phosphatic skeletons in shale. X-ray and thin-section analyses confirm a dominantly phosphatic composition, apparently fluorapatite. The most complete material includes head shields (25 mm long) comprised of oval halves divided by a median ridge. Their shape grossly resembles those of some heterostracans, but most of the surface is densely sculpted with rounded tubercles. Many tubercles are stellate with radiating ridges or nodes that superficially resemble those in astraspids (but they are fused into a shield, not tessellated). The shields are fringed by smooth dimpled raised margins. Eye and branchial openings are not evident. Additional articulated materials, up to 8 cm long, show elongate ridges along a thin continuous sheet-like skeletal layer. Some tubercles are present. The relationship, if any, between these two forms is presently unclear, but both display similar histologies. Preliminary histology, based on thin-section and SEM analyses, reveals simple skeletal tissues dominated by 1) an acellular sheet-like basal layer with multiple thin lamellae (<20 um), and 2) tubercles composed of tubular dentine, some capped by an enameloid layer. The ridged forms also appear to contain scattered ovoid tubercles with a dentine-like microstructure that are completely enclosed within the lamellar layer. A vaulted or cancellous layer is absent or poorly developed in the Winnishiek samples, unlike astraspids, arandaspids, or heterostracans, but some tubercles may show a basal cavity. The unique morphologies of the Winneshiek fossils suggest that they represent a previously unrecognized group of Ordovician fish, possibly basal to astraspids and heterostracans. The prominent interconnecting lamellar layer may be homologous with tissues of the primitive Anatolepis.