INSIGHTS FROM FOSSIL FISH TAPHONOMY INTO THE DEPOSITIONAL ENVIRONMENT OF THE MIDDLE DEVONIAN MILWAUKEE FORMATION

The fossiliferous, argillaceous dolostone of the Devonian (Givetian) Milwaukee Formation (Berthelet Member) contains abundant disarticulated skeletal elements and fragments from arthrodire and ptyctodont placoderm, sarcopterygian, and chondrichthyan fish. The complete disarticulation of these fossils has resulted in limited previous study. However, assemblages dominated by disarticulated skeletal elements still provide important insights into taphonomy and depositional environment. In this study we use a novel approach to reconstruct the paleoenvironment of the Milwaukee Formation through analyzing taphonomic characteristics of a completely disarticulated fish assemblage. Robust skeletal elements of placoderm grinding teeth, fin spines, fin plates, and armored plating (typically from shoulder, cranial, and pectoral regions) dominate the assemblage. These display variation in taphonomic attributes including corrasion (chemical corrosion and/or physical abrasion), presence of epibiont encrustation, color, luster, and the degree of pyritic pore-filling cements. Rare occurrences of skeletal elements belonging to sarcopterygian fish are notably fragile. Associated invertebrates include abundant cephalopods and pyritized brachiopods.

These observations suggest deposition on a marine shelf above storm wave base. During low-energy background periods, skeletal elements exposed on the seafloor would undergo initial disarticulation, encrustation, and color- and luster-altering corrasion. Although buried skeletal elements were ‘protected’ from encrustation and corrasion, low oxygen conditions in the substrate led to syndepositional and/or early diagenetic pyritic pore-filling cements. Periodic high-energy storm events were responsible for complete disarticulation as well as re-working and exposure of buried skeletal elements (and burial of others), resulting in a time-averaged assemblage with a wide range of taphonomic attributes. The taphonomic features of these skeletal elements suggest that robust plates were best suited to survive fossilization in this setting and, therefore, the fossil assemblage composition is an artifact of taphonomic processes and energy of the depositional environment.