LIFE IN THE DEAD ZONE: A DIVERSE MIOCENE ICHNOFAUNA PRESERVED IN VOLCANIC ASH, ASHFALL FOSSIL BEDS STATE HISTORICAL PARK, NEBRASKA, USA

Ashfall Fossil Beds (AFB) is a volcanic ash deposit containing an extraordinary Konservat-Lagerstätte of Miocene vertebrate fossils; most of which are fully articulated and preserved in their original 3-dimensional death poses. The ash bed is stratigraphically within the Ash Hollow Formation of the Ogallala Group and interpreted as the fill of a waterhole based on its lenticular geometry, sedimentary structures, and the taphonomy and paleoecology of the Lagerstätte. Colloquially, the ~3-m-thick ash bed is divided into three zones—the Skeleton Zone (lowest ~25 cm, containing mostly fossil ungulates), the Dead Zone (~2 m of pure ash in which body fossils are absent), and the Recovery Zone (ash and sand mix above ~225 cm, containing fluvially-transported vertebrate fossils). While AFB has been the site of nearly continuous paleontological research since its discovery, less attention has been paid to the diverse assemblage of vertebrate and invertebrate ichnofossils of the deposit. The Skeleton Zone and underlying sandstone contain abundant Daimoniobarax isp. (fossil ant nests), Naktodemasis bowni, cf. Cylindricum, , small (~6 cm) and larger (~16 cm) diameter vertebrate burrow networks (cf. Alezichnos), and large vertebrate tracks and trample zones. The Skeleton Zone is cross-cut by traces originating from the Dead Zone; including branching CaCO₃-rhizoliths and large diameter (~70 cm x ~37 cm), subvertical (inclined ~70°) burrows associated with bone-fragments and likely produced by canids digging for ash-entombed carcasses. The Dead Zone also contains multiple CaCO₃-cemented algal mats of various morphologies, coprolites, canid trackways, and low numbers of Daimoniobarax and vertebrate burrow networks. Such invertebrate traces as Daimoniobarax, Naktodemasis, cf. Cylindricum, and cf. Macanopsis and small-diameter vertebrate burrow networks greatly increase in abundance just below and into the Recovery Zone. The ichnofossil assemblage suggests that the Dead Zone was very much alive with multiple episodes of organisms colonizing and bioturbating the ash bed, likely during depositional hiatuses and subaerial exposure. The Ashfall Fossil Beds ichnofauna are in situ evidence of hidden biodiversity and organismal behavior not typically preserved or decipherable from the body fossil record alone.