Fossil, ichnofossil, or concretion: The enigma of “Dinocochlea ingens”

Abstract

The misidentification of biomimetic or biomorphic structures (especially helical forms) as biogenic can lead to serious errors in the interpretation of depositional environments and stratigraphic relationships, and in the detection of primitive life. “Dinocochlea ingens” was originally described as a giant gastropod and recently has been interpreted as a diagenetically-modified ichnofossil, based on dextral- and sinistral-twist bodies reported from the Lower Cretaceous Wadhurst Clay Formation, near Hastings, Sussex, United Kingdom. Morphologically similar decimeter- to meter-scale helical concretions occur with spheroidal and macrobotryoidal concretions in the shale of the Ikalukrok unit in the upper part of the Mississippian Kuna Formation that hosts the Red Dog Mine in northwestern Alaska. The mineral assemblages of both the helical and spheroidal concretions at Ikalukrok Creek are identical. The calcite of these concretions consists of cm-scale radiating aggregates of elongate poikilotopic crystals; this texture is megaspherulitic. The calcite has a relatively narrow range of δ13CVDPB values between 1.9‰ to 3.0‰ and δ18OVPDB values from −5.9‰ to −8.0‰. Growth of the concretions is considered to be pre-compaction because there is no evidence of internal layering, or inclusions of the host shale, despite their preservation of numerous microfossils. There is no indication of a causative small-scale burrow or of large-scale burrows. Analogy with laboratory syntheses of biomimetic carbonates suggests that the megaspherulitic texture of the helical and spheroidal concretions probably resulted from crystallization under strong driving forces such as high supersaturation.