Ethology and palaeoenvironmental significance of Chondrites: Revising the fossil icon of the Jurassic-Cretaceous boundary

The iconic, trace fossil Chondrites is broadly plant-root shaped and is one of the most common fossils of the Jurassic-Cretaceous boundary in terms of abundance and geographical distribution. Chondrites is widely documented from several Jurassic-Cretaceous boundary sections including those of Puerto Escano in Spain ( Pruner et al ., 2010), Nordvik in north Siberia (Russia) ( Zakharov et al. , 2014), Kashpir and Gorodische in the Volga Basin (Russia) ( Kessels et al. , 2003) and southern Mendoza in the Neuquen Basin (Argentina) ( Kietzmann et al. , 2015). Chondrites is also reported from classical units of the Jurassic-Cretaceous transition such as Maiolica ( Wieczorek , 1988), Puez Formation ( Lukeneder , 2010), Kostel Formation ( Tchoumatchenco & Uchman , 2001). Despite its abundance, the first modern revision of Chondrites was published by Fu (1991) and, since then, virtually no studies have comprehensively focused on its ethology . The behaviour of Chondrites remains unclear, limiting its application as a palaeoenvironmental and palaeoceanographic tool. Here, we address this gap by review of existing literature and analysis of novel data, including (1) macroscopic and thin section observations , with specific emphasis on specimens preserved in Jurassic-Cretaceous units (e.g. Biancone, Puez Formation); (2) Environmental Scanning Electron Microscopy observations and X-Ray microanalyses (ESEM-EDX); (3) CT-scans and resin peels of modern analogues of Chondrites , i.e. burrows of thyasirid bivalves and vermiform animals ( Fu , 1991; Hertweck et al ., 2007; Seilacher , 1990; Dufour & Felbeck , 2003); (4) computer-controlled serial grinding ( Bednarz & McIlroy , 2015); (5) morphometric analysis of 88 specimens of Chondrites (6) theoretical morphology, following the principles established in previous works ( Niklas , 1994, 2004; McGhee , 1999; Niklas , 1994). Results show that the tracemakers of Chondrites built their burrows to obtain food: chemosymbiotic bivalves produced Chondrites to provision sulfur-oxidizing symbionts with the chemical reductants they required for metabolism; asymbiotic bivalves built Chondrites for cultivating bacteria and directly ingesting them; subsurface deposit feeding annelids produced Chondrites- like traces for searching for food in the sediment. The burrowing mechanism by which Chondrites was produced depend on the physiology of the tracemaker. Bivalves produced Chondrites by pushing their extensile foot into the sediment. In the case of sulfur-pumping symbiotic bivalves, inactive burrows were actively backfilled to ensure pumping efficiency in the new tunnel. In analogy with modern annelids it is likely that the burrowing cycle of worm-like tracemakers consisted of extending the proboscis and intruding into the sediment or by ingesting the sediment particles in front of them. In the case of vermiform animals, fill of Chondrites may have been produced by selective deposit feeding with authigenic alteration of ingested clay minerals or by the withdrawal of the proboscis sucking sediment from the surface ( Ferguson , 1965). Alternatively, some examples of Chondrites may represent a passively filled open burrow system. Available evidence shows that Chondrites was continuously modified or represented a part of the producer lifespan. Morphometric data show that branch width increases through time, suggesting that the Chondrites tracemaker(s) became larger and larger over the Phanerozoic. Chondrites, as well as their modern tracemakers, is associated to a vast range of marine settings, typically including dysoxic ones. This explains the abundance of Chondrites at the Jurassic-Cretaceous boundary. In fact, the Jurassic/Cretaceous boundary interval is characterized by the widespread occurrence of black shales, often linked to anoxic events ( e.g ., Valaginian Weissert event, Hauterivian Faraoni Oceanic Anoxic Event) ( Kessels et al. , 2003; Bodin et al. , 2009). However, in line with previous works ( Bromley & Ekdale , 1984; Ekdale & Mason , 1988), caution should be exercised in using Chondrites as a proxy for dysoxia because it is also characteristically associated to well-oxygenated and space-limited environments.