Ichnology of the Lower Ordovician Landeyran Formation, Montagne Noire, France and criteria for distinguishing simple trace fossils from body fossils

Fossil sites with exceptional preservation (Lagerstätten) are instrumental in accurately reconstructing ancient ecosystems on Earth. Although Lagerstätten are typically approached from a body fossil perspective, trace fossils can also be associated with these assemblages and provide unique evidence of in situ past animal behaviours. In this study, we describe the trace fossils found with the Cabrières Biota, a recently discovered Lagerstätte from southern France. Trace fossils are reported from the mudstone-dominated Lower Ordovician Landeyran Formation and consist of simple horizontal trails (Helminthoidichnites tenuis, Helminthopsis granulata), passively filled horizontal burrows (Palaeophycus tubularis), actively filled burrows (Alcyonidiopsis longobardiae, Planolites montanus, ?Torrowangea isp.), rare vertical burrows (Skolithos linearis), isolated faecal pellets (Coprulus oblongus), and indiscriminate meiofaunal burrows. Ichnological metrics (e.g. absent to low bioturbation intensity, low ichnodiversity) suggest fluctuations between anoxia and dysoxia in sediments of the shelf (i.e. below the mean storm wave base), which favoured the preservation of soft-bodied organisms. These organisms, such as algae or sponges that can have relatively simple morphologies, are differentiated from trace fossils based on five criteria that we review here, and which can be used by other researchers facing difficulties in identifying fossilized objects regardless of the age or location of the investigated Lagerstätte.