Lower Jurassic trace fossil Zoophycos from the Ksour Mountains of northwestern Algeria and their palaeoecological and palaeoenvironmental significance

The Zoophycos ichnofacies is reported for the first time from the Lower Jurassic (Toarcian) deposits of Algeria. The trace fossil-bearing strata in the Ksour Mountains, northwestern Algeria, are an alternation of fine-grained limestones and marls. The assemblage is characterised by its low diversity, consisting of eight ichnospecies (Chondrites intricatus, C. targionii, Chondrites isp., Palaeophycus isp., Thalassinoides isp., Zoophycos brianteus, Z. cauda-galli, and Zoophycos isp.), representing the Zoophycos ichnofacies ethologically dominated by deposit-feeding behaviours of different producers. The abundance of Zoophycos and Chondrites within a low diversity assemblage suggest stressful conditions most likely indicating oxygen depletion within the sediment. Moreover, the presence of reworked burrows by Chondrites could indicate ecological succession. This is probably due to environmental conditions getting more stressful, and causing the need for exploiting the nutrient stored in the abandoned burrows after the death of the animal trace maker. This process is primarily associated with an oxygen-depleted environment, compelling the animal to engage in it as a means of obtaining energy, rather than actively searching for food within the sediment. On the other hand, the occurrence of Thalassinoides may indicate an improvement of palaeoecological conditions and short periods with partial increase of oxygenation. The ichnological, palaeontological and palaeoecological data of the studied succession indicate a relatively deep-water environment located well below the storm wave base at a deeper offshore setting.