Mass occurrence of penetrative trace fossils in triassic lake deposits (Madygen mossil site, Kyrgyzstan, Central Asia)

Bioturbation and the breakdown of organic detritus by burrowing macro-invertebrates are key factors for the energy flow in recent freshwater ecosystems due to the acceleration of nutrient cycling. According to the current state of knowledge, food webs similar to those ones of modern lakes were not operating until the Late Mesozoic, when a well-established freshwater infauna evolved. Here we describe laterally extended networks of irregularly branched burrows that constitute the most common ichnofossils in lacustrine deposits of the Middle to Late Triassic Madygen Formation, SW Kyrgyzstan. The shallow penetrative trace fossils assigned to Thalassinoides paradoxicus (Woodward, 1830) give evidence that underground exploitation of lake-bottom sediments by benthic invertebrates set up in the Early Mesozoic at the latest. Architecture and size of the fossil burrows indicate deposit-feeding, worm-like trace makers of similar morphology and behaviour to extant oligochaetes or aquatic insect larvae. Maximum intensity of bioturbation is recorded in mudstones of the transitional sublittoral to profundal lake zone which usually represents the thermocline/chemocline level in modern stratified lakes. Taking into account the low-oxygen tolerance of many recent oligochaetes and insect larvae we propose, that ecospace utilisation of deep-water lacustrine settings was originally impelled by two things: exploitation of additional food resources and avoidance of predation by carnivorous animals from well-aerated lake zones. Spatial restriction of the described fossil traces could offer a basic approach to subdivide the Mermia ichnofacies.