Population dynamics and taphonomy of the Late Triassic (Carnian) freshwater bivalves from Krasiejَw (Poland)

The Krasiejَw clay pit (SW Poland) has so far been known for yielding mass accumulations of Late Triassic (Carnian) vertebrate fossils. However, unionoid bivalves occur abundantly within the 1 metre-thick shell-rich lacustrine interval at Krasiejَw. They are most commonly preserved as shell imprints in the claystone or mudstone. A layer-by-layer quantitative sampling for shell external moulds was conducted directly in the bedding planes at two sites. The most numerous findings come from red and variegated layers in the upper part of this interval, which correspond to a well-aerated sediment. Single valves dominate at both sites and in all layers; valves in butterfly position are rare. The abundance of single valves throughout the sedimentation of the whole interval, both juveniles and adults, indicates that this interval represents the graveyard of a stable, reproducing population. The only exception is a bedding plane in the upper part of the lacustrine interval with numerous concretionary infills of closed valve interiors. They were presumably killed by a sudden load of sediment and, after the early diagenetic cementation was completed, they were winnowed from the mud. The mortality pattern is characterised by a high juvenile mortality (peak at about 13 mm), and a slight increase in subadult mortality (peak at about 30 mm; adult specimens reach 55 mm). Complete lack of early juvenile shells representing the stage with prominent ornamentation may have resulted from predation rather than from size-selective post-mortem preservation. The course of ontogeny shows virtually isometric growth. The range of variability of shell proportions within and between layers suggests the succession of monospecific populations. The Krasiejَw lacustrine interval represents the oldest insight into the dynamics and taphonomy of unionoid palaeocommunity.