Neoichnological experiments with the freshwater ostracode Heterocypris incongruens: Implications for reconstructing aquatic settings

Neoichnological experiments with freshwater ostracodes document different morphological types of traces, their associated behavior in various water depths and media (=substrates) in controlled microcosms, and the potential for their identification in the fossil record. In uncompacted, very fine- to medium-grained sand, the nektobenthic freshwater ostracode Heterocypris incongruens produced seven trace types: three crawling trails, a swimming trail, a resting trace, a burrow, and a self-righting trace. The most common crawling trails (Type 1) are randomly sinuous with infrequent looping. Two other crawling trails are observed infrequently and were self-looping (Type 2) and zigzagging (Type 7). Swimming trails (Type 3) are straight to sinuous and composed of a parallel set of appendage scratch marks. Crawling and swimming trails likely indicate exploratory or foraging behaviors associated with locomotion. Oval-shaped depressions (Type 4) commonly occur and represent a resting behavior. Burrowing produced asymmetrical U-shaped burrows (Type 5) and represents hiding from a stimulus or a foraging behavior. A fan-shaped trace of appendage scratches (Type 6) was produced when H. incongruens righted itself after landing upside down in the sediment. No traces were observed in coarse or very coarse sand, but ostracodes were observed pushing and toppling sand grains. Preservation potential of ostracode traces in freshwater environments is likely low. Traces were observed to degrade and fill in with sediment within 24 48 hours after formation or were destroyed immediately in water currents that disrupted the sediment surface. Ostracode traces are likely preserved best when formed below storm wave base and buried rapidly, or prior to desiccation in an ephemeral environment. After desiccation, only gross morphology of the traces is observed and desiccation cracks tend to follow crawling trails.