NEOICHNOLOGICAL EXPERIMENTS WITH FRESHWATER OSTRACODES

Neoichnological experiments with freshwater ostracodes allow documentation and description of different morphological types of traces and their associated behavior in a variety of water depths and substrates in controlled microcosms. In uncompacted, very fine-grained to medium-grained sand substrates the nektobenthic freshwater ostracode Heterocypris incongruens (Ramdohr, 1808) produced a variety of crawling trails, resting traces, and asymmetrical burrows that are comparable to numerous ichnotaxa. None of the ostracode traces exceeds a depth of 1 mm. Four distinct kinds of crawling trail traces occurred. The most common crawling trails consist of apparently random, sinuous trails with loops and are similar to Gordia and Haplotichnus. Traces with a medial ridge resemble Cruziana. Crawling trails with overlapping loops are comparable to Mermia. Trails with parallel sets of scratch marks are similar to Harpepus and Mesichnium, and zigzag trails resemble Treptichnus. Resting traces that occur as teardrop depressions are similar morphologically to Sagittichnus, and asymmetrical U-shaped ostracode burrows resemble Phycodes. No traces were observed in coarse or very coarse sand substrates, but ostracodes were observed pushing and toppling sand grains. Shallow bioturbation by ostracodes may produce the ghosting effect and cryptobioturbation of laminated sediments in freshwater settings. The preservation potential of ostracode traces in freshwater environments is poor. Ostracode traces are preserved best when they are formed either below wave base and buried rapidly or prior to desiccation in an ephemeral environment. Traces produced below wave base are generally narrower than those formed in sediment prior to desiccation, and they lack a medial ridge. After complete desiccation, only gross morphology of the traces can be observed. Desiccation cracks tend to follow crawling trails because they provide a zone of weakness along which the cracks forms. The study of modern ostracode traces allows their identification in the fossil record and assists with the interpretation of paleoenvironments and the makers of previously described ichnotaxa.