An excellent account of the biogenic structures from the Bay of Mont-Saint-Michel was presented by Bajard (1966). As noted by Lanier et al. (1993) and Tessier et al. (1995), some of the surface or near-surface traces illustrated by Bajard are comparable to those from Buildex, including meandering trails of the isopod Eurydice pulchra (Bajard, 1966, fig. 23), unidentified sinusoidal trails that were probably produced by nematod or ceratopoginid larvae (Bajard, 1966, fig. 30), and zigzagging, near-surface burrow systems of tabanid larvae (Bajard, 1966, fig. 36). However, these traces occur in close association with biogenic structures produced by a marine fauna, such as U-shaped burrows of the polychaete Arenicola marina and the amphipod Corophium volutator, crab trackways, dwelling structures of the bivalves Cardium edule and Tellina balthica, different types of traces produced by the polychaete Nereis diversicolor, burrows of the polychaete Polydora ciliata, and dwelling burrows of the cnidarian Sagartia troglodytes (Bajard, 1966). A diverse and abundant marine fauna has also been recorded in the bay (Larsonneur, 1989, 1994). The widespread distribution of this marine fauna and its associated biogenic structures in the Bay of Mont-Saint-Michel is consistent with the salinity of the estuary that, according to Larsonneur (1994), oscillates between 33 and 35 parts per million, being even higher in summer. The surface and near-surface traces are most likely related with tidal-flat emergence during low tide.
Despite the similarities between the Tonganoxie paleovalley and modern analogues, neoichnological information should not be extrapolated uncritically to the analysis of the trace-fossil record. In the particular case of biogenic structures, the fossilization barrier is a taphonomic filter considerably more complex than that separating living animals from body fossils (Bromley, 1996). Although surface traces are present in the Bay of Mont-Saint-Michel, their preservation potential is extremely low. Burrowing activities of the associated marine infauna will probably lead to obliteration of the surface traces. Therefore, the fossil expression of such cases is typically a burrow-dominated assemblage. Accordingly, although the Tonganoxie paleovalley may be comparable to the Bay of Mont-Saint-Michel in terms of physical sedimentary structures and bedding types (see Tessier et al., 1995), the presence of burrows of a marine benthic infauna in the latter is a substantial difference. Interestingly, differences between both cases are easily explained as reflecting brackish to fully marine conditions in the Bay of Mont-Saint-Michel and the existence of an inner freshwater zone in the Tonganoxie paleovalley.
A series of papers on the biogenic and physical structures of estuaries in the Georgia coast also provide valuable information (e.g., Dorjes and Howard, 1975; Howard and Frey, 1975; Howard et al., 1975). Dorjes and Howard (1975) recognized five animal communities in the Ogeeche Estuary, which were essentially controlled by the salinity gradient. The inner freshwater zone of the estuary hosts few infaunal organisms; therefore, the degree of biogenic disturbance was minimal. The dominant biogenic structure was represented by thin, long burrows of the polychaete Scolecolepides viridis, a gregarius suspension feeder ( Howard and Frey, 1975). Amphipods (Lepidactylus dytiscus) and insect larvae were also observed ( Dorjes and Howard, 1975). While insect larvae are important tracemakers in the Buildex assemblage, amphipod and polychaete traces have not been recorded. However, their absence in freshwater Pennsylvanian tidal flats is consistent with the evolutionary and environmental history of these groups, which only later were adapted to freshwater ecosystems (Chamberlain, 1975). This demonstrates again the importance of considering variables other than physical processes, including evolutionary adaptations of the different groups of invertebrates to the freshwater realm through time, in the search for modern analogues.
Kansas Geological Survey
Web version March 19, 1998
http://www.kgs.ku.edu/Current/1998/buatois/buatois9.html
email:lbrosius@kgs.ku.edu