SIZE-DIVERSITY TRENDS AND FEEDING-STRATEGY DISTRIBUTIONS IN BRACKISH-WATER ICHNOFOSSILS (Invited Presentation)

Size Diversity Index (SDI) is the largest burrow observed multiplied by the diversity of ichnogenera observed. The function varies markedly with salinity and in many case studies appears to provide an excellent, if un-calibrated, proxy for salinity. Here we present and interpret SDI trends from 2 modern settings (Tillamook Bay, Oregon, USA and Ogeechee River Estuary, Georgia, USA) and two Cretaceous tidally influenced settings (Bluesky and McMurray formations, northern Alberta, Canada). In the modern settings, stations were selected along fresh to marine salinity gradients. Surveyed sites were accessed in the intertidal zone and by SCUBA. In the Cretaceous units, SDI assessments were binned into 75 cm core lengths. For each 75 cm of core, ichnological diversity to the ichnogenus was determined. The burrow diameter was measured from cross sections of the burrows within the same bins.

Although it may be contended that trace diameter is only meaningful within a single ichnogenera. We disposed of that approach for several reasons: 1) that method assumes all examples of a single ichnogenera are made by the same type of animal, which is untrue; 2) single ichnogenera are commonly too sporadic in their distribution to produce an analytical dataset; and 3) some types of animals show little size response to salinity.

Observed in both modern and ancient datasets, facies-crossing trace fossils, such as Planolites, Teichichnus, Cylindrichnus, Skolithos, Palaeophycus and Thalassinoides, are persistent. In the modern examples, the aforementioned trace fossils are dominantly used as a basis for shallow-tier deposit feeding and interface deposit feeding.

The syntheses of these case studies show the following patterns: (1) SDI decreases geometrically from marine sedimentary environments (SDI 100 to 200) into brackish-water (SDI 3 to 15) and SDI in fresh water is typically zero; (2) SDI considers population response to salinity fluctuations, not those of individual forms; (3) lowered SDI are ascribed to “enforced diminution”, for which salinity stress leads to shortened life cycles and excludes animals that are poor osmoregulators; and (4) previous studies describing the trace fossil assemblages of the brackish realm as "mixed Skolithos-Cruziana Ichnofacies" inadequately describe the feeding behaviours that are deployed.