OF SHRIMP AND BEES: OVERPRINTING OF CRETACEOUS-PALEOGENE MARINE BURROWS BY MODERN TERRESTRIAL INSECTS

Within the rapidly advancing field of ichnology, most trace fossils serve as reliable in situ sedimentary structures, although their paleoenvironmental implications are occasionally revised. Detailed analysis of trace characteristics and ichnodiversity often provide the only means of reconstructing sedimentation and erosion rates, the compound nature of paleosols, and characteristics of event horizons. However, caution must be taken when overprinting by substantially more recent traces modifies or masks an existing ichnofabric. Burrowing is so intense within several sections of the Cretaceous-Paleogene greensands of New Jersey that the “Burrowed Layer” is commonly used as a marker bed. Sediments of Inversand quarry, a site famous for its vertebrate fossils, are dominated by a network of shafts and branching tunnels ranging in diameter from 5 to 30 mm. These traces are typically passively filled and are attributed to a mid-shelf Thalassinoides (Th) ichnoassemblage. A less common Th variety displays a similar gross morphology, except that it is actively filled (evidenced by backfill menisci) and ranges in diameter from 5 to 17 mm. Both are attributed to ghost shrimp, based on modern analogs. At a number of naturally and artificially exposed sections, a third type of trace is most easily recognized by its semi-lithified, oxidized lining that contrasts with the enclosing dark-grayish-green glauconite sand. These horizontal to sub-vertical traces are unbranching, have a relatively constant diameter of ~18 mm, and are occasionally hollow. Some terminate in a slightly enlarged brooding chamber. These burrows were found to penetrate up to 1 m into the outcrops and slopes and many are passively filled with unconsolidated glauconite. Most oxidized burrows exhibit no stratigraphic relationships and either cross-cut or follow the unlined Th shafts and tunnels. Field observations and occurrence of live larvae within the brooding chamber confirm that the structures are produced by modern digger bees. Where unconsolidated or semilithified deposits have been exposed for a number of years, their overprinting effect on the original ichnofabric, as well as a potential for misinterpreting the infilled insect traces as part of the marine ichnocoenosis, must be taken into consideration.