A Composite Trace Fossil of Decapod and Hymenopteran Origin in the Rice Bay Formation (holocene), San Salvador, Bahamas

Composite trace fossils are the product of two or more species of tracemakers. In many instances, such traces are also caused by different tracemaker behaviors and by tracemakers with different environmental requirements. In this study, composite trace fossils from eolianites of the Holocene Rice Bay Formation, San Salvador, Bahamas are examined. These trace fossils are likely decapod dwelling burrows that were later modified by the addition of hymenopteran brooding chambers. The structures are definable in three ways: 1) large Y-shaped vertical burrows; 2) radiating clusters of small ellipsoidal chambers (cells); and 3) a combination of the two, in which the clusters are connected laterally to the exterior of the Y-shaped burrows by thin tunnels. The size and geometry of the Y-shaped burrows coincide with those of dwelling structures made by modern ghost crabs (Ocypode quadrata) and are assignable to the ichnogenus Psilonichnus. The size and geometry of the cell clusters are consistent with brooding chambers made by hymenopterans and are likely from halictid bees. The probable ichnogenera for these trace fossils are Celliforma and Cellicalichnus. The proposed scenario for the formation of this composite trace is: 1) a sandy area in a beachdune transition was occupied by burrowing ghost crabs; 2) burrows were left open or partially buried during or after crab occupation; 3) female hymenopterans entered these burrows and constructed tunnels and brooding chambers laterally adjacent to the main crab-burrow shafts; 4) eggs were laid in chambers, which were provisioned and sealed; 5) the broods developed into adults and exited the chambers; and 6) structures were buried and cemented. The paleoenvironmental significance of these composite structures is that insect brooding always takes place above the intertidal zone, thus the ghost crab burrows were isolated from direct marine influence long enough for secondary occupation by hymenopterans, yet also before cementation. They also indicate a more narrow facies range than that suggested by each individual trace fossil.