Complex decapod burrows and ecological relationships in modern and Pleistocene intertidal carbonate environments, San Salvador Island, Bahamas

Burrowing by thalassinidean shrimp in modern tropical, shallow-subtidal to intertidal carbonate environments is widespread and a powerful agent of bioturbation, particularly within sandy substrates. Deep and intense burrowing by callianassids commonly occurs along the intertidal margins of hypersaline lagoons throughout the Bahamas, such as Pigeon Creek on San Salvador Island, where the sediments of extensive sand flats are thoroughly bioturbated by Glypturus acanthochirus. In addition to dominating the deep-tier infauna, G. acanthochirus is a true ecosystem engineer, and its prodigious burrowing activity results in a highly mounded topography. On sand flats, individual sediment cones commonly coalesce with time to form large composite mounds with surfaces stabilized by the development of microbial mats. This unique, biogenetically produced topography sets the stage for colonization by shallow-tier burrowers, particularly the upogebiid shrimp Upogebia vasquezi and several species of fiddler crabs. Burrow systems of U. vasquezi are distinctive and complex. They typically consist of a pair of U-shaped burrows in close proximity or criss-crossing, with knobs or short tunnels at their bases. The entire U-burrow pair is lined by a thick, externally pelleted wall. Inside burrow diameters range from 0.2 to 1 cm, and burrow depths are 10^15 cm, in contrast to the much deeper callianassid burrows. These complex upogebiid burrows also occur as trace fossils in late Pleistocene lagoon-margin facies of the Grotto Beach Formation on San Salvador. Initially reported as fragmentary burrow fills, whole burrow systems virtually identical to the modern upogebiid burrows recently have been found at several localities. These fossil burrows are most abundant in beds lying immediately below a terra rossa paleosol that marks the late Pleistocene^Holocene stratigraphic boundary, and the beds likely were deposited with sea-level stillstand and regression associated with the onset of Wisconsinan glaciation. With their high potential for preservation, these complex upogebiid trace fossils may prove useful as both paleoenvironmental and sea-level position indicators in the study of Quaternary carbonate sequences throughout the Bahamas and other geologically