MODIFICATION OF TROPICAL COASTAL LANDSCAPES BY BRACHYURAN CRABS: NEOICHNOLOGY OF BIOTIC INNOVATION AND SPACE PARTITIONING

Semi-fossorial brachyuran crabs are a prominent faunal element of island and mainland coasts throughout the wider Caribbean. Lineages of these brachyurans arose and evolved with the Mesozoic Marine Revolution, and the ability of these species to modify tropical landscapes represents significant biotic innovation. On the carbonate islands of the Bahama Archipelago, four species of brachyurans collectively comprise a dominant agent of bioturbation across various coastal landforms, with each crab species having a distinctive burrow form and occupying a well-defined biotope. At several study sites on San Salvador Island, the ghost crab Ocypode quadrata is common in beach berm to primary dune settings. These crabs excavate large J-, U-, and Y-shaped burrows with circular openings. The burrows preserve well as trace fossils (Psilonichnus upsilon) and serve as valuable indicators of past sea-level position. Fiddler crab burrows, such as those of Uca speciosa, are smaller in diameter (<2 cm), consist of simple, near-vertical shafts, commonly with J-shaped endings, and can be abundant in protected supratidal areas. Their fossil forms would be assigned to P. tubiformis. Burrows of Cardisoma guanhumi, the blue crab, are large, distinctive, and have circular openings (up to ~20 cm diameter) with inclined shafts leading to expansive living chambers. Occupying the supratidal zone of low, mangrove-protected coasts and the margins of tidal blue holes, C. guanhumi is a powerful bioturbator, and its burrows have high preservation potential. If discovered in the fossil record, these structures would be assigned to Macanopsis plataniformis, and would be useful sea-level points. Finally, burrows of Gecarcinus lateralis, the blackback crab, are abundant on sandy, vegetated surfaces of both modern and ancient, lithified coastal dunes. These mostly horizontal burrows are laterally extensive, have characteristic oval-shaped openings, and can generate a distinctive ichnofabric in carbonate paleosols. Given that exoskeletons of brachyurans have low preservation potential in terrestrial settings, trace fossils provide the best means to document the presence and major effects of this important decapod group in tropical, carbonate paleoenvironments.