The Bahama Islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level. The development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens. Lens position is controlled by sea level, which in stable carbonate platforms like the Bahamas is a function of glacioeustatic sea level still stands. Caves in the Bahamas that are currently subaerial must have developed during past higher sea levels. During the Late Quaternary, sea levels higher than present have been relatively short-lived, and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation. The Bahama Islands are low-lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level. Past high sea level events greatly reduced the exposed land area of the Bahama Islands, thus also limiting both the catchment for and size of freshwater lenses. Caves must be younger than the rock in which they are developed; most subaerial Bahamian caves are found in limestones that are less than 150000 years old. Development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism. The mixing of discharging freshwater with tide-pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary. Bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes. Their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands.

Odontocetes (toothed whales, dolphins and porpoises) hunt and navigate through dark and turbid aquatic environments using echolocation; a key adaptation that relies on the same principles as sonar. Among echolocating vertebrates, odontocetes are unique in producing high-frequency vocalizations at the phonic lips, a constriction in the nasal passages just beneath the blowhole, and then using air sinuses and the melon to modulate their transmission. All extant odontocetes seem to echolocate; however, exactly when and how this complex behaviour--and its underlying anatomy--evolved is largely unknown. Here we report an odontocete fossil, Oligocene in age (approximately 28 Myr ago), from South Carolina (Cotylocara macei, gen. et sp. nov.) that has several features suggestive of echolocation: a dense, thick and downturned rostrum; air sac fossae; cranial asymmetry; and exceptionally broad maxillae. Our phylogenetic analysis places Cotylocara in a basal clade of odontocetes, leading us to infer that a rudimentary form of echolocation evolved in the early Oligocene, shortly after odontocetes diverged from the ancestors of filter-feeding whales (mysticetes). This was followed by enlargement of the facial muscles that modulate echolocation calls, which in turn led to marked, convergent changes in skull shape in the ancestors of Cotylocara, and in the lineage leading to extant odontocetes.

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Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years. They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Their deep blue color, for which theyare named, is the resultof theirgreat depth, and they may lead to cave systems belowsea level. Blue holes are polygenetic in origin, having formed: by drowning of cHssolutional sinkholes and shaftsdeveloped in the vadose zone; by phreatic dissolution along an ascending halocline; by progradational collapse upward from deep dissolution voids produced in the phreatic zone; or by fracture of the bank margin. Blue holes are the cumulative result of carbonate deposition and cHssolution cycles which have been controlled by Quaternary glacioeustatic fluctuations of sea-level.Blue holes have been widely studied during the past 30 years, and they have providedinformation regarding karst processes, global climate change, marine ecology, and carbonate geochemistry. The literature contains a wealth of references regarding blue holes that are at times misleading, and often confusing. To standaIdize use of the term bluehole,and to familiarize the scientific community with their nature, we herein define them as follows: "Blue holes are subsurface voids that are developed in carbonate banks and islands; are open to the earth's surface; contain tidally-influenced waters of fresh, marine, or mixedchemistry; extend below sea level for a majority of their depth; and may provide access to submerged cavepassages." Blue holes are foundin two settings: ocean holes open directly into the present marine environment and usually contain marinewater with tidal flow; inland blue holes are isolated by present topography from surface marine conditions, and open directly onto the land surface or into an isolated pond or lake, and contain tidally-influenced water of a variety of chemistries from fresh to marine.

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The exposed carbonates of the Bahamas consist of late Quaternary limestones that were deposited during glacio-eustatic highstands of sea level. Each highstand event produced transgressive-phase, stillstand-phase, and regressive-phase units. Because of slow platfonn subsidence, Pleistocene carbonates deposited on highstands prior to the last interglacial (oxygen isotope substage 5e, circa 125,000 years ago) are represented solely by eolianites. The Owl's Hole Formation comprises these eolianites, which are generally fossiliferous pelsparites. The deposits of the last interglacial fonn the Grotto Beach Formation, and contain a complete sequence of subtidal, intertidal, and eolian carbonates. These deposits are predominantlyoolitic. Holocene deposits are represented by the Rice Bay Formation, which consists of intertidal and eolian pelsparites deposited during the transgressive-phase and stillstand-phase of the current sea-level highstand. The three formations are separatedfrom one another by wcl1-developed terra-rossa paleosols or other erosionsurfaces that formed predominantly during intervening sea-level lowstands.The karst landfonns of San Salvador consist of karren, depressions, caves, and blue holes. Karren are small-scale dissolutional etchings on exposed and soil-covered bedrock that grade downward into the epikarst, the system of tubes and holes that drain the bedrock surface. Depressions are constructional features, such as swales between eolian ridges, but they have been dissolutionll1ly maintained. Pit caves are verticalvoids in the vadose zone that link the epikarstto the water table. Flank margin caves are horizontalvoids that formed in the distal margin of a past fresh-water lens; whereas banana holes are horizontal voids that developed at the top of a past fresh-water lens, landward of the lens margin. Lake drains are conduits that connect some flooded depressions to the sea. Blue holes are flooded vertical shafts, of polygenetic origin, that may lead into caves systems at depth.The paleokarst of San Salvador is represented by flank margin caves and banana holes formed in a past fresh-water lens elevated by the last interglacial sea-level highstand, and by epikarst buried under paleosols formed during sea-level lowstands. Both carbonate deposition and its subsequent karstification is controlledby glacio-eustatic sea-level position. On San Salvador, the geographic isolation of the island, its small size, and the rapidity of past sea level changes have placed major constraints on the production of the paleokarst

Banana holes are circularto oval voids with diameters ranging from 2 meters to more than 10 meters, and with depths up to 5 meters, which are found throughout the Bahamas. They are named for a specialty crop sometimes grown in the thick moist soils that accumulate in them. They commonly have verticalor overhung walls, and exhibitphreatic dissolutional morphology. Occasionally, banana holes are found with complete or nearly complete roofs.Banana holes are the result of shallow-phreatic dissolution in the top of a fresh-water lens supported by the last interglacial sealevel highstand (ca. 125,000 years ago). Their current surface expression is the resuh of the partial. or total coBapse of their thin roofs. They did not originate by progradational collapse from depth, or by vadose processes. Once expressed on the surface by roof collapse, however, banana hole floors are modified by vadose waters with clevated CO 2 concentrations derived from the organic material that collects within them.

Bahamian caves formed mostly by dissolution resulting from mixing of fresh and saline waters at the margin of an island ground-water lens have been defined as "flank margin caves." The location of these caves provides an indication of sea-level position at the time the caves formed. Dissolution of the caves was rapid compared to the rate of inversion of aragonite to calcite, as indicated by the presence of up to 40% primary aragonite in the wall rock of some Bahamian caves. Bahamian cave chambers with volumes in excess of 14,000 m 3 , which formed during a sea-level highstand that lasted no more than 15,000 yr, provide evidence for chamber dissolution rates as high as 1 m 3 /yr. Slope and scarp retreat of ridges eventually unroofed flank margin caves, and may have ultimately produced features that appear remarkably similar to abandoned coastal bioerosion notches. Pit caves and banana holes are also common in the Bahamas. The former are vadose pathways dissolved from the surface; the latter are phreatic pockets dissolved at the top of the fresh-water lens.In contrast, Bermudian caves are mostly collapse caves. The original dissolution that led to these collapse caves occurred during sea-level lowstands and was produced by vadose cave streams perched on the contact between limestone and volcanic rocks. Flank margin caves are small and uncommon on Bermuda; pit caves and banana holes are also rare.Interior topographic depressions are also dissimilar in Bermuda versus the Bahamas. Bermuda has a positive water budget (precipitation exceeds evapotranspiration) so ground-water flow is either through or outward from interior marshes; this flow transports CO 2 -enriched water into the surrounding limestone. Dissolution of the carbonate bedrock is accelerated by this acidic water, and as a result topographic depressions have deepened and expanded over the course of multiple glacioeustatic sea-level changes, Lateral expansion of these inshore basins has erased evidence of earlier flank margin caves. Alternatively, in the southeast Bahamas conditions are more arid, resulting in a negative water budget, and ground water is discharged from adjacent land areas into interior saline lakes. Both waters are saturated with respect to CaCO 3 , even after mixing, and bed-rock dissolution does not occur. As a result, interior basins have maintained their original depositional morphology.Mylroie, J. E.