To refer to this article use this url: http://www.scriptageologica.nl/07/nr135/a04
Discussion
The living brachiopod fauna of the Caribbean Basin is relatively diverse. Cooper (1977) reported over 50 species of brachiopod assigned to some 20 genera from the Caribbean Sea and Florida Straits. Miocene horizons have yielded 25 species assigned to ten genera (Harper, 2002a). By contrast, only four taxa are, to date, known from the Pleistocene (Trechmann, 1930, 1937; Harper, 1993, Harper et al., 1995; Harper & Donovan, 2002). This association is a continuation of a similarly depauperate Pliocene brachiopod fauna that, where developed, is also of low diversity, characterized by species of Argyrotheca, Terebratulina, Tichosina and Lacazella (Logan, 1987). Thus, assemblages with Argyrotheca, Terebratulina, Tichosina and Lacazella apparently continued from the underlying Pliocene in deep-water facies commonly associated with fore-reef deposits (Donovan & Harper, 1998). By comparison the Pleistocene fauna of the Mediterranean Sea was more diverse with some eight species belonging to seven genera, and including Argyrotheca, Gryphus, Lacazella, Megathiris, Megerlia, Platidia and Terebratulina (Logan et al., 2004). Both faunas have Argyrotheca, Lacazella and Terebratulina in common, whereas Gryphus is very similar to Tichosina. It is possible that either a sufficient range of sedimentary environments has not yet been collected to appreciate the full range of the Pleistocene fauna or the Plio-Pleistocene interval represents a survivor fauna following local extinction events in the Caribbean Basin at the Miocene-Pliocene boundary.
Establishment of the depth and environments of depositions of the two successions is based on both faunal and sedimentological criteria. In particular, brachiopod data have already proved useful in studies of the Pleistocene rocks of Jamaica (Donovan & Harper, 1998). Modern brachiopods display a considerable depth range from the intertidal zone to abyssal depths in excess of 6 km, but with about 40 % of all living brachiopods occurring at depths between 50-400 m; the majority occupy depth bands between 100-400 m. Tichosina species generally occur at depths greater than 100 m in the modern Caribbean (Cooper, 1977) while Terebratulina is still common at depths in excess of 300 m (Logan, 1987). The faunal arguments for the depositional depths of the Manchioneal Formation on Jamaica were discussed in detail by Harper et al. (1995) and Donovan & Harper (1998). Low current velocities, low light intensity and substrate preferences are the main controlling factors on initial larval attachment, and thus the subsequent distributional patterns of the brachiopod animal. At low latitudes micromorphic brachiopod species would be more suited to cryptic habitats associated with reefs and other cavernous carbonate buildups (Jackson et al., 1971); the strategies of larger species may be associated with deeper-water environments where larger patches of substrate are available in darker, less turbulent waters.
Asgaard & Stentoft (1984) established precise figures for the depth range of a number of brachiopod species in a more focused study of the distribution of living and dead modern brachiopods on the shelf and slope off Barbados. Species of Argyrotheca and Terebratulina preferred depths in the ranges of approximately 90-100 m and 90-140 m, respectively, whereas Tichosina occupied deeper water from approximately 200-250 m. Observations on both the reproductive behaviour and the associated predators of all three taxa may have also impacted on their ecological and geographical distributions. Asgaard & Stentoft (1984) noted the development of brood pouches in Argyrotheca, and possibly Terebratulina, where fertilized ova and larvae are retained within the mantle cavity of the female. When expelled, the resulting distribution of the larvae is local and patchy. This may partly explain the morphological differences apparent between the Barbadian and Jamaican Argyrotheca and Terebratulina species. Moreover, during early stages of attachment, the larvae and larval shells are commonly removed from open and accessible substrates by grazing herds of the black sea urchin, Diadema antillarum (Philippi). Clearly, a restricted, cryptic life style would be a considerable advantage. In this context it is significant that predatory borings are relatively rare in our collections of Pleistocene brachiopods (Donovan & Harper, 2007). Either by occupying cryptic habitats (Argyrotheca and Terebratulina) or the deep sea (Tichosina), predators were probably avoided.
In summary, the basal and probably deeper-water facies of both the Coral Rock of Barbados and the Manchioneal Formation of Jamaica are characterized by high dominance brachiopod faunas with abundant Tichosina. Stratigraphically younger units within each formation have fewer Tichosina, but more abundant Argyrotheca and Terebratulina. The move from large, deeper-water generalists to more micromorphic cryptic faunas may have been a reflection of adaptations in the brachiopod biofacies to increasing water turbulence, light intensities and predation pressures within a regressive sequence. Nevertheless, the shallower-water faunas were still in the range of 80-150 m water depth, implying relative uplift in this order for these parts of Barbados and Jamaica since the early Pleistocene.