Comparative Ecology and Taphonomy of Shallow- to Deep-Water Bivalve- and Brachiopod Assemblages from the Modern Red Sea and Gulf of Aden

A continuous transect ranging from coral reefs on shallow-shelf down to 1900 m-deep bathyal environments in the Red Sea and Gulf of Aden provides unique opportunity to evaluate bathymetric trends in ecology and preservation of bivalves and brachiopods. Proportional abundances of bivalves and brachiopods were studied from 159 quantitative samples of these shallow marine, deeper shelf and bathyal settings.

Low abundance, moderate diversity (7 species from 7 families in 5 orders) and small-shell sizes appear to characterize modern Red Sea brachiopods, whereas bivalve assemblages are highly diverse (281 species from 46 families in 10 orders) and characterized by a broad range of shell sizes. Multivariate analysis reveals similar depth- and substrate related bivalve- and brachiopod assemblages. For both taxa, water depth seems to affect assemblage composition more strongly than substrata. A striking difference between bivalve- and brachiopod assemblages is, however, that non-reefal shallow-water sediments are virtually devoid of brachiopods. Combined analysis of brachiopods and bivalves confirm the depth- and substrate related pattern observed in separate analysis of bivalve assemblages. These results are stable at the genus- and species level. Ecological comparisons suggest that brachiopods tend to co-occur mainly with septibranch- and filibranch-, but not with eulamellibranch bivalves.

The rhynchonellid brachiopods are more affected by surface alteration than terebratulids that are more bored. In bathyal environments, terebratulids are cemented, and are less frequently fragmented and bored than in shelf environments. Rhynchonellids probably degrade more rapidly than terebratulids because their fragments are significantly more degraded, in contrast to terebratulids. Differences in preservation of impunctate rhynchonellids and punctate terebratulids in tropical carbonates are thus reversed comparing to temperate siliciclastic environments, implying the negative role of low thickness and absent shell ornamentation rather than of high organic content.