HABITAT HETEROGENEITY AND SPECIATION AMONG DEEP-SEA TRIASSIC BENTHIC BIVALVES FROM THE EXAEROBIC ZONE

High speciation rates among pelagic and deep-sea biota have been difficult to reconcile with their inferred homogeneous habitats and large populations. Population and diversity dynamics of Triassic halobiid and monotid bivalves, however, offer a model example of the linkage between high speciation rates in deep-sea benthos and habitat heterogeneity. Morphologic, taphonomic and population demographic evidence coupled with their facies occurrence suggests these bivalves were likely benthic opportunists adapted to dysaerobic deep-sea settings that other shelly epibenthos found uninhabitable. Species richness, raw and per-taxon rates of origination and extinction and taxic turnover exceed that of other bivalves with species durations averaging less than 1 m.y., making them comparable to pelagic ammonoids and/or conodonts. Speciation rates of halobiid and monotid bivalves also appear to be decoupled from evolutionary rates at higher (e.g., generic and familial) taxonomic levels. Elevated speciation rates are likely coordinated with high-frequency changes in the physical and chemical benthic conditions along the edge of oxygen minimum zones and possibly enhanced by high within-species genetic diversity. Although gradual phyletic series are known, punctuated speciation events are common in these bivalves and are correlated with abrupt colonization events of non-continuous patchy populations at shifting benthic oxygen thresholds. Metapopulations of sister and sibling species are replaced without competition in subsequent threshold conditions. Although other factors including H₂S gradients, pH, substrate characteristics, and variations in the organic matter content may also have contributed to habitat heterogeneity, high-frequency (ka) variations in benthic oxygen levels were likely the primary factor in controlling speciation of halobiid and monotid metapopulations.