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Prevalent concepts of foraminiferal paleobathymetric interpretations are based on the modern distribution of living benthic faunas and assume that species depth habitat have not changed with time. Recent studies by Streeter (1973), Schnitker (1974), and Lohmann (1978), suggest that the distribution of benthic foraminifera is not as static in space and time as had been previously believed. Present-day deep sea benthic foraminiferal assemblages appear to be controlled more by the distribution of bottom water masses than by bathymetry.

Melonis pompilioides (Fichtel and Moll) is a benthic foraminifer presently inhabiting lower bathyal to abyssal environments. The species is described by Bandy and Chierici (1966) as a typical example of an isobathyal species. The use of M. pompilioides as a bathymetric indicator in modern and ancient environments rests upon two assumptions: First, that it is an isobathyal species and its upper depth limit is approximately the same in different oceans. Second, that the depth habitat of the species, at least its upper depth limit, has not changed with time.

Data from the southern California Borderland support a different model, one which explains the Pleistocene distribution of M. pompilioides as the result of faunal migration. Late Pleistocene and Holocene occurrences of the species in basins with sill depths shallower than 2,300 m, the present-day upper depth limit of the species, is evidence of water-mass changes in the last 16,000 years. The apparent bathymetric change of the species is a faunal response to changing environmental conditions rather than evidence of tectonic uplift of the seafloor.

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