Four distinct faunal zones are present in otolith assemblages of bottom muds from the eastern Gulf of Mexico, between depths of 300 and 2,000 ft. At least 2 more zones can be projected on the basis of the literature and known water-circulation patterns in the Gulf, and the outer limit of workable faunas may be far out on the abyssal plain. Within the documented depth range, surprising precision is obtained by measuring

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the ratio of known forms which inhabit different sonic scattering layers in mid-water. The lowermost layer represented in each fauna is in part a function of water depth, and also commonly indicates the boundary between water masses of sedimentologic importance.

The 4 zones detailed in this study are (1) mid-shelf; (2) cold bottom and epipelagic, or shelf-edge; (3) lanternfish, or oxygen minimum; and (4) bathypelagic, or Antarctic water. All these zones may be subdivided by later investigations.

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