Fossil holoplanktonic mollusks (pteropods and heteropods) are so far little known from Trinidad and have not yet been employed in paleoenvironmental studies in this region. Specimens >125 µm were examined in samples from two sections of the Middle Miocene Brasso Formation of Central Trinidad. Samples from the type section at Brasso Village were younger (planktonic foraminiferal Zones N11–N12) than those from Nollis Tunnel, near Tabaquite (N8). Both sections yielded abundant holoplanktonic molluscs, but all were juveniles. At Brasso Village, yields were highest in samples collected near a bathyal oxygen minimum zone (OMZ) that was revealed using benthic foraminifera. The percentage of the fauna as Vaginella sp. did not vary significantly throughout this section, while Heliconoides sp. was concentrated in rocks deposited near the OMZ base and Creseis spina above the OMZ core. This implies depth stratification among the pteropods that limited the flux of Heliconoides sp. to shallower water samples. The fauna from Nollis Tunnel, where recovery was restricted to eight out of nine samples, was used to infer the position of the OMZ at this site. The assemblage was more abundant at Nollis Tunnel than at Brasso Village. Simultaneous confidence intervals show that the proportional abundance of Vaginella sp. did not vary significantly throughout the Nollis Tunnel section, while Creseis roesti and Heliconoides inflatus inflatus? showed depth stratification. This demonstrates that fossil holoplanktonic gastropods have potential as a tool for paleoenvironmental interpretations, especially in detecting fossil OMZs.
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Vol. 49 • No. 2-3