Deoxygenation has profound effects on marine biota and delivery of ecological functions in benthic systems. Globally, coastal and oceanic hypoxia is rapidly increasing due to anthropogenic activities including climate change and eutrophication. Little is known about the response of marine ecosystems to deoxygenation over long timescales and the consequences this will have for functioning. This study presents results from biological traits analysis (BTA) of 21 time averaged benthic palaeocommunities from the Wessex Basin, Dorset, UK representing ˜8 million years of fluctuating regional hypoxia during the Kimmeridgian (˜ 148–155 Ma). BTA assesses ecosystem functioning using biological traits expressed by species and has not previously been applied to palaeocommunity data.

The preserved remains of the palaeocommunities contained gastropods, brachiopods, scaphopods, bryozoans, serpulids, hydroids and crustaceans, but were dominated by bivalves. Significant changes in species composition are shown within periods of less intense hypoxia, but during these periods trait composition did not significantly differ implying conservation of ecological function. However, significant changes in functioning occurred between periods of extremely different palaeoredox state. Proportionally more surface living or shallow burrowing species with traits suggestive of opportunists occurred during periods of low oxygen availability. Morphological differences of hypoxic communities included higher relative abundance of organisms with thinner skeletons (< 0.5 mm) composed of less soluble forms of calcite that may be linked to acidity. These changes are similar to those for modern benthos exposed to hypoxia.

Investigation of functional changes that occurred during ancient hypoxic events can be used to infer the magnitude, thresholds, and rates of long‐term functional change in modern communities. Results from this study suggest that during de‐oxygenation delivery of normal benthic functioning will initially be maintained, but will collapse once thresholds are reached. This is consistent with the patterns emerging for contemporary systems where functional collapse is associated with hysteresis and threshold effects.