Ophiomorpha group trace fossils occur abundantly in a range of Eocene-aged deep-marine environments of deposition in the Basque basin, northern Spain. The morphology and dimensions of these trace fossils, observed in off-axis submarine lobe deposits, are discussed. The reported specimens display a highly organized and systematic burrowing behavior preserved on turbidite bed bases that display interconnecting Y-shaped (hexagonal-polygonal) morphologies. This observation, together with the cross-cutting relationship with tool marks, suggests construction of postdepositional agrichnial burrow networks. The networks probably harvested microbes that broke down cellulose-based organic matter providing an exploitable nutrient source for crustacean trace makers of Ophiomorpha. Therefore, the Ophiomorpha group–related traces discussed herein are postulated to represent an ethological response to changes in deep marine environmental conditions driven by global climate change during the early Paleogene, including the early Eocene hyperthermal events.

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