Shallow-water methane-seep faunas in the Cenomanian Western Interior Seaway: No evidence for onshore-offshore adaptations to deep-sea vents

Sulfide-rich environments in shallow water were considered as sites where animals acquired preadaptations enabling them to colonize deep-sea hydrothermal vents and seeps or where they survived extinction events in their deep-sea habitats. Here we document late Cenomanian shallow-water seep communities from the Tropic Shale in the Western Interior Seaway, United States, as a possible refutation of these hypotheses. The late Cenomanian was a time of extremely warm deep-water temperatures, which supposedly facilitated adaptations to the deep sea, and of widespread oceanic anoxia (oceanic anoxic event 2) that supposedly extinguished deep-water vent and seep faunas. Contrary to the expectations, the taxa of the Tropic Shale seeps were not found at coeval or younger deep-water seep or vent deposits. Furthermore, a cluster analysis of faunal similarity among Cretaceous vent and seep faunas revealed no distinction between pre- and post-Cenomanian seep faunas, but instead strong similarities among Aptian to Late Cretaceous seep faunas. This suggests that a low temperature gradient from shallow to deep water did not facilitate invasions of deep-sea vents and seeps from shallow water and that preadaptation for living at deep-sea vents and seeps did not evolve at shallow-water methane seeps. The vast majority of adaptations to successfully colonize deep-sea vents and seeps were most likely acquired below the photic zone.