THE SULFATE HYPOTHESIS AND OTHER EMERGING PATTERNS
IN THE EVOLUTION OF METHANE-SEEP FAUNAS

Steffen KIEL

Swedish Museum of Natural History, Department of Palaeobiology, Box 50007, 10405 Stockholm, Sweden.

Corresponding author: steffen.kiel@nrm.se

The origin and evolution of the faunas inhabiting deep-sea hydrothermal vents and methane seeps
have been debated for decades. These faunas rely on a local source of sulfide and other reduced
chemicals for nutrition, which spawned the hypotheses that their evolutionary history is independent
from that of photosynthesis-based food chains. Trends in body size, relative abundance, and
epifaunal/infaunal ratios of seep-inhabiting mollusks track current estimates of seawater sulfate concentrations through the last 150 million years. Furthermore, the two main faunal turnovers during
this time interval coincide with major changes in seawater sulfate concentrations. Because sulfide
at seeps originates mostly from seawater sulfate, variations in sulfate concentrations should directly
affect the base of the food chain of this ecosystem and are thus the likely driver of the observed
macroecologic and evolutionary patterns. The results imply that the methane-seep fauna evolved
largely independent from developments and mass extinctions affecting the photosynthesis- based
biosphere.