How has the modern vent and seep fauna evolved? This question has been the matter of controversial debate for many years. Various theories have been proposed, ranging from a Paleozoic origin (> 250 Million years ago) and subsequent in situ radiation, a Cenozoic radiation triggered by the evolution of whales 40 Million years ago, to the hypothesis that these deep-sea communities became extinct during the `oceanic anoxic events` in the Mesozoic and Cenozoic and were subsequently repopulated by shallow-water taxa.

Using a detailed analysis of the geologic ranges (lifespans in geologic time) of mollusks at methane seeps, I could show that living methane seep inhabitants are indeed relatively young at an average age of 34-54 Million years. However, they are significantly older than mollusks in marine shallow-water environments, which are only 23-34 Million years old on average.

Because mollusks at methane seeps are almost of the same average age as deep-sea mollusks in general, the seep inhabitants probably follow the same ‘onshore-offshore trend’ as other deep-sea animals. The ‘onshore-offshore trend’ describes the observation that new clades of animals first appear in shallow water and then, over geologic time-spans, colonize the deep-sea. This was surprising, because the inhabitants of methane seeps have their own, independent food source. Interestingly, the results also indicate that deep-sea anoxic/dysoxic events did not affect seep faunas, casting doubt on the suggested anoxic nature and/or global extent of these events.

My work has also challenged the hypothesis that the radiation of whales had a significant impact on the evolution of methane seep mollusks. Large quantities of sulfides are released during the decay of these giant marine mammals on the deep-sea floor. These serve as food for a very similar suite of animals as found at methane seeps. It has been suggested that whale carcasses are used as ‘stepping stones’ by methane-seep inhabitants, which help them to cover the often large distances between isolated seep sites. According to this hypothesis, the appearance of whales ca. 40 Million years ago made this ‘island-hopping’ possible and opened new habitats for the methane-seep inhabitants, and thereby resulting in their diversification.

However, whales evolved in shallow water in the Eocene, but acquired the ability to cross open oceans only in the Oligocene, long after the majority of the modern methane-seep fauna appeared. Hence it is unlikely that the first whales had an impact on the evolution of the seep fauna, because only ocean-crossing whales could have sunken to the deep-sea floor.

 

Kiel, S. and Little, C.T.S. 2006. Cold seep mollusks are older than the general marine mollusk fauna. Science 313: 1429-1431.

Kiel, S. and Goedert, J.L. 2006. Deep-sea food bonanzas: Early Cenozoic whale-fall communities resemble wood-fall rather than seep communities. Proceedings of the Royal Society B 273: 2625-2631.