fourth stage of ecological succession at whale fall sites, called the "reef stage". A whale fall enters this stage once the organic compounds have been exhausted and only minerals remain in the bones, which provide a hard substrate for suspension and filter feeders. Methanogenesis A process called methanogenesis can also occur around whale falls. Archaea that produce methane can be abundant in anoxic sediment, but are typically not found in co-occurrence with the sulfur reducing bacteria found at whale falls. Whale falls do however support both sulfur reducing bacteria and methane producing archaea, leading to the conclusion that the area is not electron donor limited, and/or there is minimal or no competition for suitable substrate. Concentration gradients of both sulfide and methane can be found around whale falls, with the highest concentration coming within one meter of the carcass, which is several orders of magnitude higher than the surrounding sediment concentrations. Methanogenesis appears to only occur in sediments as opposed to sulfur reduction, which occurs both in sediments and on the bones of the carcass. The addition of sulfur reduction in both sediments and high lipid whale bones is a key factor for why whale falls are able to sustain deep-sea communities for extended periods of time. Paleontology Whale fall fossils from the late Eocene and Oligocene (34–23 MYA) in Washington and from the Pliocene in Italy include clams that also inhabited non-chemosynthetic environments. Chemosynthetic-only animals do not appear until the Miocene (23–5 MYA) in California and Japan. This may be because the lipid content of early whale bones was too low. As prehistoric whales evolved to live in pelagic waters and dive deeper, structural changes in their anatomy included increased size, reduced bone density and higher lipid content. It is this increased lipid content that led to the establishment of chemosynthetic communities in the deep sea. The discovery of the limpet Osteopelta in an Eocene New Zealand turtle bone indicates that these animals evolved before whales, including possibly inhabiting Mesozoic (251–66 MYA) reptiles. They may have survived in seeps, wood-falls and vents while waiting out the 20 million year gap between the reptiles' extinction and whales' emergence. Another possibility is that these fossils represent a prior, dead-end evolutionary path, and that today's whale fall animals evolved independently. Anthropogenic effects It has been suggested that the whaling industry has had an effect on the biological pump through the elimination of