An overview of Miocene to Pleistocene methane-seep faunas from Taiwan: Paleoecology and paleobiogeographic implications

The invertebrate macrofauna of four methane-seep deposits of Miocene to Pleistocene age from Taiwan is taxonomically evaluated. The Neogene faunas, with one example each from the early and late Miocene and the Pliocene, consist exclusively of one or two large, infaunal lucinid bivalve genera per site, namely Meganodontia and Lucinoma. The former includes the large, charismatic Pliocene ‘Loripes’ goliath, which is here identified as belonging to the widespread, Neogene to Recent genus Meganodontia. The Pleistocene seep fauna is more diverse, includes extant species like the mytilid Gigantidas horikoshii and Meganodontia aff. acetabulum, unidentified species belonging to Lucinoma and the vesicomyid Isorropodon, and specimens resembling the enigmatic bivalve Sisonia frijellanae, described so far only from the late Miocene of the Philippines; the gastropods could not be identified below family level. The two Miocene seep faunules formed on the shelf of the Eurasian plate, the Pliocene faunule in an upper offshore setting in the Tainan foreland basin, whereas the Pleistocene site with its more diverse fauna formed closer to the shelf edge of the Tainan foreland basin. We suggest that water depth is the most likely driver of this difference in species diversity. The very negative δ13C signature of the seep carbonates of all fossil sites indicates the seepage of mostly biogenic rather than thermogenic methane, analogous to the extant seep sites around Taiwan. Biogeographically, the Miocene seep faunas show, with Meganodontia, links to seep faunas from lower to mid-latitudes worldwide (Caribbean Sea, Mediterranean Sea, Philippines, and New Zealand). From the Pliocene onward, biogeographic ties are more restricted to areas in the central Indo-West Pacific Ocean.