Living Monoplacophora: morphological conservatism or recent diversification?

Kano, Y., Kimura, S., Kimura, T. & Warén, A. (2012) Living Monoplacophora: morphological conservatism or recent diversification? — Zoologica Scripta , 41 , 471–488. The molluscs of the class Monoplacophora are classic and probably the most striking case of a ‘living fossil’ in the deep sea. Until the discovery of the first living specimens in 1950s, the group was thought to be extinct since the Devonian period, almost 400 million years ago. Morphological conservatism over a long span of time in a ‘living‐fossil’ lineage may theoretically result in distant but closely resembling taxa, and this idea apparently led some authors to recognise as many as six families and three superfamilies for less than three dozen living monoplacophoran species. However, no genetic or fossil data have been available regarding the history of their diversification. Here we describe Veleropilina seisuimaruae sp. n., the first member of the class from the north‐western Pacific, based on the shell, radular and anatomical characteristics. Phylogenetic analyses of 6‐kb DNA sequences estimate that the divergence of V. seisuimaruae and Laevipilina hyalina , the only other monoplacophoran available for genetic studies, dates back only to the Late Cretaceous, despite their significant morphological differences among the living members of the class. The recent Monoplacophora might have radiated fairly recently from a cryptic lineage with ordinary rates of morphological evolution, possibly after the global deep‐sea anoxia at the Cenomanian/Turonian boundary.