Monoplacophora

Monoplacophora meaning 'bearing one plate' is a class of molluscs. They have a cap-like shell and live on the bottom of the sea.Menzies R.J. et al 1959. Ecology of the Recent Monoplacophora. Oikos 10 (2): 168–182.  

They were well-known as a fossil group from the Cambrian to the Devonian. One species was dredged up from the Pacific Ocean off Mexico in 1952. It was found to be a Monoplacophoran, and given the genus of Neopilina.Lemche, Henning 1957. Nature 179, 413–416.  This was one of the most remarkable modern discoveries of a 'living fossil', and a Lazarus taxon. 

More than that, they are the most extreme example of the pull of the recent, which is a term in palaeontology. All fossil groups have a first and last appearance in the fossil record but, for living species, their last appearance is the present day. This may be very much later than their last appearance as a fossil. The discovery of the living monoplacs extended their time range by 400 million years.Hallam A. and Wignall P.B. 1997. Mass extinctions and their aftermath. Oxford University Press, Oxford.

The anatomy of Neopilina shows "serially repeated structures" such as gills. Serial repetition of anatomical structures such as gills and muscles may have evolved once in the common ancestor of chitons and monoplacophorans. It suggests the ancient ancestor of molluscs had bilateral symmetry, and segments.Giribet, Gonzalo et al 2006. Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons. PNAS 103, 7723–7728.  

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category:Molluscs