Living Monoplacophora: morphological conservatism or recent diversification?

Abstract

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.

The Continuing Debate on Deep Molluscan Phylogeny: Evidence for Serialia (Mollusca, Monoplacophora + Polyplacophora)

“…The chiton girdle is also covered by a cuticle with embedded calcareous and organic sclerites, similar to the body cuticle of the shell-less aplacophorans, but according to our results, this is convergent and may reflect the different, single versus multicellular spicule formation in these taxa [50]. That chitons cluster with monoplacophorans rather than aplacophorans is congruent to previous molecular approaches that included monoplacophoran exemplars [4, 9, 10, 13]. The exception is the phylogenomic study by Smith et al [8], in which a single monoplacophoran, Laevipilina hyalina , robustly clustered with cephalopods in the main analyses, though parts of the genes used also showed signal supporting an association with chitons.…”

Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’

“…Instead, throughout their long evolutionary history, notostracans have undergone multiple global radiations and high species turnover. Recent, time calibrated, phylogenetic analysis of other traditional ‘living fossils’ such as cycads (Nagalingum et al, 2011), nautiloids (Wray et al, 1995), horseshoe crabs (Obst et al, 2012) and monoplacophorans (Kano et al, 2012), have also revealed that extant species are more recently diverged than suggested by fossil data alone. We therefore caution against drawing conclusions about patterns of diversification based on fossil data alone in groups where widespread morphological conservatism may obscure rampant cryptic speciation.…”

“…Since Darwin’s first use, the ‘living fossil’ term has been applied to groups which appear to have diversified little and are morphologically stable over long periods of evolutionary time, with examples including cycads, tuatara, coelacanths, horseshoe crabs and Ginkgo biloba . However, morphological stasis can obscure the patterns of species diversification, and recent time-calibrated phylogenetic analyses of some ‘living fossils’ have indeed revealed that extant species are in fact only recently diverged (Kano et al, 2012; Nagalingum et al, 2011). …”

Deep-sea video technology tracks a monoplacophoran to the end of its trail (Mollusca, Tryblidia)

“…As in other deep-sea molluscs, monoplacophorans are apparently reasonably tolerant to the transition to surface pressure and temperature, and species in several genera have been observed alive after recovery to the surface (Lowenstam 1978;Marshall 1998;Urgorri et al 2005;Schrödl et al 2006;Ivanov and Moskalev 2007;Wilson et al 2009). Molecular sequence data have been published for seven species (Wilson et al 2010;Kano et al 2012;Stöger et al 2013) including one partial and two complete mitochondrial genomes to date (Stöger et al 2016).…”

“…400 million years ago), with no further fossils until the Cenozoic (Warén and Gofas 1996). Based on molecular clock estimates, living monoplacophorans share a common ancestor as recently as the Late Cretaceous, 83.2 Ma (Kano et al 2012;Stöger et al 2013). This gap of some 300 million years in the fossil record provides room for doubt about whether even Tryblidia is monophyletic.…”