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The origin of a phylum: soft-bodied fossils and early mollusc evolution

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How did the molluscs originate? Molecular and morphological studies give different answers to this question. The traditional morphological story envisages worm-like primordial molluscs that eventually gained a shell, ultimately diversifying to produce the complex anatomies of cuttlefish, snails and clams. But molecular studies are converging on a different topology, where shell-less molluscs represent a highly derived rather than primitive state. This concept finds support from recent fossil finds, and a new 965-character morphological supermatrix suggests that the traditional model represents an artefact of the techniques used to analyse morphological data. When a more appropriate model of evolution is employed, morphological data reproduce the “molecular” topology. This tree indicates that modern molluscs are all highly derived, so extant taxa can tell us little about molluscan evolution – the fossil record offers an invaluable window on the origin of molluscs. Shelly fossils are often ambiguous, so we must turn to exceptional “soft-bodied” fossils if we are to identify early molluscs. The contentious Wiwaxia, known from the Cambrian Burgess Shale (~500 Ma), has been interpreted as an annelid worm. It lacks a shell, and is covered in carbonaceous spines and scales. New observations reveal similarity between its mouthparts and sclerites and those of molluscs, and reveal a molluscan creeping foot. Quantitative analysis shows that its detailed anatomy strongly supports a molluscan affiliation, and places the taxon near the base of the molluscan tree. Molluscs seemingly evolved from a Wiwaxia-like ancestor – but how fast did the modern groups arise? Most molluscan classes leave their first shelly fossils late in the Cambrian period, suggesting a gradual diversification. But another soft-bodied taxon from the early Cambrian, Nectocaris, strikingly resembles the derived coleoid cephalopods. Could molluscs really have diverged so early? Nectocaris’s interpretation has been controversial, but detailed anatomical observations uphold its similarities to the cephalopods. Whether this resemblance represents common ancestry or profound convergence, the molluscs rapidly attained body plans as complex and disparate as those seen today.

References

1 Smith, M. R. 2012 The mouthparts of Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula. Proceedings of the Royal Society B: Biological Sciences 279, 4287–4295. (doi:10.1098/rspb.2012.1577)

2 Smith, M. R. & Caron, J.-B. 2010 Primitive soft-bodied cephalopods from the Cambrian. Nature 465, 469–472. (doi:10.1038/nature09068)

3 Smith, M. R. & Caron, J.-B. 2011 Nectocaris and early cephalopod evolution: Reply to Mazurek & Zatoń. Lethaia 44, 369–372. (doi:10.1111/j.1502-3931.2011.00295.x)

4 Smith, M. R. 2013 Affinity, ecology and diversity of the early “cephalopod” Nectocaris. Paleobiology in press.

5 Todt, C. & Wanninger, A. 2010 Of tests, trochs, shells, and spicules: Development of the basal mollusk Wirenia argentea (Solenogastres) and its bearing on the evolution of trochozoan larval key features. Frontiers in zoology 7, 6. (doi:10.1186/1742-9994-7-6)

6 Telford, M. J. & Budd, G. E. 2011 Invertebrate evolution: bringing order to the molluscan chaos. Current Biology 21, R964 –R966. (doi:10.1016/j.cub.2011.10.029)

7 Sutton, M. D., Briggs, D. E. G., Siveter, D. J., Siveter, D. J. & Sigwart, J. D. 2012 A Silurian armoured aplacophoran and implications for molluscan phylogeny. Nature 490, 94–97. (doi:10.1038/nature11328)

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