EVOLUTION OF MOLLUSKS

The big three

Annelids, arthropods, and mollusks are three major invertebrate phyla, with coeloms and blood vascular systems, that have successfully expanded from probable marine origins into freshwater and terrestrial environments.  Their close relationship was suspected for over a century because of the annelid cross.  The annelid cross was a peculiar relationship of four cells found in at least some of the early embryos of all three phyla.

Annelids and arthropods are easily seen to be closely related because of their obvious metamerism, cuticles, and general relationships of some systems.  But all but some recently discovered mollusks lack obvious metamerism.  So which came first, mollusks or metamerism?

Annelid origin of mollusks

        Ventral view of the external anatomy of Neopilina

The discovery (Lemche an Wingstrand, 1959) of a living member of the Monoplacophora, Neopilina, in 1952 helped answer the question.  It had obvious remnants of metamerism in paired nerves, blood vessels, and muscles.  The single pair of ventricles straddled the posterior portion of the gut and helps illustrate the way the unusual perirectal ventricle of bivalves evolved to enclose that portion of the gut.

McAlester (1964) provided further evidence of the monoplacophoran-bivalve connection with the intermediate fossil, Babinka.  Figure 9-41 (above), on page 479 of

Invertebrate Zoology, 3^rd ed., by Engemann and Hegner (1981) illustrates the transition of muscle scars those of modern bivalves such as the clam, Anodonta.

The fossil studied by Sutton et al. (2001) helped fill the gap between polychaete annelids and mollusks.  They reported it had a polychaete-like body with 7 small calcareous dorsal valves spaced along the dorsal surface.   They named it Acaenoplax hayae gen. nov. and sp. nov. in Phylum Mollusca, and the fossil they “interpret as a plated aplacophoran.”  Structure was determined by computerized reconstruction of serially ground sections.  The only internal structure was a tube in some sections thought to be a gut.  A space may indicate the position of an undeveloped valve.  The posterior 7^th valve has a ventral portion as well as a dorsal portion.  About 18 ridges circling top and sides bear setae and give a polychaete-like appearance in their photos.

   

The molluscan shell as a cause of loss of metamerism

The success of the cone-shaped shell and the muscular foot may well have made the survival value of repeated structures less valuable for survival.  Hence the loss of appendages and segmentation was promoted by the protective shell.  

It is easy to make a transitional series of gastropod shells from limpets to elongated and spiral shells arising from the monoplacophoran type.  From gastropods with siphons to cephalopods with their jet propulsion is less clear but quite likely.

References

Engemann, Joseph G., and Robert W. Hegner.  1981.  Invertebrate Zoology, 3^rd ed.  Macmillan Publishing Co., New York.  746 pp.

Lemche, H., and K. G. Wingstrand.  1959.  The anatomy of Neopilina galatheae, 1957.  Galathea Report, 3:9-72. +56 Pl.

McAlester, A. Lee.  1964.  Transitional Ordovician bivalve with both monoplacophoran and lucinacean affinities.  Science, 146:1293-1294.  Babinka has muscle scars intermediate between the monoplacophoran, Neopilina, and modern bivalves.   

Sutton, Mark D., Derek E. G. Briggs, David J. Siveter and Derek J. Siveter.  2001.  An exceptionally preserved vermiform mollusc from the Silurian of England.  Nature, 410:461-463.

Joseph G. Engemann   August 3, 2013