The worm-like molluscs, the Aplacophoria, are not well known to most of us, and they are a difficult group to study because they live in deep waters. Their taxonomy has been somewhat controversial, with little agreement on their relationships with other molluscan groups. However, the consensus has been that they are the “most primordial molluscs to be found on earth”. There is an evolutionary story that helps to promote this understanding: the other molluscan groups appeared during the Cambrian Explosion, and they have armour of some kind. It is reasonable to postulate a simpler kind of mollusc that had no armour – which appears to be exactly what we have in the aplacophorians. New research, however, has brought a change of view: these naked molluscs are now considered to have evolved from the ancestral shelled forms.
“A fossil unearthed in Great Britain may end a long-running debate about the mollusks, one of life’s most diverse invertebrate groups: Which evolved first, shelled forms like clams and snails, or their shell-less, worm-like relatives? The small new fossil, found in marine rocks along the English-Welsh border, provides the best fossil evidence yet that the simpler worm-like mollusks evolved from their more anatomically complex shelled brethren, rather than the other way around.”
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