An aplacophorian mollusc with armour

  • Julia Sigwart

Press/Media: Research


Science Literature [news website]

by David Tyler

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."

The experimental background to this find is worthy of note. Although the fossil was discovered a decade ago, its detailed structure has only been revealed recently - by serial grinding at intervals of 30 micro-metres and recording each step digitally. Then, the images have been processed to reconstruct a 3D model using physical-optical tomography. This has revealed a worm-like body covered by seven protective plates reminiscent of a chiton. The body is that of an aplacophoran, whereas the armour is polyplacophoran. The research paper says it is "the first unambiguous combination of palaeoloricate valves and and aplacaphoran body" (p.960). One of the authors of the research paper refers to it as a "kind of missing link".

"This is a kind of missing link with a worm-like body, bearing a series of shells like those of a chiton or coat-of-mail shell," said Derek E. G. Briggs, director of the Yale Peabody Museum of Natural History and one of the paleontologists who studied the new fossil, Kulindroplax perissokomos.

There are two other fossils that hint at affinities between these two groups. Considered as a whole, the authors consider that we have good evidence for the "flexibility and disparity of the palaeoloricate bauplan" (p.95). Their cladistic analysis suggests that these two groups come together in a natural clade: the Aculifera. This is perceived as a sister group to the Conchifera. The emerging phylogenetic analysis may signal an "end to a long-running debate about the mollusks" (source here). The authorts also point out that their work is complementary to some other studies involving molecular evidence (such as that of Kokot, et al. cited below):

"The recovery of Aculifera and Conchifera in all variants of our analysis demonstrates the strong support that fossil data bow offer for the Aculifera hypothesis, paralleling recent molecular evidence favouring this topology." (p.96)

One implication of this study is that the aplacophorans are not the "most primordial molluscs to be found on earth". No longer should they be thought of as appearing on Earth during the Cambrian explosion. These animals are derived, having shed their armour. The Press Release puts it this way: the new fossil "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." The research paper concludes:

"Aplacophorans were primitively shelled molluscs; the few living forms represent the survivors of a diverse valve-bearing Palaeozoic clade." (p.96)

The research points us to the value of using the bauplan paradigm in our scientific work. The Darwinist mindset tends to lose sight of the higher taxonomic categories, and its adherents focus on the origin of new species by natural selection acting on natural variations. New biological information is said to be generated by this species-level process. However, bauplan thinking is far more open to flexibility and disparity arising from existing biological information by permutation and combination. This creates a major issue for cladism to address: how do we assess whether characters are primitive or derived? This study gives us a practical example: a character everyone once thought was primitive has turned out to be derived. If most species are permutations within their bauplan, then the weight given to natural selection acting on natural variations is greatly overplayed.

Furthermore, if bauplan thinking is given due weight, the quest for evolutionary linkages is an agenda for finding false positives. Diversification is unlikely to be linear, but fractal, depending on environmental factors. Classic examples of relevant Darwinist misinterpretation are the alleged horse lineage, the mammal-like reptiles, and hominids. Palaeontologists have been much more aware of the "bushy" nature of diversification trends than evolutionary biologists!

The description of "missing link" should also be considered in the light of bauplan thinking. The new fossil has pointed to a common ancestry for the aplacophorans and the polyplacophorans. These are united in the Aculifera, a natural clade. The newly described fossil is a "link" between the two groups, and it has previously been "missing". In this sense, the significance of a missing link is to unite biological groups with a common bauplan. This is quite different from the Darwinian use of the term, whereby a fossil organism marks a key transition in an evolutionary lineage. The two usages of the phrase should not be confused.

Period08 Oct 2012

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  • TitleAn aplacophorian mollusc with armour
    PersonsJulia Sigwart