The original molluscan radula and progenesis in Aplacophora revisited

Abstract

Radulae of two Middle Cambrian Burgess Shale stem molluscs Odontogriphus and Wiwaxia have recently been imaged by new techniques. Images show both species had a central, or rhachidian, tooth and mirror-image, separately articulating lateral teeth as found in the radulae of Polyplacophora and Conchifera. The hypothesis from earlier radula images of these two fossil genera is that their radulae were like those of Aplacophora (Solenogastres [or Neomeniomorpha] + Caudofoveata [or Chaetodermomorpha]) where there is no central tooth. From these earlier fossil images it was concluded that Aplacophora, often considered basal molluscs, had retained the condition of the original molluscan radula. Support for considering that the early mollusc radula lacked a central tooth rested on embryogenesis in several Polyplacophora and many Gastropoda species in which the rhachidian tooth develops after the first lateral teeth. Recent molecular studies and fossil vermiform chitons indicate that Aplacophora, rather than being basal to Mollusca, branched from chitons in the Ordovician, after development of the rhachidian tooth in molluscs. The arrested development in Aplacophora of a central tooth supports the hypothesis that Aplacophora are progenetic, rather than reflecting the earliest molluscan radula. As a vermiform, shell-less taxon, Aplacophora appears to be preadapted to a deep-sea floor turbidite, changed from the prevalent biomats of Precambrian and Cambrian times.

Keywords:

• rhachidian
• Kimberella
• Wiwaxia
• Odontogriphus
• Aculifera
• Ordovician

Acknowledgements

This is the first recent aplacophoran paper authored by me that does not include Chris Schander’s name, for we worked together closely from the time he was a postdoc in my lab. He always gave me a hard run for the money when I theorised, and I sorely miss not only that, but his warm presence and email support. Our last paper together (Scheltema, Schander, Kocot. 2012. Hard and soft anatomy in two genera of Dondersiidae (Mollusca, Aplacophora, Solenogastres). Biological Bulletin 222:233–269) sadly was posthumous for Chris.

Thanks go to both Julia Sigwart and Christiane Todt who honoured me as one of the organizers of Who are the ‘Aculifera’? (WCM 2013, Azores) and arranged with Kevin Kocot that I could attend the symposium by Skype. I thank Martin Smith for providing me with the original Wiwaxia radula image before publication and for helpful discussions. Dmitry Ivanov gave me a difficult choice to make among his many fine caudofoveate radula images (Figure 2B), as well as discussions about the caudofoveate radulae. Jacob Vinther, Christiane Todt, and another reviewer gave helpful advice and corrections on several aspects of the text; their attentions have strengthened many a weak spot. This is a contribution to the collection Who are the ‘Aculifera’? in memory of Christoffer Schander.