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.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call