A FOSSIL RADULA FROM THE LOWER CAMBRIAN MAHTO FORMATION, JASPER NATIONAL PARK, CANADA

Delicate processing of a shelf-facies mud/siltstone from the Lower Cambrian Mahto Formation, south-central Canadian Rocky Mountains, has yielded an abundance of denticulate and spinose carbonaceous microfossils. Isolated structures can be arranged into a number of broad morphological categories, including: 1) linear arrays of up to 13 hook-like denticles with a fibrous microstructure, sometimes showing signs of terminal wear; 2) pointed, sometimes bifurcating elements with a bipartite fibrous base; and 3) batteries of densely packed, linearly arranged Y-shaped elements. Regular co-occurrence, similarity in microstructure, and instances of in situ articulation allow these structures to be reassembled as a complex, multi-element feeding apparatus, directly comparable to the molluscan radula. There is no evidence of molluscan shell, or indeed any shelly material, in this sample, suggesting that the source organism was non-biomineralizing. Co-occurring hooks, spines, and patterned/perforated organic sheets may or may not represent other preservable components. Reasonably close comparisons can be made between this early Cambrian radula and those of extant aplacophorans and/or gastropods, though there is no particular reason to assume it derives from a crown-group mollusc. Even so, the radula has long served as the principal synapomorphy of the Mollusca, and this Lower Cambrian occurrence substantially corroborates SSF evidence of their early diversification.