Sclerite-covered taxa and early trochozoan evolution----Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences

Reconstruction of Orthrozanclus elongata n. sp. in life

Orthrozanclus is a shell-bearing, sclerite covered Cambrian organism of uncertain taxonomic affinity, seemingly representing an intermediate between its fellow problematica Wiwaxia and Halkieria. Attempts to group these slug-like taxa into a single ‘halwaxiid’ clade nevertheless present structural and evolutionary difficulties.

Recently, Prof. ZHAO Fangchen and colleagues from Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, with Dr. Smith Martin from Durham University, report a new species of Orthrozanclus from the early Cambrian Chengjiang Lagerstätte, published in Scientific Reports.

The scleritome arrangement and constitution in this material corroborates the link between Orthrozanclus and Halkieria, but not with Wiwaxia — and calls into question its purported relationship with molluscs. Instead, the tripartite construction of the halkieriid scleritome finds a more compelling parallel in the camenellan tommotiids, relatives of the brachiopods and phoronids.

Such a phylogenetic position would indicate the presence of a scleritome in the common ancestor of the three major trochozoan lineages, Mollusca, Annelida and Brachiozoa.

On this view, the absence of fossil Ediacaran sclerites is evidence against any ‘Precambrian prelude’ to the explosive diversification of these phyla in the Cambrian, 540–530 million years ago.

Orthrozanclus elongate from the Cambrian Chengjiang Biota

This research was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences and the National Natural Science Foundation of China.

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