Comparative Palaeoecology of Chancelloriids in the Maotianshan Shale and Wheeler Shale Biotas: Non-Adaptation to the Cambrian Substrate Revolution
Tristan J. Kloss1†, Stephen Q. Dornbos1 and Junyuan Chen2
1 Department of Geosciences, University of Wisconsin, Milwaukee, Wisconsin, USA
2 Institute of Geology and Palaeontology, Nanjing, China
Chancelloriids were a taxonomically enigmatic group of Early and Middle Cambrian animals noted for their unusual sponge-like morphology. They lived in the midst of the Cambrian substrate revolution, when increasing bioturbation in shallow subtidal unlithified seafloors led to a transition from firm Proterozoic-style substrates to soft Phanerozoic-style substrates with a well-developed mixed layer.
We compare the ecology of two exceptionally preserved chancelloriids: Allonnia, from the early Cambrian Maotianshan Shale biota, southwest China; and Chancelloria eros, from the middle Cambrian Wheeler Shale of Utah. Both these localities exhibit an extremely low level of bioturbation, with an ichnofabric index of 1. This suggests that firm Proterozoic-style substrates still dominated both environments.
Allonnia is interpreted as a shallow sediment sticker adapted to firm substrates. Its morphology reflects this lifestyle: specimens often exhibit tapered blunt basal ends, and lack structures for attachment to hard substrates. These features are shared with Chancelloria, suggesting that it too was a shallow sediment sticker. It is possible that the chancelloriids were restricted to firm substrates, and that their failure to adapt to the changing substrate conditions of the Cambrian substrate revolution led to their extinction in the late Cambrian.