Comparative Paleoecology of Chancelloriids In the Early Cambrian Maotianshan Shale Biota and the Middle Cambrian Wheeler Shale Biota: Non-Adaptation to the Cambrian Substrate Revolution

Chancelloriids were a taxonomically enigmatic group of early and middle Cambrian animals noted for their unusual 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. Paleoecological studies of the exceptionally preserved chancelloriid Allonnia from the early Cambrian Maotianshan Shale biota of southwest China suggest that Allonnia was a shallow sediment sticker adapted to firm Proterozoic-style substrates, and that this lifestyle partly contributed to its morphology. Extremely low bioturbation levels within the Maotianshan Shale (average ii=1) and specimens of Allonnia exhibiting tapered blunt basal ends, while lacking structures for attachment to hard substrates, support this interpretation. Specimens of the chancelloriid Chancelloria eros and the rocks in which they are preserved were collected from the middle Cambrian Wheeler Shale of Utah for comparative analysis. The morphology of Chancelloria shows similarities with Allonnia, exhibiting a blunt basal end and lacking attachments for hard substrates, making it likely that Chancelloria was a shallow sediment sticker. Several meters of strata from the Wheeler Shale were analyzed using the ichnofabric index (ii) method. Results indicate extremely low levels of bioturbation (average ii = 1), suggesting that firm Proterozoic-style substrates still dominated this environment during the middle Cambrian. These results suggest that Chancelloria is morphologically very similar to Allonnia, probably due to adaptations to firm Proterozoic-style substrates that were still present in some settings during the middle Cambrian. The adaptations of Chancelloria also suggest that middle Cambrian chancelloriids had not adapted to the changing substrate conditions of the Cambrian substrate revolution. It is possible that this non-response to changing substrates contributed to the extinction of chancelloriids by the late Cambrian, although this hypothesis remains to be tested.