Paleoecology and taphonomy of the Early Cambrian Maotianshan Shale biota chancelloriid Allonnia junyuani: Adaptation to nonactualistic Cambrian substrates

摘要

The Cambrian radiation marks a period of extensive ecologic innovation within metazoan communities. Among these innovations were increasing levels of vertical bioturbation and a subsequent transition from Proterozoic-style firm unlithified substrates with very low levels of bioturbation to Phanerozoic-style soft substrates with a high water content and well-developed mixed layer. This transition, termed the "agronomic revolution" had a significant impact on benthic organisms already adapted to firm unlithified substrates, leading to evolutionary and ecological changes in these organisms known as the Cambrian Substrate Revolution (CSR). Chancellodids are morphologically unusual sessile suspension feeders restricted to the Early and Middle Cambrian, and as such are a likely candidate to have been affected by the CSR. Examination of exceptionally preserved specimens of the species Allonnia junyuani from the Maotianshan Shale of Yunnan Province, China, in addition to a study of the rocks in which they are preserved, indicate that A. junyuani was likely a shallow sediment sticker that used a tapered blunt basal end for insertion into firm unlithified substrates. Bioturbation levels in the Maotianshan Shale are very low (ichnofabric index average= ii1) suggesting that Proterozoic-style firm unlithified substrates dominated this environment, consistent with the morphological adaptations of A. junyuani. These results suggest that A. junyuani was adapted to nonactualistic environments, and this may have partially contributed to its unusual morphology and extinction by the Late Cambrian. Complete, well-preserved specimens from the study group were very rare (3.7% of total), whereas partly or completely disarticulated specimens dominated (77.5% of total), suggesting that, despite the Burgess Shale-type preservation apparent in the Maotianshan biota, A. junyuani typically underwent significant pre-burial decay.