TAPHONOMY AND PALEOECOLOGY OF THE EARLY CAMBRIAN MAOTIANSHAN SHALE BIOTA CHANCELLORIID ALLONNIA

The early Cambrian soft-bodied Maotianshan Shale biota of Yunnan Province, China provides a high-fidelity view of the Cambrian radiation. One of the more unusual animals in this biota is the taxonomically enigmatic chancelloriid Allonnia. In order to better understand the taphonomy and paleoecology of Allonnia, 626 specimens housed at the Early Life Research Center in Chengjiang, China were examined. These specimens were categorized based on extent of decay as follows: 1) articulated with sclerites well-preserved; 2) articulated with sclerites poorly preserved; 3) partly disarticulated; and 4) completely disarticulated. The majority (77.5%) of specimens were partly or completely disarticulated, suggesting that most of the chancelloriids experienced significant pre-burial decay. All of these specimens do exhibit some degree of soft-tissue preservation, indicating that post-burial decay was minimal however. Complete specimens were very rare (~ 3.7%). This portion of the study indicates that despite being preserved in a Burgess Shale-type taphonomic setting, Allonnia typically underwent significant decay before final burial. Examination of its substrate-adaptive morphological features reveals that the basal end of Allonnia tapers to a round blunted point, with no structures for attachment to hard substrates or stabilization, making it likely that it lived as a sediment sticker with its lower end inserted in the sediment. Bioturbation levels will also be assessed using the ichnofabric index (ii) method on 2.3 meters of strata from Maotianshan Shale core samples. Initial observations of these core samples indicate extremely low (~ii1) levels of bioturbation, suggesting that firm Proterozoic-style substrates still dominated this environment. The extremely low levels of bioturbation are also consistent with the interpretation of Allonnia as a sediment sticker. The results of the paleoecological portion of the study suggest that Allonnia was adapted to nonactualistic early Cambrian substrates, a lifestyle that may have partly contributed to its unusual morphology. The Chancelloridae in turn were part of a larger community of early Cambrian benthic organisms that evolved partly in response to the presence of nonactualistic substrates and were not simply failed evolutionary “experiments”.