Community palaeoecology of the early Cambrian Maotianshan Shale biota: Ecological dominance of priapulid worms

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The early Cambrian soft-bodied Maotianshan Shale (Chengjiang) biota of Yunnan Province, China provides a critical glimpse of animal life during the heart of the Cambrian radiation. The Shankou biota is the focus of this current work, and 9963 specimens from it have been examined and tallied. This collection of fossils represents a time-averaged assemblage of unknown duration from a 10-meter thick sequence. The predominantly benthic community recovered from this section is presumably autochthonous. This community was buried through several hundred millimeter to centimeter thick burial events. Only specimens interpreted as buried alive, based on either soft-part preservation or fully articulated skeletons, were counted. Algae were too fragmented to be counted. To avoid including synonymous species, diversity was evaluated at the genus level. A total of 57 genera from 9 phyla, encompassing 14 ecological groups, were found in this assemblage. The results reveal that the three most abundant genera comprise 43.2% of all specimens: the tubiform priapulid worm Paraselkirkia (16.0%), the diminutive priapulid worm Sicyophorus (14.3%), and the brachiopod Heliomedusa (12.9%). No other genera total more than 9% of specimens. At the phylum level, there is an interesting dichotomy between taxonomic diversity and ecological dominance. The arthropods are the most diverse phylum (37% of genera) and rank second in relative abundance of specimens (26.3%). The priapulids, however, with only 17.5% of genera is the most abundant group in the assemblage (43.2%). All other phyla, excluding brachiopods (19.6%), represent only 10.9% of the assemblage. Because of their sheer numerical abundance, this study indicates that priapulid worms may have exerted more influence on energy flow and community structure than other phyla in this particular trophic web. This result contrasts strongly with both traditional views of Maotianshan Shale biota palaeoecology, which often claimed arthropod dominance based solely on taxonomic diversity, and the palaeoecology of modern priapulids, which are relegated to marginal marine settings. This result also demonstrates the importance of collecting quantitative relative abundance data when performing palaeoecological investigations.